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GURU SOUGHT - DIODE ALTERNATOR TEST

Vauxhall Astra 1.4i MPi I realise theres a possibility that I can probably fix this problem easily by fitting an alternator, but I'm really just curious if there is any gurus who can explain this to me. After a journey of about 30 minutes, my battery warning lamp comes on very very faintly. This has been happening for about three years. When I first noticed it I fitted a new battery because there were a few occassions when the starter sounded a bit lazy. The light still came on after a 30 minute journey. I did do an alternator swap for another second hand one and the same thing happened exactly. The car has kept running since, so I never really worried too much about it. A short time ago I thought I would investigate again, since I was planning a longish trip. I studied the wiring diagram and could see that there is a 12v supply to the instrument panel, which supplies current to the battery warning lamp, the lamp having its negative terminal connected to the small terminal on the alternator. Before cranking, this terminal is more or less a ground if you like. The warning lamp obviously lights therefore before cranking. When the engine is started this alternator terminal becomes the equivalent of the charging voltage, hence the supply to the warning lamp positive terminal and negative terminal are now equal, ie theres no P.D. across the bulb. Anyway to get to the point, I got suspicious of what was happening, and decided to put a diode in the line between the warning lamp negative terminal and the small connection on the alternator. I had a hunch that the current flow was actually in the opposite direction to what I had generally assumed to be the norm. I was proved right. I fitted the diode with pointed end of the triangle (the kathode) pointing up toward the instrument panel, coming from the alternator. In other words, when the car had been run 30 minutes, there was lets say 1 or 2 volts more on the small pin at the alternator, than there was at the supply voltage to the instrument panel. Obviously the general condition that illuminates the warning lamp is when alternator output is less than battery voltage, but in this case it was the exact opposite. Anybody ever seen this before? The bit which adds some mystery is that even after a new battery was fitted, and another alternator, it still behaved the same. I can imagine that the second hand alternator I fitted maybe wasn't as good as a new one, but I would be surprised if both the old and new alternators had exactly this same characteristic, ie after 30 minutes driving, putting the light on slightly. Only for that bit of confusion I would stick a new alternator on, but I have my doubts if indeed this would sort the problem out. I am presuming the alternator with its voltage regulator and diode pack is a self contained unit. There is no other regulatory circuitry anywhere else outside of the alternator is there? The alternator is the Bosch one, where the brushes and the voltage regulator are a single unit, held onto the rear of the alternator by two screws. I would really like to understand whats going on if anyone can help. Thanks

Reply to
Alabastar
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That implies ground theft to me. I see a lot of corroded ground straps to the body and/or engine of the vehicle because I live in the rust belt of North America.

These vehicles steal grounds through other components like the alternator or the gas pedal cable or gear shift or even the radio or stereo.

Heat can affect a corroded connection or the ground path as well.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

Ref: Mike Romain

Thanks for the advice. I cleaned all the ground connections I could find, battery to chassis ground, chassis to engine ground etc and even put an independent earth strap directly to the alternator from the inlet manifold. Guess what. No change. I measured some voltages which make it even more peculiar.

With a cold engine and engine not runniing, I measured the voltage from

Battery V+ to the small terminal on the alternator (D+). It was about

10.5 volts.

The battery itself was 11.9 volts.

I measured from the battery ground post to the engine ground (inlet manifold) and measured 0.1 volts.

I started the engine.

The D+ terminal was now 0.3 volts higher than battery V+ terminal, but the dash warning lamp was not lit.

I did the 30 minute warm up routine and the warning lamp began to glow slightly.

I measured the D+ to Battery V+ again. D+ was now only 0.15 volts higher than V+.

The Battery ground to D+ now measured 14.1 volts, while the battery itself 13.95 volts

How can the voltage drop of 0.3 volts not light the lamp, but a drop of

0.15 does. I'm baffled
Reply to
Alabastar

The state of your battery charge (11.9V) is indicating a bad connection or a bad alternator or a bad battery.

The power drop between the battery and alternator output is pretty large if I am reading this right.

I like to 'walk' the meter to try and find the source of the drop. It is best with the engine running for this. I start off on the battery posts with the probes, then from one post to the clamp on the other, then the other end of the battery cable, say at the engine or chassis if the negative, then to the engine block, then to the alternator bracket, then to the alternator case.

Walking the other way goes from the negative post to the positive clamp, then the end of the positive cable at the cable loop, then the bolt, then to the alternator plus wire's loop, then the bolt.

Where I find a drop, I fix it. There should be very little.

I hope this helps a bit...

Mike

Alabastar wrote:

Reply to
Mike Romain

Thanks again. I will take my meter for a "walk" tomorrow all being well. I must also "walk" it up to, first the input, then the ouput side of the fuse box, since I am possibly, incorrectly, presuming, the supply to the dashboard is 100% battery voltage. Maybe in fact there is a drop somewhere between the battery V+ and the fusebox? I'm thinking that simply because I can't explain why, with the 0.3v drop D+ to V+ , the warning light is off, but with 0.15v drop D+ to V+ the lamp is on. I will post an update once I've been for a "walk". Thanks Mike.

Reply to
Alabastar

Ref: Mike Romain

Well Mike that's it finally sorted. I only got at it today, Tuesday, because its been so windy here. I would have lost my bonnet before today. (I don't mean a dainty we hat that I have, I mean my hood, as you probably call it). Well, I took the meter out for a walk and quickly discovered that there was a drop of 0.15 volts between the alternator earth strap and the alternator body. The best about it is, that the very first thing I did ages ago was fit an independent earth strap from the inlet manifold to the alternator adjustment clamping screw. I obviously never scraped back to clean aluminium on that occassion. Once I gave it a scrape today and clamped it all up there was 0.0 volts drop from the alternator earth strap to the alternator body. I ran it for thirty minutes and no warning lamp. I then did a alternator body back to the battery ground post to see if it was also 0 volts all the way back from the alternaor body to the post. It measured

0.05 volts. I walked the meter again and was amazed to find that the 0.05 volt drop was actually taking place between the engine block and the cylinder head. I would have thought that the connection there through all the cylinder head studs etc. would have been fine. Just shows us how wrong it is to make assumptions. Anyway thats an easy one to put right. Thanks for your help. Its good to put that light out after all these years. Regards. Neill
Reply to
Alabastar

Cool, thanks for the update.

Not the first failure I have seen at that point. I live in the rust belt in North America....

Mike

Alabastar wrote:

Reply to
Mike Romain

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