What voltage should I have on the alternator exciter wire?

[Slowmobile]

Hello All. 

Could anyone tell me what voltage I should be getting at the alternator exciter wire, with the ignition on? 

I’ve currently got no output from the alternator and I’m thinking the exciter wire could be the culprit. I seem to be getting no voltage on the thin purple exciter wire at the moment. Is there any reason other than a broken wire that could cause this?

I’ve bypassed the exciter wire temporarily and run a wire direct from the positive battery terminal. This stimulates the alternator to produce only 3.5v output, which suggests a faulty alternator too? 

The fact these two problems happened simultaneously makes me wonder if I’m missing something obvious??

Thanks in advance! 

2006 Berlingo M59 - 1.9D

[Sol]

Normally, the exciter is fed from the "charge" warning lamp. I'd expect it to be 12v when the ignition is on. Once the alternator starts to charge, the exciter coil also goes to 12v internally so extinguishes the lamp. (until then it goes to earth / 0v so the lamp will light up to begin with)

Your little charge bulb in the dash could be the issue, but make sure when you jumper 12v to the exciter terminal, that the original wire is taken off, as if there is a fault on that circuit (low resistance to earth) then it could affect the alternator output from working, and make you think the alternator itself is bad.

But could also be the voltage regulator on the alternator is bad too.

[Slowmobile]

Thanks for your reply Zion. Here’s an update…

The purple exciter wire is actually occasionally throwing out an amount of power between 4-10v, but for a fraction of a second before returning to 0v. It does this repeatedly at apparently irregular intervals of a few seconds. It’s certainly not a constant 12v or anything like it.

I’ve traced the purple wire back to the fuse box in the engine bay and continuity seems fine between the alternator end and the fuse box end.

After disconnecting the purple wire at the fuse box, I tested the fuse box pins for voltage, and they too are flashing 4-10v at irregular intervals.

The battery warning light on the dashboard is operational. Is this what you mean by “charge warning light”?

[Sol]

Yea that's the light I meant. Maybe however on the Berlingo, the alternator exciter is triggered by the BSI as that sounds like odd behaviour.

Have you tried disconnecting the battery for a while and see if it settles down? If that didn't work, maybe try a BSI reset. It's well documented on here and on YouTube.

Not sure where to go if that doesn't work. Maybe get a Haynes manual and look at the diagrams to see where this circuit is driven from. Could be a bad relay?

[Slowmobile]

Thanks again Zion.

As far as I can tell, the exciter wire is triggered by the BSI. I had the battery off for four days while it charged, but it hasn’t changed the symptoms.

At the moment the alternator is half off (seized bolt holding me up). Once I can get the new alternator on, I’ll see what happens and try the BSI reset. Thanks for the tip. With a little luck that’ll fix it.

If anyone else has any ideas for me to try in case that doesn’t work, please let me know. I’ll update again once the new alternator is on and I see what happens.

[brodfather11]

looking at a haynes it seems the ECU is also involved as well as the BSI

[jj9]

. . . Your little charge bulb in the dash could be the issue, but make sure when you jumper 12v to the exciter terminal, that the original wire is taken off, as if there is a fault on that circuit (low resistance to earth) then it could affect the alternator output from working, and make you think the alternator itself is bad.

Did you try the above that Zion advised, this would show if the alternator was working.


.

[Slowmobile]

Hello again all - and thanks for your help. Here’s another update and a resolution.

The alternator definitely wasn’t working. The voltage was steady at something like 11.7v with the engine running. Also a multimeter reading directly from the alternator confirmed no output. The ‘charge’ light on the dashboard was working correctly.

Thankfully the problem is now resolved. Here’s what I found.

The exciter wire was not a constant 12v with the ignition on, but was intermittently pulsing some power between 4-10v. It did this at both ends of the wire.

When I last updated I was halfway through replacing the alternator. I also had to replace the ring terminal on the alternator to battery lead, and fix a partial break in the exciter wire. However I’m not convinced the wires were the problem, just a coincidence.

With the new alternator fitted, normal service is resumed and the car is now charging itself again. I’m blaming a faulty alternator.

For the sake of completion I’d like to check again what the exciter wire is doing but it’s chucking it down now and I’m just happy to have a working car again. At some point I’ll check it and update this thread, and maybe even dismantle the old alternator to diagnose what went wrong.

For now though, it’s fixed - so thank you all again for your help!

[Sol]

Glad it's working.

[lofty99]

Hi

I’m getting the exact same problem. Did you or anyone find out what the exciter wire voltage is on a working car. Mine is on off at 4v then 10v then nothing.

A mechanic friend is adamant that exciter needs to be 12v before swapping the alternator.

Things I’ve checked/changed for testing
ECU
Bsi
Wire from exciter to engine bay fuse box
Wire from engine bay fuse box to internal fuse box - tested with a bulb to make sure it can carry a load

Any help would be appreciated