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XJ-S Cruise Control

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Thanks for the following procedures for troubleshooting the Jaguar Cruise Control system go mainly to Tom D. Graham.

Description

An electrical signal from the drive train provides the signal about how fast the car is going. On early XJ-S's there was a dedicated sending unit near the input flange on the differential unit. Later XJ-S's split a signal from the speedometer transducer on the transmission. On still later models, reportedly the transducer is built into the differential unit itself.

The signal is processed by the cruise control electronic speed control unit. The speed control unit directs the amount of vacuum in the bellows unit and the bellows unit operates a cable to the gas pedal/throttle.

Within the bellows assembly are two solenoids, one which normally (cruise control off) vents the bellows to atmosphere and the other which normally seals off the vacuum line from the intake manifold. The "vent solenoid" is connected to the yellow/white and black wires, and the "vacuum solenoid" is connected to the yellow/white and yellow/black wires. When the control unit calls for speeding the car up, 12V is applied to the yellow/white wire, activating both solenoids. The vent is sealed and the vacuum line is opened, and the resulting vacuum within the bellows pulls the cable, applying throttle. The speed control unit modulates the ground connection of the vacuum solenoid to apply the proper vacuum to maintain a constant speed.

On the brake pedal housing is a switch that breaks the yellow/white wire whenever the brakes are applied, effectively shutting off the system and allowing the throttle to return to idle. This is actually a back-up feature; the power from the brake light switch is also sent to the speed control unit, which is supposed to drop the power to the yellow/white wire.

As the result of a recall, there is yet another back-up device installed in the vacuum line to the bellows unit. This device is designed to seal the vacuum line and vent the bellows unit in addition to the solenoids.

Adjustment

If the cruise control is adjusted correctly, when the set switch is pressed the system will maintain the speed the car was doing at the instant the button was pressed. If adjusted incorrectly, it will maintain either a higher or lower speed. Hence, the test procedure is drive the car on a straight and level road and press the set switch, allow the speed to settle about ten seconds, and press it again. If adjusted properly, it can be set over and over and still maintain the same speed. If incorrectly, the repeated sets will result in gradually higher or lower speeds.

If adjusted incorrectly, adjust the speed control unit until correct; it is a simple matter to have the speed control unit hanging under the dash (it is located above the passenger side footwell) and the necessary tools along for the test drive, and possibly even an assistant driving. Although the repair manual describes a method of adjusting the cruise control, some of these units have no obvious adjustments. If you pry the box open, however, there are two adjustable pots on the circuit board. The one to adjust is the one in the corner, farthest from where the wires enter the box. NOTE: The adjuster is VERY sensitive; it is difficult to move it a small enough increment.

Bellows Unit

If you have an ohmmeter, check the resistance across the bellows solenoids, yellow/white to black and yellow/white to yellow/black. Each solenoid should register 25-30 ohms.

Unplug the connector to the bellows unit, and connect the black wire from the unit to chassis ground and the yellow/white wire to 12V using a jumper wire. You should hear a click. Disconnect the black wire from ground and connect the yellow/black wire to ground, and once again connect the yellow/white wire to 12V. You should hear another click. This verifies that both solenoids are moving.

Connect both the black and yellow/black wires to ground and the yellow/white to 12V to activate both solenoids, and suck on a hose connected to the bellows unit. You should be able to move the bellows. If you can't seem to suck anything, the vacuum solenoid isn't opening (or the hose is kinked or plugged). If you seem to suck air easily without accomplishing anything, either the vent solenoid isn't closing or the assembly is leaking.

The following comments apply to the older cruise control actuators that had a bellows that looked like a bellows, and a flat metal disk that the cable attached to. Newer cars use a different actuator, and some even attach directly to the bellcrank -- a no-no with the older throttle cable design.

This cruise control actuator can be disassembled easily. Remove one bolt at the front end and disconnect the throttle cable from the disk at the rear end and remove it from the car. Then peel the bellows away from the disk the cable attaches to, and away from the solenoid housing the same way.

These units seem to have two common failure modes. The first and most obvious is that the bellows itself develops a tear or leak; even a small leak is enough to render the system totally inoperative. You can easily check if the rest of the system is operational by patching the leaks, using a bicycle tire patch kit, tape, or whatever. It may not last, but it will tell you if the rest of the system is OK. Most Jaguar supply catalogs indicate you must replace the entire actuator unit, but John's Cars sells the bellows itself. It ain't cheap (US$60+), but it beats paying for the whole unit. When installing, a little vacuum grease or gasket sealant around the edges might be helpful.

