IanM

When I first start my car, the dash reading sits at about 13 volts. After driving for awhile, it slowly drops down to just above the 12 volt line. No warning lights come on, and the car starts and runs fine. What causes this? I know once the car heats up, the fans will kick in, so maybe that could affect things. But it doesn't rebound once the fans shut off, and turning the headlights, stereo etc off doesn't make much difference either. Could the under-hood heat somehow affect a poor ground connections? Dying alternator voltage regulator? Do other people see their voltage drop when their car warms up?
Thanks,

Cass944

when your engine gets warm your radiator fans turn on. its the same thing as pushing the snowflake button on your dash. those fans make my gauge drop. but it could also be your alternator or a bad ground. mine did that for a long time untill it finaly just died one day/ then i had the alternator replaced and its been fine ever since.

IanM

I'm interested - what voltage are other people seeing on their dash gauge when they first start their car? When the car is at operating temp? With headlights, stereo etc. on?

IceShark

There are 3 problems. One likely small one is resistance will increase the hotter a conductor gets. Probably not a big issue.

The biggest causes of the drop are: (1) your alternator diodes getting hot and (2) the voltage regulator has a circuit that drops output so you don't cook things like the battery, it will usually drop a maximum of about 3/4 volt when it gets really hot. This looks like it could explain nearly all your voltage drop.

However, your wiring is crummy as you should be starting out cold at ~ 14v. Also, dropping the max 0.75 volts tells me you may not have your alternator cooling scoop connected to the heat shroud on the back of the alternator.

IanM

IceShark - I'll check, but I'm almost certain that cooling scoop is still there. Perhaps it's blocked or something. Do you think I might need a new voltage regulator? Is that why the alternator diodes get hot? Where are the chassis ground connections located?

IceShark

Ian, make sure the hose from the dam scoop to the shield is clear and no holes in the walls. You could suck something in there that is cutting down flow. The diodes get hot from high current output and hot ambient air temp, voltage regulator shouldn't heat them up by itself.

New voltage regulator set at 14.4 volts should help. Then do something like my battery cable project and you will get the voltage up.

The main chassis ground connections that are causing your problem in the cabin are the two that come off the battery. One to inner firewall and other to bellhousing. Also helps to add a supplemental one from the front of the block to the forward frame rail where the lights and such ground.

You could also be losing in the positive from alternator to starter and then from there to battery.

The next weakness is the run from the battery over to the fuse box. This is shows up when you load the electrical system up.

IanM

IceShark,
I checked last night, and the plastic shroud is in place behind my alternator. However, I have a sneaky suspicion that this hose you described that connects to it is likely not there. Should I buy some tubing and make up a hose? How about flexible heater duct tubing? What diameter would be best?

As far as hot ambient air temps go, that certainly has nothing to do with it. It's winter here in Vancouver, and in the evenings it's probably been hovering around 40F. I'm concerned about the grounds, I think I should at a minimum check them and make sure they're secured properly. The engine was out of the car about a year and a half ago, so who knows if the mechanic tightened everything back down properly.

I did some tests last night. I checked battery voltage using three methods:
(1) using a voltmeter directly onto battery posts
(2) voltmeter on battery, w/ ground lead on cam cover
(3) inside car voltage gauge

These are the results (1/2/3):
Car cold, not running - (12.57/12.58/11.5)
Car cold, running - (13.92/13.94/13.2)
Car warm, running - (13.71/13.74/12.5)
Car hot, running, lights&fan on - (13.25/13.3/12.2)
Car hot, running, accessories off - (13.4/13.44/12.5)

So, what do you think? With all accessories off and car running, I have a max of over 13.9 and a min of about 13.4. With lights and heater on I have a min of about 13.25. Would you say this is normal? Time for a new voltage regulator, or just time to attach a cooling tube to back of alternator?

Why is the voltage consistently lower when I put the voltmeter ground lead directly onto battery negative post? Does that indicate a bad ground connection coming off my battery? Is it normal for the in-car volt gauge to read so low? How might I make my volt gauge read more accurately?

Thanks!!

bs

the resistance of a poor ground connection can vary with a lot of things, including engine temperature. i had bad grounds that were causing a whole host of problems, and when i got them fixed, my voltage gauge behaved accurately and consistantly for the first time.

get some dryer vent hose and fix the duct to the alt. and use the method i describe in <a href="http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=013694" target="_blank">this thread</a> to clean the two engine grounds. clean the instrumentation grounds under your dash. they are right underneath the main fuse box.

the voltages you gave are consistant with some resistance between the battery and alternator.

IanM

Ben,
What's the easiest way to get at the ground connections that lead from the battery to behind the engine? Can you get at them if you jack up the car and go in from underneath? I peaked in behind my oil filler tube, and it doesn't even look possible to get tools down there. Does the intake manifold have to come off? Do you have any pics?

dermass

question, is the gauge reading alternator output or battery voltage. if its reading alternator output isnt it behaving correctly? when you start the car the alternator is working extra hard to charge the battery. once the car warms up, the alternator is continually charging the battery. once the battery is fully charged, the alternator relaxes, explaining the voltage drop.

jspearin

IanM - Did you ever resolve this issue you were having? I'm having the exact same issue right now. I recently replaced the voltage regulator on the back of the alternator and that improved things a little bit but I'm still seeing the voltage drop when the engine warms up. I'm thinking the grounds are my next thing to investigate but I'm curious what you found. Thanks in advance for any help or advice.

Kurt R

You realize you're replying to a 14 year old thread, don't you?
IanM sold his turbo in 2012.

Check your grounds.

jspearin

Thanks Kurt R, super helpful!