Battery, Alternator or something else?

[Martinr]

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.

 

[melsg5]

some other Fiero specialist will answer your question in more detail later but my guess is you have 2 different problems. First your voltage regulator doesnt seem to be kicking in causing the low charge and second you have some sort of issue with idle control.

 

[Oreif]

If the alternator jumps up with a rev of the engine, It could be one of two things. The regulator inside the alternator is going bad or you have a dirty connection/missing ground wire. The latter is a very common problem.

First thing to do is get an ohm meter and with the car off measure resistance from the alternator case to the negative side of the battery. If it is higher than 3 ohms that is the sign of dirty or missing connections.

If the resistance is high, check the ground straps. There should be one attached to the engine block right near the battery and hooked to the right side decklid hinge. The second one is hooked to a bolt that holds the transmission to the engine on the firewall side and it attaches to a frame rail just under the air cleaner housing. If either is missing or badly corroded, replace it. On manual cars they sometimes get broken or not hooked back up when the clutch is changed. After those are checked, disconnect the battery then disconnect the wires on the alternator and make sure the thick red wire is clean and tight when you hook it back up. Also make sure the alternator bracket is clean and unpainted. Some folks like to paint the brackets for looks but this hinders the alternator grounding and keeps it from charging properly. One last thing to check would be the wires on the starter and the battery cable wires (at both ends battery side and where they go.) The negative battery cable needs to be attached to the engine block not a bracket. The odds are this will correct your issues.

If the alternator still acts the same way then the regulator is going bad. Replace alternator and go thru the reset procedure.

Once you verify that the charging system is good, Then you need to reset the computer. The reason being is faulty grounds can give the computer weird readings and make it not run efficiently or do strange things. To correct it the computer needs to be reset so that it can re-learn how to manage the engine.
You do this by doing the following:
Disconnect the battery for at least 5 minutes. (this may happen when you disconnect it for cleaning wires and cables above)

Then hook it back up and drive the car for 15-20 minutes. During the drive you will need to go over 40 mph for at least 15 seconds (a quick run down a local highway is the best way to do this) this process lets the computer reset the running parameters of the engine including setting the idle. Your idle should be 850 to 1050 (typically about 950 rpm) when you are done. Turn it off wait a few seconds than start the car again. It should start and run smoothly with normal idle. Then let the car cool down and start it up.
When cold (below 40*) the idle should kick up to around 1200 for a little bit until the engine warms up then return to normal idle.
All should then be good.

Let us know what you find out.

 

[Martinr]

Thanks

Thanks for all that info, I'll need to go and pinch my dads meter, then I'll get cracking on resistance checks etc.

I'll come back with any info I get

______________________________

 

[Martinr]

Turned out to be totally simple. Houdini on another forum. Stated that the resistance of the alternator warning light in the voltmeter is used to excite the alternator. I checked and sure enough the bulb was blown. Replaced it this morning and started her up. The needle shot straight up to 14v.