One more thing, I did not switch computers from the 2003 XJ8 donor to my 2001 XK8. Could that cause a problem?

 

Yes, it sure could. That is why the question early on as to which ECM you were using.

 

Damn. I thought there was a decent chance working on the grounding would produce a fix.

I'll check your in-dash gauge readings against mine tonight. I know that gauge is not very accurate, but it should be pretty repeatable from car to car.

 

I installed the ECM from the donor car (2003 XJ8) and put it in my 2001 XK8. The car would turn over but not start. I put the original ECM back in and the engine fired right up. Might replace the alternator plug next, however it looks very good as does the wiring coming out of it.

 

My dash gauge shows 13V also, but I never checked the actual voltage with a meter. Will do so next time I take it out.

 

My in-dash gauge shows readings pretty close to Noterman's.

This gauge appears to be dummied-up much like the temperature gauge. It looks as though anything over 13 volts displays as 13. Battery voltage, ~ 12.5, displays as 12. The gauge does show the voltage dropping toward 10 volts while the starter runs, as it should.

Does anyone know if the in-dash voltage gauge ...
- is fed by a single wire pair (from which it would determine the voltage to display and also whether or not the battery is charging) or ...
- are there two signals sent to the gauge, one for the voltage and another for whether the battery is charging?

 

The in-dash gauge is a 4-wire stepper motor, driven by a microcomputer and a step-motor driver chip on the instrument cluster circuit board.

I believe that it is either monitoring the instrument cluster voltage supply or the CAN message sent by the ECM with the voltage. I had never determined which. Whichever it is, it significantly dampens variations using software, so it is not an accurate measure. Also of importance is that since it does not monitor across the battery directly, it sees significant voltage drops in the wiring from front to back when under load.

There is a separate signal directly from the voltage regulator which drives the charge warning light.

 

So the suspect connector at the alternator carries the signal that determines whether or not we get the charge warning?

 

Correct

 

Aha. So we should really ignore the fact that the not-charging light happens to be housed in the same gauge as the voltage needle. Separate stuff entirely.

Noterman, how would you feel about scratching a little insulation off of the wire down-stream from the connector at the alternator to see if we have signal there? (There are probes that will poke a hole in wire insulation for such readings ... that's better if you have one.)

 

I have two wires on the plug that goes to the alternator. One is yellow and the other is white and blue. I will do as suggested probing the wire and let you know what I find. I do have an extra harness with the alternator plug if needed. Thanks again

 

Pretty sure it's the yellow one that we want here.

 

Started today with a overnight hard reset of the ECM. Battery voltage was 12.80, after the car was started the voltage went to 14.3. I stripped the yellow wire from the alternator it showed 13.3 volts, the white /blue wire showed 13.8 with the engine running at idle. Gauge in car read 13.0 volts with the engine running, 11.5 during the start. Still have the red light in the amp gauge and battery not charging message.

 

Hrumph! I'm not sure what we should see there, but sort of expected that yellow wire should match the alternator output (14.3 V, you said). I'll see if I can get readings from my car to compare. Please also take an AC reading from the yellow wire.

Confusing.

edit: From the MY 2003 electric guide ... the yellow wire signal should appear at the ECM, pin EM80-079. Worth a look to see if it's getting there.

 

Hi Dennis, Not sure what you mean. I do not know how to check AC power on the yellow wire. I did not think autos had AC power.

 

He means switch your VOM to AC and get a reading of AC voltage in the output.

 

Please forgive me I am very challenged with a Multimeter. I set my meter to ac volts 200. With the engine running I read 6.0 volts, on both the yellow and blue/white wire. If I touch the battery it shows 26.6 volts. Thanks everyone.

 

OK, don't worry about the AC measurement ... in reality just a fishing expedition anyway, which I am inclined to do when I don't know what the %^&* is going on.

If I'm reading the MY 2003 schematics right, the yellow wire on the alternator is only for sensing battery-not-charging and it goes to one and only one place ... ECM pin EM80-079. We would learn a lot by seeing if the 13.x volts you measured at the alternator is getting to that spot. See post #42. This wire routing does not apply to MY 2001, and probably some other MYs.

I've never had occasion to poke into an ECM. Is it feasible to get to that spot while the unit is powered? Second best would be to open the ECM connector and test for connectivity (just of the yellow wire of interest) between the alternator end and ECM end.

 

Can you tell me how the pins are numbered on the ECM. Not sure which one is EM80-079

 

Sort of on the fly this evening, but if you Google something like "Jaguar XK8 2001 Electrical Guide" you should find a PDF with all of this stuff including pin positions on the connectors.