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Cold Start Circuit Checklist, Jaguar XJ6 Series III

Cold Start Circuit Checklist, Jaguar XJ6 Series III

Doug Dwyer

The purpose of this checklist is to describe of the cold start system and provide a diagnostic procedure to isolate faults. A 12 volt test light (or multi-meter), small alligator clips, jumper wires, and ordinary hand tools are required. Please note that this information applies to 6 cylinder models only.

Circuit description

To provide easy cold starting an intake manifold mounted cold start injector is provided. This injector provides a timed spurt of raw fuel into the intake manifold. This injector operates independantly of the regular fuel injection system and is controlled by a heated "Thermotime" switch, mounted in the forward- most postion of the water rail. This circuit is operational only while the starter motor is engaged. The duration of operation ranges from 0 seconds with a coolant temperature of 70F or higher to about 7 seconds when the coolant temp is below 0F degress.

Fault diagnosis

This checklist assumes that the fuel pump operates correctly and provides fuel pressure whenever the starter motor is engaged. If not, see the "Fuel Pump Checklist". The engine must be *cold*. Warm coolant will skew your test results.

1) Disconnect the connector to the Thermotime ("TT") switch. If left connected during these tests it will "time out" and you'll have to wait for it to cool down.

2) Locate the starter relay and disconnect the white/red wire. This will disable the starter motor but leave the rest of the circuit operational for testing.

3) First we'll determine if the injector itself is operational. Remove the cold start injector. The injector is mounted directly into the intake manifold between the middle two intake runners. It will be easier to remove the injector if you first remove the harness and the fuel supply hose. Be prepared for some spilled fuel when you remove the hose ! Once the injector is out reconnect the fuel supply hose and momentarily turn the key to start to repressurize the fuel rail.

4) Using your small alligator clips and jumper wire ground the terminal of the injector which is associated with the white/green wire. Now, using another clip and jumper, briefly apply 12 volts to the terminal associated with the white/purple wire. You should hear the injector click and quite a strong spurt of ready ! If you don't get any spurt, then the injector is dead and needs to be replaced. If it operates OK at this point, go to step 5.

5) Remove the connector at the cold start injector. Find the connector terminal associated with the white/purple wire. Notice that the connector for the TT switch also has a terminal for a white/purple wire. Using your test light check for voltage at both of these terminals whenever the starter circuit is engaged. If you have voltage at both of these terminals, go to Step 7. If no voltage at both or either of these terminals, see Step 6.

6) Voltage for these two terminals originates at the silver, firewall mounted starter relay. A white/yellow wire from the relay carries voltage to terminal 3 of the red, firewall mounted diode pack. From the diode pack voltage, still on the white/yellow wire, is carried to a connector. After the connector the wiring color changes to white/purple. So, if voltage is not present at the terminals tested in Step 5 you must trace and repair a wiring or connector fault somewhere along this circuit. If the wiring and connectors check out OK, it is possible that the starter relay may be defective and not sending the voltage.

7) You are here because you have confirmed the the voltage side of the circuit is operating correctly. Now we will check for a ground path. This gets a bit tricky so plan ahead. The ground "event" is timed and will only last a few moments. Arrange your test light so it will check for *ground* instead of voltage (attach the clip to a voltage supply and use the probe to test for ground). Leave the cold start connector off but reattach the connector to the TT switch. You will be checking for ground at the terminal of the injector connector which is associated with the white/green wire. Insert the probe of your test light into the terminal and (get ready now !) have a helper turn the key to "start". Your test light should light up (indicating a ground circuit) for about 2 seconds (or as much as about 7 seconds if coolant temp is below 0F degrees). If the test light flashes then the ground is OK and the circuit is functioning as designed. If the light does not light then the TT switch is on the fritz or you have a wiring fault on the white/green wire between the TT switch and the injector.


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