If a longer actuator cable can be rigged, it may be a good idea to relocate the actuator to the space between the radiator and the grille. This will not only keep the actuator cooler (perhaps helping the rubber last longer), but it will make working on the distributor easier.

It has also been suggested that using Son Of A Gun or some similar substance on the bellows may help protect the rubber from aging.

Another common failure is that the tiny rubber seats on the solenoid plungers fall off, and the solenoids no longer seal the ports. To repair this failure, merely glue the rubber seat back on the end of the plunger. After removing the bellows, the solenoids can be removed by prying them out of the housing to get at the plunger. If you've lost or damaged the rubber seat, use a hole punch on a bicycle inner tube to make a new one. Make sure the sealing surface is flat and smooth before reassembly.

It must be pointed out that a failure of a cruise control could be dangerous, although that would typically require the two openings to atmosphere to fail closed AND the opening to vacuum to fail open -- highly unlikely. You should make repairs carefully, using a very reliable glue (such as JB Weld) to hold the rubber seat on the rod. Use of unreliable glue such as cyanoacrylate (superglue) is not recommended.

Bellows Relocation

To get that bellows assembly to last longer, it would probably be a good idea to relocate it somewhere cooler. Ron Whiston relocated his to the area in front of the radiator, mounting it on the same bolt that holds the horns. He fabricated a new cable, similar but longer. He notes that removing the mount bracket, which is held to the back of the A/C compressor with the same bolt that holds the hoses on, will require recovering and recharging the freon circuit -- a lot of extra effort and expense. Therefore, he suggests you can either wait until you're working on the freon system anyway, or you can fabricate a new bracket and abandon the original one in place until the opportunity arises to remove it.

Checking Wiring and Switches

To check the wiring, disconnect the connectors wires at the speed control unit. The speed control unit is located just under the fuse panel above the passenger footwell. It has two electrical connectors, a nine pin connector and a single pin connector. Pull the connectors apart and perform the following checks on the wires going out to the car, not to those going into the speed control unit. Make sure that the bellows unit three wire connector is connected properly if it was disturbed by previous testing.

Connect one lead of a volt/ohmmeter (VOM) to a convenient car chassis metallic ground, and test to make sure you have a good ground using the ohm setting on the meter. The other meter probe will be used on the pin connectors. Unless stated, all measurements are made with the ignition off.

1. Set VOM to ohms and test the black wire; it should check as zero ohms, since it is a ground wire.
2. The yellow/purple wire is connected to the "inhibit switch" mounted on the gear selector; this switch opens to allow the cruise control to work only when the selector is in D. The same switch serves to allow the kickdown solenoid in the GM 400 transmission to be activated only when the selector is in D. As a result, when the selector is not in drive, the wire is not truly grounded; it is routed through the kickdown solenoid in the transmission to ground, which is effectively grounded as far as the sensitive speed control unit is concerned. With the selector in P (or anywhere other than D), check that the yellow/purple wire reads low ohmage; it might not be zero, but it should be low. With the selector in D, it becomes open; check that it measures infinite ohms.
   If the console is opened, the physical operation of this switch can be observed. These type microswitches, complete with the rollers, are available at electronic shops.
3. The black/pink wire is connected to the set switch on the turn signal stalk. It normally reads as open (infinite ohms) but should indicate zero ohms while the set switch is pushed. See below.
4. The black/slate wire is connected to the master control switch just behind the selector lever on the console. It should read open (infinite ohms) when the switch is in either ON or OFF position and zero ohms when the switch is moved to RESUME.
5. The yellow/white wire connects to the two bellows unit solenoids through the brake-operated switch which acts as a backup to cut out cruise control when brakes are applied. It should read the 25-30 ohms of the vent solenoid. When the brake pedal is pressed, it should read open.
   This switch is located on the brake pedal housing within the engine compartment, right next to the driver's side bonnet latch. It is threaded into the housing with a locknut. The locknut can be loosened and the switch screwed in or out until it works properly.
6. The yellow/black wire is connected to the vacuum solenoid. It should read 5060 ohms; this is because both solenoid coils are in series to ground referenced to this wire.
7. The yellow/orange wire actually connects to two different pins at the speed control unit connector. It is connected to the master control switch. Turn the ignition on, select a suitable voltage scale on the VOM, and check that this wire reads 12 volts with the switch in the ON position (center) and zero volts with the switch in the OFF position.
8. The green/purple wire is connected to the brake light switch. Normally, this wire should read zero volts; when the ignition is on and the brake pedal is pressed, it should read 12 volts.
9. The yellow wire to the single connector is the signal from the pulse generator mounted on the transmission. Pulses from this wire signal the cars' speed to the speed control unit. This pulse signal also works the speedometer, so if the speedometer is working it is likely that the pulse is also getting to the speed control unit. However, the presence of the pulse at this connector can be checked. Connect a voltmeter to chassis ground and the connector pin. Drive the car, and even at very low speed the signal will measure around 4 volts AC. And the voltage does not increase with speed.

