Intoduction: 1985 XJS HE sat for 15 years. Changed fuel. New battery. It started and ran for about 15 minutes. Next day, it started, idled for 2 minutes and died. Would not start thereafter.
Troubleshooting: Pump runs with direct power to it. Tested Main relay #16 and Pump relay #17. Both working. Double checked voltage to pump terminals by unplugging ECU and jumping terminal #15 (orange wire from terminal #85 on Pump relay) directly to ground. Battery voltage exists at pump terminals. But when ECU is plugged back in, only .253 volts exist at pump terminal. Double checked with a second known good ECU, same .253 volts. Both ECUâs are 16CU with lambda.
Question: Setting aside the Inertia Switch as possible culprit, what other switches in the fuel wiring circut exist that would interupt the grounding of the orange wire terminal #15 into ECU?
As you can see the circuit for the pump feed is completely separated from the pump control circuit.
You should check if the ECU grounds terminal #15 when you turn the ignition on.
Otherwise, a bad connection, a bad ground (check also the ECU ground), a broken wire or a bad relay.
Aristides,
Thank you for the answer and schematic. I am currently working on the Jag at this moment. Once again, I followed through with the explanation and added the test of the Inertia switch. All is functioning to power the pump with ECU disconnected.
There must be another relay going through the ECU that would disconnect terminal #15 (the orange wire), preventing the ground the pump needs.
Perhaps the Cold Start Relay (US Jag).
I will study tge schematic you kindly forwarded. My Hanes is a little shy of specifics.
Ă+
Ah! My Haynes has mislead me. The schematic has the orange wire going to #19. The schematic you sent has the orange wire at #15âŚwhich incidentally I have determined to coincide with the physical connection. Yes the harness side goes to ground from terminal #15, as conclusive of my testing stated in the original post. I must edit it (-:
So this confirms my suspicions that there must be another relay or switch that the ECU relies upon to connect the terminal #15 to ground.
The Gnd shunt will be an internal output chip.
Ensure the Ecu has a good Gnd on Pins 1, 16, 17, 34.
Manually shunt Pin 15 to Gnd and see if the pump runs.
The main relay is supplied Batt+ and is permanently grounded. The coil is energised by the Ign +ve via inertia switch.
The fuel pump relay is supplied +12v from the main relay and is switched by Pin 15 being grounded.
The FP relay sends Batt+ to the fuel pump, which has a permanent Gnd.
You fault sounds like the Ecu has a poor Gnd. Output chip failures tend to be complete, not partial.
Iâm not personally aware of any additional external devices or switches. But neither my brain nor the schematic shown accounts for every possible build variation. Is yours a USA market car?
However, just in case were not already aware, the ECU has an internal âkey onâ timer for the #15/orange wire circuit. That is, if you simply turn the key âonâ the ECU will ground the fuel pump relay (via the orange wire) for only 2-3 seconds. You have to check the circuits pretty darn quickly to catch that event!
The ECU will also provide the orange wire ground when the starter is engaged. You can check it then. A helper would be useful.
Failure of [what I call] the âorange wire ground circuitâ is rather common with these ECUs so this ^^^^ is the easiest and most common way of checking the system. With the orange wire grounded the pump should run continuously whenever the key is âonâ. If it doesnât then obviously the problem is elsewhere.
Electrical Gremlins!
Firstly I extend my appreciation and thanks to the helpful insight answers to my posts. Because of this help I eventually wandered back to the basics. Sometimes we (I) get to caught up in troubleshooting, the simple is often ignored.
After examining and eliminating any possibility that another relay or switching sensor was the cause, I went to the connector at the harness end that plugs into the ECU. It is there that the solution was found do to loose contacts. I tweaked #15 (and for insurance all others) so that it would be tight, clamping the spade pin(s) on the ECU.
THE JAG RUNS! it fired right up and settled into a nice idle.
Next problem. It appears after about 30 minutes, it gets warm. Will remove thermostats and check them. Also will test coolant for combustion gasses.
Thanks for the helpful advice and schematic. Brilliant!
Also check the viscous coupling is engaging the fan when it heats up.
One trick is to stall the fan with a length of wood, then once it is properly hot pull the wood out and the fan should start up with a definite whoosh.
The viscous clutch can leak the oil out, but also the bimetallic spring can be covered in dirt, stopping it heating up.
Depending on the type of contact, the Ecu contacts should be available separately. Search for âtimer contactsâ.
Thanks for the advice.
Sadly, it suffers from a blown head gasket. Discovered the leak about 4 hours ago, bank A number 7 or 9. Leaking into the valley (injector side of head).
I recall it was running warm, when I buried it into storage 15 years ago.
Now I have a good excuse to renew the paint on the car, when the motor comes out(-; But that project is now a year awayâŚto many irons in the fire at this time.
