Low voltage / battery light on

[jon_quinn]

Hi,

I have a 1985 924 lux, and the battery light is on with the engine running and the voltmeter reads low at about 11v. I noticed that the battery light dims a little as the revs increase, but the voltmeter does not seem to move.

I had assumed that the alternator was damaged so I had it tested and the brushes and regulator were replaced and I was told that it was working, but when I installed it back in the car the problem is still there.

I can remember when it was working OK that the battery light would stay on (and a low voltage reading on the voltmeter) after the car was started until the revs had passed a certain point. Does anyone know why this is as I am wondering if whatever makes this happen is now not working properly and is keeping the alternator output off?

Thanks in advance for any help.

Cheers, Jon.

 

[vitesse]

Sounds like the regulator isn't working properly-my e-bay 924S is the same -once started ,I have to really blip the throttle to "kick" the regulator into action,then the ammeter jumps from 11 volts to 14.3 volts & after that it's OK.

 

[vincematthews]

If you had it properly tested? by an auto-electrician? then they should have checked the regulator is working - double check this with them. You should be able to check the output yourself with a voltmeter but do it on the actual alternator terminals (rather than anywhere else) when the engine is running - usual percautions apply []

Start with simple things that don't cost like cleaning the battery terminals properly with emery paper to remove any corrosion on the posts and inside the caps.
Then check the earth from the alternator to the engine block and block to chassis earth for condition and undo and clean any corrosion off before refitting.
On 924s the wires that run from the battery across the front of the firewall down to the starter and alternator, especially where it is routed near the exhaust, can be a problem. From underneath the car trace the wires back from both components and check their condition making sure the insulation is intact along their full length.

Finally let us know how you got on.

 

[jon_quinn]

Hi,

Thanks for the responses. Yes, the alternator was teted on a dyno, but I will take it back and get them to double check it.

I am now suspecting that the cable may be at fault, the earthing looks ok. I will check the voltage with the engine running, do you know what it should read on each of the terminals? There are 2 on the back of the alternator, plus the earth to the engine block.

Cheers, Jon.

 

[jwindwood]

98% sure this will be the wiring from the bulkhead to the starter/alternator as has been mentioned. I have had three 2.0 924s and ALL have needed this replacing. Two options-buy the loom from Porsche (It plugs in near the coil ) for about £90 or remove the old loom and make your own replacement with wire/terminals and reuse the insulation and block plug.
Removing this loom is a pain because the heatshields must be removed from underneath and the fixings are not easily accessable. I have made my own looms on all of my cars-cost about £8 !

 

[vincematthews]

ORIGINAL: jon_quinn
I will check the voltage with the engine running, do you know what it should read on each of the terminals? There are 2 on the back of the alternator, plus the earth to the engine block.
Cheers, Jon.

From memory, you should get about 14V on the terminal with the big insulator (relative to earth).

As John says its relatively easy to replace the loom, I've done it too - getting to it is the problem and reusing the plug connector. Make sure you use heavy enough gauge wire.

Don't forget the battery posts and caps - they can give you similar symptoms.

Cheers

 

[jon_quinn]

Hi guys,

Thanks for your help with this one, the car is now up and running again.

I gave up trying to replace the wiring myself. I found the access to the starter motor was so limited that I took the car into the local garage and asked them to do it for me.

Anyway, it's fixed now.

Cheers, Jon.

 

[colin944]

If the regulator is gone and your car is a 2lt.Then take it out and show to a good vw man I got one for mine I think it was audi100 paert and cost £12 as opposed to £45 from porsche etc.