Battery warning light at low revs

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Marko Washington

Marko Washington

Member
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26
Location
Oxfordshire
I've just started getting a battery warning light at low revs, below 2000rpm. My guess is it is going to be the alternator, but is there anything else it might be save me the cost and effort of an alternator change?

Cheers.
 
Marko Washington said:
I'll check the drive belt, will it be fairly obvious if it's slipping?
Click to expand...
Run the engine for a minute, revving it a couple of times so the light goes out when revved. Turn the engine off and put the keys in your pocket. Then check how hot the alternator pulley is. Be warned, if it's slipping, it will be really hot. If it's really hot, then the belt is the first thing to change.
If the pulley feels as cool as the belt, the chances are that it's the alternator that has failed.
 
If it's a TD4, the crank pulley can also delaminate, which can cause the same symptoms as you are getting.
 
Marko Washington said:
It is a TD4 yeah. I'll have a look at that as well then. This might be a down the garage job.
Click to expand...
The crank pulley is easy enough, although the bolt can be an absolute b.....d to undo sometimes. Use a powerful torch to check around the crank pulley for silver glittery stuff. This is the pulley striking the engine, stripping off metal as it does so.
I often mark the inner and outer sections with timpex, run the engine, then see if the marks change position in relation to each other.
 
Also, and I know this may not going to be an exact science, but how urgent is this?
I need the car this afternoon to go check on horses and a couple of trips up and back tomorrow, but then I can do without it in the week as I've another car back on the road on Monday.
 
IIRC, the water pump on the TD4 is driven by the same belt - if this lets go, you'll have no drive to the water pump, and this is unlikely to have a happy ending. The possibly fried alternator will be the least of your worries at this point. But, as Nodge says - check the crankshaft damper....
 
In case it will help future searches, I did manage to get under and check the voltage coming from the alternator and it wasn't putting out anything at low revs and then just about 12 or 13 at revs higher but intermittent. My guess is that the brushes in the alternator have worn, so at high revs it just pushes out enough to make a vague connectin but nothing stable. New alternator going in tonight, after a crawl around underneath doesn't look too bad a job. I'll do the belt I reckon as well as I don't need the car for a few days and can take my time. Thanks for the advice.
 
You can just replace the regulator and brushes in these cases - costs about £7. Too late now but you could do up the old one and keep it for a spare or sell it.
 
Marko Washington said:
In case it will help future searches, I did manage to get under and check the voltage coming from the alternator and it wasn't putting out anything at low revs and then just about 12 or 13 at revs higher but intermittent. My guess is that the brushes in the alternator have worn, so at high revs it just pushes out enough to make a vague connectin but nothing stable. New alternator going in tonight, after a crawl around underneath doesn't look too bad a job. I'll do the belt I reckon as well as I don't need the car for a few days and can take my time. Thanks for the advice.
Click to expand...
Usual fault and classic for those symptoms is actually one phase of the rectifier diode pack failed. Changing the reg / slip ring brush pack will do absolutely nothing if that is that case. Just FYI ;).
So, before spending any money on it - check the diodes in the rectifier.
:)
 
Joe_H said:
Usual fault and classic for those symptoms is actually one phase of the rectifier diode pack failed. Changing the reg / slip ring brush pack will do absolutely nothing if that is that case. Just FYI ;).
So, before spending any money on it - check the diodes in the rectifier.
:)
Click to expand...
Rectifiers also available on the site! The chap there is very helpful if you email him, and probably would be able to help you diagnose the problem you have.
 
Hi GF,
It is really really easy to diagnose a rectifier issue - all you need is a multi-meter and the rectifier pack removed - 6 diodes - 2 for each phase.
As said, classic rectifier single phase failure symptoms.
It all depends on your level of skill, alternators are actually very simple devices. If you can take one apart properly it is easy to identify the fault (any fault) with a multi-meter and a knowledge of how an alternator works. Some can be a pain to disassemble as parts are heavy soldered - some are easy.
You can check the field and stator windings at the same time although no symptom of any issue there exists. A few years back I programmed commercially for external high current multistage regulator units for marine use. Still (Electronically) a fairly simple device (electronically) that sounds complicated - lol ;)

Often though if basic skills are in doubt it is far simpler to just get a recon alternator or one from a scrap yard.

Good site though !
Joe
 
Joe_H said:
Hi GF,
It is really really easy to diagnose a rectifier issue - all you need is a multi-meter and the rectifier pack removed - 6 diodes - 2 for each phase.
As said, classic rectifier single phase failure symptoms.
It all depends on your level of skill, alternators are actually very simple devices. If you can take one apart properly it is easy to identify the fault (any fault) with a multi-meter and a knowledge of how an alternator works. Some can be a pain to disassemble as parts are heavy soldered - some are easy.
You can check the field and stator windings at the same time although no symptom of any issue there exists. A few years back I programmed commercially for external high current multistage regulator units for marine use. Still (Electronically) a fairly simple device (electronically) that sounds complicated - lol ;)

Often though if basic skills are in doubt it is far simpler to just get a recon alternator or one from a scrap yard.

Good site though !
Joe
Click to expand...
Hi Joe

I have a diode tester on my multimeter which proves handy for fault-finding battery charger output boards and the like, but I've just bought an old Hameg 20 MHz scope and would like to get some use out of it on the car. Would a duff output rectifier on an alternator show up as a ripply output?

Also, what else could it be used for? - I was thinking ABS sensors, MAF, MAP, etc as well as injector and solenoid PWM signals.

When I get some spare time I'd like to start scoping stuff all over the car so I know what normal looks like. Any advice you might have would be much appreciated.

Cheers, GF
 
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