I have a 1985 with a voltage problem.

I took my car for a ride and while driving my volts were 14 volts however when I stopped at a traffic light the dashboard displayed that I went down to 11.5 volts. I had stopped for roughly 2 minutes. I arrived at home and sitting in the driveway with the car running in drive position and foot on brake the volts went from 13.5 volts to 11 volts within three minutes. As I pulled it into the garage and took my foot off the gas the lights dimmed several times and the volts went to 10.9 volts. I have no voltage problem with while I'm driving it or when I have it idling in park position. I have a new alternator and a new battery. The engine has been recently rebuilt.

Any suggestions of what it might be?

 

Sounds like a bad diode (one of three). With 2/3rds of the field it can't produce enough current at idle.

 

Hi
What is the idle speed? 600 rpm or does it slow below that?
I know with my 85 the display never showed the actual voltage always shows up to a volt lower than actual battery volts, prolonged idle was always an issue when a/c and headlights were on it would occasionally show a low voltage alert.

The ecm will try to slow the RPM down, i remember the factory setting was 560 with temperature offset so it would idle at 600 rpm but can drop to around 570-580 hot while idling for long periods in drive.
With tunerpro and an ostrich emulator i have since set the idle to 650rpm, due to the alternator not charging much at low rpm when bumping the rpm up the charge would improve a lot.

Check what your idle speed is when hot and sitting at traffic lights etc, also check your loads cooling fan's, lights radio etc.

I ended up using the 86-91 alternator due to space requirements with a blower and can recommend powermaster alternators they have a very high amp rating at idle.
The 84-85 had a 140 amp alternator the 86-91 a 105 amp i remember running into problems going from the 85 si alternator, to the 86-91 cs type with it not charging as well as the 85.
Also some new alternators offer only the 105 amp for the 85?!?

 

Check the thick red charging wire coming off the alternator. If it looks a little burned at the crimp, you could have a bad connection there. I had the same symptoms you describe, and it was a failing crimp which caused the volts to jump around until the wire broke off.

 

Yep..

the idle speed is a big part of this. You need to be @ 650 in gear.
Other issues include the IAC and a/c. The IAC should bump the idle UP 50 rpm when the a/c is engaged so rpm does not drop any lower. A/c uses energy AND adds drag to the motor so its a double whammy to the system. Idle can get real low voltage at time due to this.

At 10.8 you can idle most all day with the accessories running and still be ok. The .2v deficit is not so much to cause problems.

As I've stated many times, GO thru ALL the misc electrical connections, contacts, plugs and junctions. Computer cars are extremely sensitive to dirty connections. The fact that elec current is flowing thru a "contact" connection means that there WILL be corrosion forming (green/wht crud) between the contacts. This 'stuff' (oxidized metals from both contacts) does NOT conduct, so when there is a good layer or the wire itself has been saturated inside the plastic insulation, the conductivity is lost...resistance goes way UP, conductivity goes down, and the car starts to perform poorly...

Early C4s have a "jumper pole" behind the battery. That's where ALL the magic takes place...where ALL the power originates for the digital dash, the ECM, the EFI, everything that makes a C4 run. Take the wires OFF and wire brush them till shiny and reinstall.

As previously mentioned, the red wire on the alt is good for arcing in the contact if it gets the least bit loose. That one right there will make it hard to push 14v thru the system...

Rule of thumbs.. ( never knew if it were the right or the left for automotive rules)
At start up you should see 14.1 + on the display for the first few minutes after the starter has been used. That's the regulator replacing the stored battery voltage that's been used to start up.
Then....

The display should show 13's most of the time,
all the time when above 1000 rpm,
and should only fall below the mid 12's with everything ON and running.
That includes lites, a/c, fans, stereo, brake pedal, the fax machine and everything else running....when you sit at idle. THATS the only time you should see <12.

Brake lites use a BUNCH of voltage. Doesn't seem normal, but they DO. Your foot on the brake sucks a LOT out of the battery @ idle. Try going to neutral as an experiment to see the difference. Takes a few seconds to respond but it WILL go up without the brake lites being lit.

On extremely hot days...you might see a bit less output. Maybe even 10.9v. As long as its brief the .5v to .9v loss is not an emergency.

If you start seeing <12v at rpm over 750....then its time to investigate. Again, the connections are the #1 problem with the charging system.

BTW...

this system is sensitive to these connections and the resistance that can come from dirty/loose connections.
I have witnessed a very poor running engine that misfired, would not find a stable idle and just ran like crap, CURED by simply cleaning the connections at the jumper pole so the ECM and other things that depend on "clean" power sources could operate as designed with the voltage and resistance they were designed to work with.

It just good housekeeping with the electrical system.

 

I really appreciate all the responses.

I next tried to start the car and place the car in neutral without my foot on the brake and I had a reading of 14 volts. With my foot on the brake while in neutral was 14 volts and with the lights turned on 13.2 volts. I drove the car approximately 6 miles and it began giving voltage problems again. Today, I'm going to check the wires to and from the alternator and in addition check and clean the jumper pole wires.

These suggestions were all very helpful.

Thank you,
Wheels26

 

You could have a grounding issue. You might try running a separate ground wire from the alternator case to some point on the body as a test.

 

Diodes if everything else is good. Been there, done that.