Hi, I have been reading over numerous posts as I have the battery light coming on in the dash.
I have replaced the +ve and -ve cables.
The battery was a fair bit low on acid so i filled it up and charged it. The battery has been on since new.

When starting the jag, the bat light doesn't come on for about 10 to 20 seconds, with or without revving the car. Some times while having the car in test mode and also testing with a multimeter the charge to the battery is up to 14v, but other times there is no charge at all ie 12v.
It appears as if one time the alternator works and the next it doesn't.

I have been advised to check the ECU, but I have no idea where this is? I'd rather rule out all possibilities before replacing the alternator, as this looks like a job and a half.

Thanks

Phil

 

PHil, based on what you are seeing, i would be questioning the field wire between the ECU and the alternator. If this wire has a break in it, it can be affected by heat and vibrations and cause the voltage to fluctuate between 12 and 14 V like you are seeing.

If you want to check the wire yourself, let me know and I will tell you which wire to look for and you can take it from there.

 

Hi Thermo,

Thanks for this, if you could tell me which wire to check I would be very grateful.

Phil

 

I have tried to remove the 3 pin plug on the alternator to test the field wire, but I cannot get the thing off.

Is there a specific way to get these off as I didn't want to go my usual heavy handed approach.

Thanks

 

PHil, if you look, you should see an orange/green wire going to the alternator on the 3 wire plug. This is the field lead. To measure this lead, I would say to get a sewing pin and then slide it along the insulation of the wire into the plug. You should feel it hit the pin inside the plug. From there, you can use a set of alligator leads to hold the pin and then with the motor running, push the pin in to get a reading. Under minimal load, you should be seeing in the neighborhood of about 7-8 VDC. If you turn on a lot of electrical stuff (dash fan, headlights, rear defroster), you should see it jump up to about 10 VDC. As you are watching the voltage, if you can have someone else wiggle the wiring harness, if the voltage starts jumping wildly (more than 1.0 VDC), then that part of the harness has an issue with it. If you get 0 VDC out and the dash is reading 12 VDC, then odds are the wire between the generator and the ECU is bad or the ECU is bad. If the wire is up at 12 VDC along with the dash, then your problem is internal to the alternator.

Hope this helps.

 

Hi Thermo,

I managed to test all three of the 3 Pin wires as the colors are slightly different:

White wire = 8v to 10v
Orange Wire = 3v to 4v
Brown Wire = 1v to 1.5v

The other odd point I noticed was that the Orange wire read 4v while the car was off, not sure if this is meant to.

Also, I recharged the battery and measured the output before and during the car running: Before starting the battery read 12.6v but while running dropped to 11v then back to 12v. Seemed there was no output at all this time from the alternator.

Any suggestions?

Many thanks

 

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