Set Switch Ground

The set switch on the stalk has a ground wire within the column that connects with other grounds. This wire has a nasty tendency to break off right where it attaches to the stalk, rendering the entire cruise control system inoperative. Any scheme that will provide a ground wire connection to the stem of the stalk itself will fix it. The broken wire can be spliced, or a new wire can be clamped onto the stalk just inside the column housing.

Troubleshooting (In Car)

If the bellows unit is OK and no problems are found with the cruise control wiring or switches, the only remaining component to check is the speed control unit. The tester shown in Figure 15 is simple to make, requiring only two flashlight bulbs, two resistors and some wire. It is also used for the bench test of the speed control unit.

Notes

1. Length of the three wires is about three feet each.
2. The two bulbs are 3V flashlight type PR2.
3. The 33 and 22 ohm resistors are 1/2 watt or larger.
4. The bulbs and resistors are soldered as shown.
5. Small alligator clips will help in connecting.
6. All components can be purchased from Radio Shack, about US$6.
   Mount this assembly on a piece of cardboard about 4 by 6 inches.

Figure 15 - Cruise Control Test Circuit

Disconnect the connector from the bellows unit and connect the three wires from this tester to their respective colors on the wiring harness (not to the bellows unit). The bulbs and resistors take the place of the two bellows solenoids so that we can observe the operation of the speed control unit. Route this assembly out from under the hood and use a windshield wiper to hold it against the windshield so you can observe the bulbs while driving.

Next go for a drive. At about 30 to 40 mph push the "set" cruise button. The bulb on the left (33 ohm) should light but rather dimly and stay lit. This bulb is taking the place of the yellowwhite solenoid that closes the bellows to the atmosphere.

The right bulb should light but flicker, and, as you slow down the bulb will light brighter, and as you speed up it will grow dimmer. This bulb is taking the place of the yellowblack solenoid that controls the vacuum from the engine.

If this bulb test circuit works, then the speed control unit and associated wiring/switches are good. If this test fails then the speed control unit is possibly at fault.

Troubleshooting (On Bench)

Bench testing the speed control unit is not complicated but you do need some experience working with electronic circuits. Essentially, what we do is connect up the speed control unit with power and grounds as it would be in the car, then feed it a pulse generation to simulate speed and observe its operation with the above twobulb tester. Thus, to perform this bench test you will need:

1. 12V battery source
2. about 8 alligator clip leads, minisize with boots
3. a sine wave frequency generator.

FIRST, fuse the 12v battery supply with a 1 amp fuse; this will hopefully save you from any nasty smokings. The speed control unit should only draw around 1/2 amp.

Working with the speed control unit and its associated nine pin connector make the following connections:

1. Black/red and brown go to battery ground (negative).
2. Brown/black is not connected (simulating the inhibit switch in drive).
3. Brown/white is not connected (used only for "resume speed").
4. Brown/yellow is attached to a clip lead but left open (it is the "set speed" switch).
5. The twobulb tester is connected, black to battery ground, middle wire to yellow/white, and outside wire to yellow/black.
6. Yellow and black/green are connected to positive 12 volts.

This completes the basic wiring of the speed control unit. Next a pulse must be sent into it via the single green wire connector to simulate the car speed. A simple sine wave generator that can put out 5 volts at 60 to 200 Hz will work. The generators' signal lead is connected to the green wire connector and the ground to battery ground.

