1985 C4

My battery voltage is good both on volt meter in dash and multimeter and they correlate.
When I drive and use windows the volt meter in car says 14.4V as it should.
However, if I turn car off after 10-15 minutes of driving I have noticed that when I come to a stop light, it idles low and can die and, at times, when I use the power windows the car near dies or dies and I have noticed dash flickering slightly.
It doesn't happen on the first start and drive but this does occur after I start it again as I described above.

I suspect that it is still the alternator that functions differently at times especially after driving for a while.

Is my suspicion correct?
Is my alternator, not yet dead, but dying?
It is the original alternator. The batter is new and has been maintained.

Thanks,

C

 

As a test, if the RPM is dropping after a full warm up--try increasing the idle RPM and see if the issues are the same.
Has IDLE RPM dropped from previous normal/satisfactory operation?
If so, may suspect RPM is too low after getting to normal operating temp.
Would look at IAC operation, or anything which affects idle RPM at normal operating temp.
May want to manually adjust base idle RPM as a TEST only.

If you have access to "clamp on ammeter" would be a good time to look at amp draw/charge during operation.
Should be neutral draw/charge at idle at normal engine operating temp.

 

If RPM drops low the alt can stop charging or fail to charge fully

The battery should be able to keep the engine running even without an alternator, for a while at least.

Your symptoms don't quite add up the way you describe them as an alt failure- as previous poster mentioned perhaps the RPM is dropping and the alt is cutting out.
another possibility is the battery grounds are deteriorating which will cause both charging and battery power issue at the same time

I would
1. keep multimeter on the engine and compare RPM with Charging voltage when the issue occurs. Report findings
2. Inspect grounds for battery, and grounding for alternator, and grounds for chassis. Basically all of them. Don't trust your eyes either- feel for breaks and damage, perform continuity tests when everything is fully warmed up. Heat causes resistance so it maybe fine cold then shorting when hot.

If its the old alt, ancient, good chance it could be failing anyways, whether or not this is the issue I would investigate or at least prepare to replace with a brand new unit eventually. I always keep a brand new alternator spare handy for my daily drivers, Not an issue if its not your daily though because it can sit while you wait for a new one. Never buy or trust a rebuild unit- yes sometimes they are fine and you get lucky, But these days I strongly recommend simply buying a brand new unit with a warranty and some even come with roadside free towing and stuff.

 

Thanks for the responses. It is an unusual thing. With first start and drive the car is fine. Its only after it is at operating temp and restarted that this occurs. I have adjusted the idle screw but may need to adjust more.

In park, sitting in driveway where should the idle be on the dash?


C

 

I agree with the above answers. And it may be a long shot, but check your wires for corrosion, rust, or even looseness. My buddy just bought a vette a few days ago, although a C7, his alternator wasn't charging on the way home (54miles), and died as a result of it. Anyhow, I figured his alternator was going bad too. Turned out his ground and power wires are shot to hell. They looked like Freddy Kruger installed them lol. Keep us posted chief

 

Thank you for the response. So are you leaning more towards the alternator?
Someone else suggested that my MAF was not functioning correctly when up to operating temperature.

Thoughts?

C

 

Ok, so here is some more information. I went to drive care to test the voltage. My battery via mutlimeter is 13V. The voltmeter in the car on the dash was reading 13.6-13.9V. The dash lights were flickering. When I actuated the window switch to put it down the car died and the voltmeter on the dash nosedived. It started right up again. And when I used the window switch as a load, same thing happened. I think it is the alternator. The wiring looked ok on the unit.

I think, under load, the alternator is faulty and this is why the AC or the window loads are too much for the alternator at times.

Thoughts?

C

 

Depends how far it dives and where you measure. If its dropped to near battery voltage everywhere, then Likely alternator. But if dropping below battery then I will suspect something else, perhaps the battery or connections between fusebox battery and the lights that cause the dimming or the battery connection itself.

Voltage is like water pressure, imagine a pipe with flowing water under pressure. If you see a water pressure drop way at the far end but the main source (alternator or battery) is still plenty of pressure, then the problem must be down stream.

For example if the battery is showing 13.5v while the dash and interior (say voltage measured from the ciggarette lighter) voltage drops to 10v or 11v then it must be a connection issue between the fusebox and interior stuff to the battery. But if you hit the window switch and the interior drops to 10v and the battery drops to 10.5v that is suspect of battery since the drop occurred all the way back at the battery.

At this point it is critical you tell us where you are measuring voltage when the drop occurs. I recommend measure at the alternator, and battery, at the same time using multimeter. That would narrow it down pretty good. For example if alternator voltage says 14v but the battery drops to 12.5v while the alt is 14v then you can clearly see voltage is not able to reach the battery properly, connection issue between them. Sorry about all the words I just want to help and this is one of those issues where its really easy to just say "yeah replace the alt" when it could easily be a wire or some fuse related issue

13.0v at the battery though is pretty low. If you see 13.5 at the alternator and 13.0v at the battery that would indicate alternator too small or unable to charge properly.

 

Thanks King. I know where to measure battery voltage at. Where to I put the multimeter leads to measure the alternator voltage? I am assuming that the voltmeter on the dash is measuring the alternator. Is that accurate?

C

 

I take the lug off the back of the alternator, attach a (thin) wire to it, and run that into the car so I can read the lug on the back of the alt using a multimeter from inside the car during a drive and loading.

I never trust electric in dash OEM gauges. It COULD be accurate, but that is kind of rare, honestly. Depends on the type of car. Corvette probably more accurate than most i would think. But C4 is kind of aged, so... I don't like to guess.... be sure. It could also be a fault somewhere between the alt and that gauge, you know like a wire behind the dash or something.

 

I doubt it is the alternator, the car should run fine without an alternator until the battery gets discharged to the point that it will not run. It appears your alternator is charging but when you put a load on the car it stalls. That leads me to believe that your minimum idle is too low and the IAC cannot recover fast enough at the low 550 RPM idle to prevent a stall. The minimum idle needs to be set at 550 RPM when it is fully warmed up in park at 550 RPM to prevent that stall from happening, Years ago there was a service bulletin for setting the minimum idle speed.

-Warm car up
-Turn key off
-Jump the A-B terminal at the ADL port
-Turn key back on
-Disconnect the IAC control at the throttle body
-Turn key off, pull jumper, start car
-Set the idle at 550 RPM with the throttle screw
-Turn car off
-Plug IAC back in
-Turn key back on
-Reset the TPS to .54 volts
-Start car and idle should be good and stable

This assumes IAC is good and timing is set correctly with no mechanical issues.