Hi all, my Fiero (1986 2.8 GT Auto) battery was flat this morning, so I jump started it off my MG. It fired up fine.

Problem I've been having for a while is when I first start the car, the volt meter shows low (9V I think). Then when I rev the car hard, the needle jumps up to 13.5 - 14 Volts then stays there, if I turn the car off and start it again, the needle goes down to (I think) 9V again.

I assume that when I start the car, the alternator is not charging the battery, but then when I rev the car, the alternator cuts in and then continues to charge.

The car has been idling high at 1200 revs since I got it 5 months ago, I've noticed today though, the car settled to 800 revs when I left it with the volt meter low i.e whilst the alternator wasn't charging, as soon as I revved the car, the volt meter went up to 14 volts and the idle speed went back to 1200 revs and stayed there.

I haven't got a clue when the alternator should kick in etc. If anyone has any insight or knowledge that may help me to diagnose the problem I would appreciate it.

By the way I put the battery off my MG onto the Fiero and started it, the volt meter behaved the same, so although the battery was flat this morning, I don't think that is the problem.

[This message has been edited by Finale-gt (edited 01-13-2012).]

There is a voltage regulator built into the alternator that decides if it will charge the battery and when. It may be malfunctioning. It may also be the current draw that flattened your battery in the first place. Anything more than 50 milliamps for the whole system means something is not right.

The voltage regulator takes a reading using a "sense line" could be something problematic there as well. Thought typically a sence line issue comes in the way of an overcharging. I would suspect teh voltage regulator might be going, but do a test to see what the draw is with the car off. When I first start up my 84 and not matter where its idle is I have a strong 13 volts min with the new alternator and new wiring.

Check out theorge's cave for info on the sense line.

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Rescued & resuscitated 1984 Indy Fiero SE Pace Car Replica #770 presently reversibly modified!
SE: Stormtrooper Edition


Everything is the Previous owners fault

Usually when the volt meter does what you are describing - the belt is slipping on the alternator.

Reving it up and the the engine slowing down lets the alternator pulley catch to the belt.

Any chance you are getting a squeely noise when the voltage is low and that noise goes away once it catches?

[This message has been edited by phonedawgz (edited 01-13-2012).]

I have installed dodge runners auto tensioning alternator pulley kit so it can't be a slipping belt.

I've the same year/trans setup, and it usually takes a half a second to a second before the alt kicks in and starts putting out juice. If its taking logner than that, it's probably having trouble building the magnetic field, due to a bad connection or a weak voltage regulator. If your battery voltage is low, it'll make it that much more difficult for it to overcome any resistance from bad/worn/weak/corroded connections. I was occasionally having the same issue, but I don't think I've seen it happen since addind a secondary ground strap to the engine. Most things ran better after doing that.

If a diode in the alternator fails it can allow the battery to drain off when the engine is off. The alternator can still charge the battery but will then drain it flat after the engine is switched off. I would suspect the alternator for sure. Larry

If I don't rev the engine up, then the alternator doesn't kick in at all. Once I rev it though it starts and carries on. I've tried another battery and it's the same result. I have also added an additional ground strap. It certainly sounds like the voltage regulator or alternator.

Either way with shipping to uk it's not going to be cheap. I need to know which is at fault before I shell out on either. Can the voltage regulator be purchased seperatly or is it part of the alternator?

I've tried to price a replacement alternator already but as they are between 75 and 500 dollars I'm a bit confused on which one I need!

The alternator warning light bulb in the voltmeter is burnt out. The Fiero uses the resistance in the bulb to excite the alternator.

I have to rev my 1985 S10 Blazer to get the alternator to charge. It has no warning light at all.

Interesting theory, I haven't noticed a warning light so I'll go and check that, thanks.

Mart.

quote Originally posted by Hudini: The alternator warning light bulb in the voltmeter is burnt out. The Fiero uses the resistance in the bulb to excite the alternator. I have to rev my 1985 S10 Blazer to get the alternator to charge. It has no warning light at all.

The problem also could be a bad connection at the alternator plug. Check the non-red wire. It usually is white or brown.

Spot on with the alternator warning bulb. Haven't started the car yet though as it's under my daughters bedroom and it's 11pm here. I'm going out in it tomorrow so I'll update then,

Don't know about Hudini but your definately Einstein. Replacing the bulb cured it. Thanks again, you may have just saved me a small fortune!