With all connections in place and the signal generator set at 80 Hz, and 5 volts, touch the brown/yellow wire to ground for a second and remove it. This simulates pushing the "set speed" switch in the car. The two bulbs should light with the left one remaining steady while the right one changes its flickering as the signal generator frequency is lowered or raised. If not, replace the speed control unit.

Speed Sensor

Ron Whiston points out that the speed sensor for the cars fitted with trip computers is different than the sensor for cars without computers. The differences are obvious, the color is different, the plug is different, but the dealer may still hand you the wrong one. It's best to take the old one with you when buying a replacement.

Replacement Cruise Control Units

Larry Lee sends this description on how to replace the Jaguar cruise control system with a generic Dana unit:

The particular unit I installed was purchased from Sears, Model 318.20309. This model came with a turn signal lever that can replace an existing one (if no other switches are on it), but Model 318.20308 included a clamp-on control switch. Other than the control switch design, the two units are identical. The manual for this unit does not say "Dana" anywhere in it, but I recognized the system as very similar to other Dana units (also purchased from Sears, but marked "Dana Electronic Speed Control Kit 250-1000") I had installed on other cars as far back as 1978. There have been some changes over the years -- such as a change from two driveshaft magnets to one, and deletion of an inertial deceleration switch -- but all of them that I have seen are installed essentially the same way.

The installation manual is quite complete and easy to follow. It includes an electrical schematic and a pictorial drawing that shows how the various components and cables connect. The cable plugs are all different, so improper connections are difficult to make.

Follow the manual instructions to install:

Driveshaft Magnet

Road Speed Pick-up Coil

Servo

Disengagement Switch & Valve Assembly (Brake Switch)

Some judgment must be exercised when mounting the various components.

The Disengagement Switch & Valve Assembly mounts on the brake pedal. When the brake is applied, it breaks an electrical connection to ground, killing the cruise. Besides that, it also vents vacuum to the system, making doubly sure it is off. Considering the fact that the stock XJ-S already has a switch mounted on the brake pedal housing for disabling the cruise control, it may be tempting to discard the Dana unit, plug the vacuum dump line, and just connect the wire to the stock Jaguar switch. Note, however, that the Jaguar switch works the wrong way; it breaks contact when the brakes are released, and connects to ground when brakes are applied. Therefore, a relay would be required to use this switch. Considering the effort required as well as the reduction in safety from defeating the vacuum dump, it's probably better to simply use the Dana parts.

The driveshaft pickup is normally mounted right behind the transmission. However, considering how difficult it is to get around the transmission mount on the XJ-S, it may be preferable to mount it at the rear end of the driveshaft adjacent to the differential. If possible, the pickup should be attached directly to either the transmission or differential in order to move with them on their soft mountings; if mounted on the surrounding bodywork, the transmission or differential -- and hence the drive shaft with magnets on it -- may move around relative to the pickup, possibly interfering with the signal.

The Dana kit Lee describes attaches only one magnet to the drive shaft. While he reports no noticeable balance effects from attaching the small magnet to one side, those who are concerned may easily add a dummy weight to the opposite side when installing.

Note that any cruise control servo should be attached to the throttle linkage in a similar manner to the original. On the early XJ-S, it may be helpful to unbolt the pedal assembly from the firewall and lift it out to look at it. If the cable connection is relocated to the bellcrank at the top center of the engine, operation of the cruise control may cause the throttle cable to come undone or jam; however, later cars were designed to permit attachment here.

Locating the servo unit itself may be challenging. The original mounted just forward of the distributor -- and you are replacing it, presumably because it didn't survive the heat in this area. If the cable is long enough, the best place is probably out in front of the radiator. You could get creative and make up your own extra-long cable, or find a way to connect the end of the Dana cable to the end of the Jaguar cable and use both!

Whichever Dana kit is purchased, the control switch assembly won't look all that great in the XJ-S interior. Worse yet, the original set switch (on the turn signal stalk) and ON/OFF/RESUME switch (on the console) will no longer be used, leaving either unused switches or gaping holes where they once were. The following is based on Lee's method for using the original Jaguar set switch and ON/OFF/RESUME switch in place of the control switch that came with the kit. This will maintain the appearance of the Jaguar interior, but it requires the purchase of a relay. Note: the illustration shows a standard automotive relay, but any 12V SPDT relay will work. If the standard automotive item is used, make sure it has the real 87A connector (normally closed contact). Many standard automotive relays, notably driving light relays, actually have two 87 connections and no 87A connections, which won't work.

