Bad alternator?
Thread Starter
Navigator
Joined: Mar 2018
Posts: 9
Likes: 0
Bad alternator?
I have a 1985 automatic it has 51,000 miles. when I put it under load with headlights radio and max A/C or heat the rpms get low about 500 and the volts on the dash will start dropping slowly from 14.4 and go down into the 12’s until the engine eventually stalls. The headlights will dim and then get bright again and the car seems to chug when in reverse. It does it more so in reverse than in park or drive. When I start turning things off the volts will climb back up to 14.4 on the dash and the rpms will run in the 600s and car seems to run fine. Is this because of the alternator and it just is struggling to run everything at the same time or could it be something else?
Pretty new to working on cars mine specifically I know a little but always trying to know more any help is appreciated
Pretty new to working on cars mine specifically I know a little but always trying to know more any help is appreciated
Le Mans Master
Joined: Aug 2015
Posts: 6,128
Likes: 2,003
From: St. Charles MO
I'm no electrical expert, but it seems that if the battery were healthy and charged, it should run everything even with no alternator for a while. So you may have bad battery, or alternator, or both. 14.4v is probably at least tolerable, but something (or multiple things) are no responding well to load. Test both the battery and alternator on a load test.
Thread Starter
Navigator
Joined: Mar 2018
Posts: 9
Likes: 0
I’ll give it a try myself in the morning I got the battery tested at autozone in December and it came back a good battery wasn’t all the way charged but was close and then two weeks ago went to advanced auto parts and they tested both the battery and the alternator came back good but started to turn stuff on put the radio on and heat and died almost instantly. Changed the spark plugs after that because he suggested it told me to do that then told me to maybe adjust the tps haven’t messed with that yet. My buddy is a pretty car savvy used to be a mechanic but not to into the electrical part of things he seems to think it’s the alternator but I figured I’d get an opinion from people who could have experienced the same thing with the same type of car.
Team Owner
Joined: Sep 2003
Posts: 25,414
Likes: 2,745
I’ll give it a try myself in the morning I got the battery tested at autozone in December and it came back a good battery wasn’t all the way charged but was close and then two weeks ago went to advanced auto parts and they tested both the battery and the alternator came back good but started to turn stuff on put the radio on and heat and died almost instantly. Changed the spark plugs after that because he suggested it told me to do that then told me to maybe adjust the tps haven’t messed with that yet. My buddy is a pretty car savvy used to be a mechanic but not to into the electrical part of things he seems to think it’s the alternator but I figured I’d get an opinion from people who could have experienced the same thing with the same type of car.
Safety Car
Joined: Sep 2001
Posts: 4,163
Likes: 555
From: Glenbeulah Wi
Your idle is dropping too low. I suspect your iac has failed or is dirty or your minimum idle rpm setting is too low. When you put the higher electric load on the car the iac should open and keep the car idling at 600 rpm. Since your drops to 500 rpm or less the iac appears to not be woking. The alternator will not hold 14 volts at 500 tpm or less with a high load. If you rev it up some does the voltage climb back up to 14 volts? You had the battery and alternator tested, look at the low idle problem.
Thread Starter
Navigator
Joined: Mar 2018
Posts: 9
Likes: 0
Your idle is dropping too low. I suspect your iac has failed or is dirty or your minimum idle rpm setting is too low. When you put the higher electric load on the car the iac should open and keep the car idling at 600 rpm. Since your drops to 500 rpm or less the iac appears to not be woking. The alternator will not hold 14 volts at 500 tpm or less with a high load. If you rev it up some does the voltage climb back up to 14 volts? You had the battery and alternator tested, look at the low idle problem.
Safety Car
Joined: Aug 2009
Posts: 3,752
Likes: 278
From: Melbourne, Victoria, Australia
Your idle is dropping too low. I suspect your iac has failed or is dirty or your minimum idle rpm setting is too low. When you put the higher electric load on the car the iac should open and keep the car idling at 600 rpm. Since your drops to 500 rpm or less the iac appears to not be woking. The alternator will not hold 14 volts at 500 tpm or less with a high load. If you rev it up some does the voltage climb back up to 14 volts? You had the battery and alternator tested, look at the low idle problem.
The ECM detects the a/c is running and bumps the idle up, there is a diode across the compressor plug check its there.
The 85 hot idle is set at 580rpm, it should start run 1,200rpm a few moments then settle to 800rpm cold and when hot 600rpm.
I find at idle hot with a/c on 580 is what it idles at.
I recommend the base idle setup be carried out, NOTE base idle 400 rpm was changed to 500 rpm for better idle.
The procedure needs the top right hand ALDL terminals under dash to be shorted by paper clip, turn ign on 30 seconds and disconnect the IAC plug and turn off ignition then remove paperclip.
Start the car and set to 500rpm in drive with handbrake on engine hot,
check TPS volts 0.54vdc.
Turn engine off and connect IAC connector and start engine, it should run 1200 rpm a couple of seconds then drop to 600rpm.
Pro
Joined: Jan 2003
Posts: 706
Likes: 47
From: Denver Colorado
OP, do you have a "performance" pulley set up with a larger alternator pulley and a smaller than stock crankshaft pulley? Such a set up will give problems at idle with a lot of accessories turned on. The gain in power to the wheels is offset by a loss of electrical generating power at lower rpms.
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