Hey all,

I have a 1986 3.4l pr and I replaced my alternator (cs conversion) about two months ago with an AC DELCO reman. Everything was working fine, but about a month in, my volt gauge wouldn't show voltage (but would slightly move when ignition is on) and the volt light would be on. I got out the voltmeter and even under full electrical load it's charging 13.58 volts (14 with no load) when measured at the battery. I've reseated the alternator connector and verified the single wire with the bolt is nice and tight.

I took it to Advance Auto yesterday and they ran a test on it - they had the same values as far as voltage. However, it did come back with "excessive ripple" with the range being 210mV. Would that cause the volt light to come on and the gauge not show any voltage?

Greg

Please clarify; do you mean you see almost no deflection at all on your Fiero's voltmeter needle, and a separate Volt-Ohm test meter is showing 13-14 volts? If so, what is the engine-running voltage at any battery tap in the fuse block in the driver's footwell?
Ripple: your alternator is producing 0.2 volts of AC on top of 13.5 to 14 volts DC.

Likely One or more Diodes in the Rectifier have problems.

Most ACdelco parts only have 30-90 day warranty. Go check and if possible return under warranty.
If not... I use AZ parts w/ lifetime warranty that just need phone # to track in the system. IOW you won't need a receipt months to years later if have problem again.

alt should be above 14v, close to 15vdc.

quote Originally posted by Rexgirl: Ripple: your alternator is producing 0.2 volts of AC on top of 13.5 to 14 volts DC.

close enough but may not quite right.
1 diode dead means 1 or 2 stator winding is "dead" and can cause a volt drops on alt's output and big noise problem.

SI CS and most other alt's are 3 phase AC generator w/ 3 phase "full wave" rectifier to output DC.
SI and some others are infamous for shorted diodes... when so does get AC on the output term, way more then 0.2v, and quickly kills the battery and more.
CS often fails w/ open diodes not shorted diodes and shouldn't see real AC when rectifier dies.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Checked the voltage at the fusebox. With the multimeter on the open "IGN" leads I get 14v. Unplugging the gauges fuse and reading there, I get 11.5v.

When I turn the ignition to on, the voltage gauge needle moves very slightly, but not in any of the normal volts range - just sits near the rest spot.

I guess my overall question is - what is it that's making the volt light come on and my gauge to not register any voltage? Because with my tests at the battery and fusebox, and Advance Auto's check of the alternator, it's producing well above 12v.

When the voltage output of the alternator is lower than the voltage of the battery, the light will be on.

Does your oil pressure gauge read normally?

quote Originally posted by Rexgirl: When the voltage output of the alternator is lower than the voltage of the battery, the light will be on.

That's the thing - the alternator is charging at 14v and the battery at rest is at 12v.

The oil pressure gauge reads normal.

Visit the Ogre's Cave. There are some wiring issues with the CS Alternator.

Electrical/Charge & Start/CS Alternator

'A fully charged battery should read 12.6 volts. CS alternators won't turn on if the battery drops much below 12V.'

[This message has been edited by Gall757 (edited 11-08-2019).]

If the oil pressure gauge is working as normal then the voltmeter should be as well since they are wired in parallel. Gall757 and Ogre are correct in suspecting an alternator issue.

Replaced the alternator under warranty. Not the problem.

Pulled the gauge pod and discovered the connectors simply were not making good contact. Whatever combination was giving signal to the oil pressure gauge, the backlights and the volt light, but somehow not pushing the volt gauge (despite some slight movement on startup).

I think in summary here - if I realized that the volt gauge should read the battery voltage when in accessory mode, I would have figured out the alternator wasn't the problem - especially when it was pushing out 14 volts.

gif of the problem

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