Remove the 4-wire plug from the Dana control switch cable, and connect the Jaguar switches to the plug as shown in Figure 16. This sketch shows ONLY the wiring that differs from the Dana (Sears) schematic in the area of the control switch; basically, the section of the Dana schematic from the four-connector plug onward should be replaced with this sketch.

Figure 16 - Wiring for Dana Aftermarket Cruise Control

How the Jaguar switch functions replace the Dana switch functions should be clear from the Dana schematic. The Jaguar switches are physically located in two places, but that makes no electrical difference. A good ground for the Set Speed switch is essential (as is a good ground for the Servo), but it can be located just about anywhere.

For clarity, Lee describes the operation of the Dana control switch that is being replaced:

It is a single piece that contains two separate switches, but they share a couple of wires. (For convenience, I'll refer to the wires by the numbers shown on the connector.) One is a momentary push button that is NC across wires 2 and 3. When pressed (SET/COAST), it opens between 2 and 3 and closes between 2 and 4. Actually, releasing this button is what engages the control and determines the speed setpoint. The second switch is a slide switch that has three positions. In OFF, no connections are closed. In ON, a detented position, wires 1 and 2 are closed. This same switch can be pushed momentarily (spring return) into RESUME/ACCEL position, which maintains the 1 - 2 closure and also closes between 1 and 4.

For those whose parts don't look exactly the same as those in the illustration, a description of the contacts made may be helpful. First, the relay: 85 and 86 are the coil connections, 30/51 is the common contact, 87 is the normally open contact, and 87A is the normally closed contact. On the ON/OFF/RESUME switch, placing the switch in the ON position connects contacts 3 and 7. Pressing the RESUME maintains the 3-7 connection and momentarily connects contacts 6 and 8.

Once the unit is installed, the check-out procedure described in the manual should be followed without any changes. Use the electrical schematic to sort out any errors in the new wiring. Hopefully, this will not be necessary.

The pictorial drawing in my manual showed one UNLABELED adjustment screw on the Regulator." Note: the adjustment screw referred to here is on the side of the regulator box, to the left of the "Centering Adjust" screw. "It should be labeled 'Minimum Speed Adjust.' The manual describes how to make all set-up adjustments. I set the Minimum speed at 30 mph, which enables the unit to maintain a set speed anywhere between about 30 mph and 90 mph. Others may prefer a different range.

Once everything has been tested and adjusted, push the Regulator and excess wiring up behind the underscuttle panel below the steering wheel. I have not found it necessary to fasten the Regulator to anything.

One should now be able to enjoy miles of foot-off driving!

Note that while this installation maintains the interior appearance of the Jag, the Dana cruise control does not work exactly the same; it has more features. First, the stock Jaguar set switch is just that, but with this Dana system it becomes a set/coast switch; if pressed and held when the system is engaged, the car will slow down, and a new cruise speed will be set when the button is released. Second, the resume position on the control switch becomes a resume/accel; if held down, the car will speed up.

One other issue: The original Jaguar cruise control system includes a switch on the shifter to ensure that the car is in D before engaging the cruise control. The Dana aftermarket cruise control lacks such a safety provision, probably because there is a built-in electronic control to prevent the engine from racing if the cruise control is engaged while in neutral; there is a step in the instructions for testing the regulator to ensure engine racing does not occur. The system also supposedly will not permit engagement below an established minimum speed, so engaging in neutral would evidently require attaining highway speed and then moving the shifter to neutral.

If the safety features that are good enough for Dana are good enough for you, the YP wire from the shifter switch may be simply abandoned. However, this same switch also ensures that the car is in D before sending power to the kickdown switch on the GM 400 transmission. Therefore, the switch itself and the other two wires connected to it -- a LGW wire to the kickdown switch and a BW wire to the solenoid within the transmission -- must be left in place for the kickdown to function properly.

If, on the other hand, you insist upon restoring this safety lockout, another relay will be required -- a DPDT this time. A 12V power supply should be fed through the shifter switch to operate the relay. When in D, one pair of contacts on the relay should provide the same connection between the kickdown switch and the transmission solenoid that was provided before. When in any position other than D, a totally separate set of contacts should be wired in series with the disengagement switch on the Dana system.

 

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