Voltage Drop

[cdillon12]

Hey guys. Issue I’m having is when I turn on my a/c (does not blow cold air) my voltage drops from 13v to 9/10v depending on fan blower speed. I turn it off voltage jumps riget back to 13v. When I turn my headlights on voltage drops to 11volts. Turn off right back to 13v. Alternator and battery have been tested and test good. Battery cables look good. What could it be? 87 GMC/350. Thanks!

 

[75gmck25]

Based on my experience, the alternator is bad, regardless of how they tested it. It puts out the right voltage with minimal load but cannot keep up under higher loads. Mine had similar symptoms when I ran the blower and headlights at the same time, and the fix was a new rebuilt alternator.

I gave them the part number for a 12si instead of the original 10si, so I upgraded from 63 amp 10si to 94 amp 12si at the same time. They didn’t seem to care that the Lester number was different.

 

[fast 99]

If a digital voltmeter is available put it on AC. Across the battery posts [engine running] if voltage is near .100 or more alt is bad. There will always be some ac voltage but normally it is in the .025-.050 area. There isn't a hard number for this test. Battery condition and RPM will vairy the result.

 

[Challis350]

It drops to 9/10 volts even when you're in motion?

At worst mine will drop to under 12 if I have every accessory on and i'm sat at a red light.

 

[SirRobyn0]

Hey guys. Issue I’m having is when I turn on my a/c (does not blow cold air) my voltage drops from 13v to 9/10v depending on fan blower speed. I turn it off voltage jumps riget back to 13v. When I turn my headlights on voltage drops to 11volts. Turn off right back to 13v. Alternator and battery have been tested and test good. Battery cables look good. What could it be? 87 GMC/350. Thanks!

Is that at idle engine running or off?

If running at idle, bad alternator, to low of an idle speed, or a ground issue are the most likely.

 

[cdillon12]

It drops to 9/10 volts even when you're in motion?

At worst mine will drop to under 12 if I have every accessory on and i'm sat at a red light.

Yes it drops when I’m driving and turn on ac.

 

[Snoots]

That alt may test good on the bench but, did they put a load on the output?
That sounds like a bad regulator to me.

 

[cdillon12]

That alt may test good on the bench but, did they put a load on the output?
That sounds like a bad regulator to me.

Yes it was still in the truck.

 

[SirRobyn0]

Yes it was still in the truck.

IF you are sure the alternator is good, check the ground from the block to the battery. Check the charge wire from the alternator to the battery. Start by a visual inspection, then do a resistance test on the cables and/ or a voltage drop test. Should be no more than 0.20 volt drop, preferably it'll be less than 0.10. If it is occurring at idle check your idle speed, if it's idling to low or the A/C kicker isn't working the alternator isn't going to produce power very well at 450RPM. You can get an alternator that passes testing, but at idle doesn't produce like it should.

 

[Matt69olds]

First things first, do a simple voltage drop test.

Put the POSITIVE probe of your meter on the charge stud of the alternator. Put the NEGATIVE probe on the POSITIVE post of the battery. Start the engine, turn on anything electrical. Read your meter, whatever the meter reads is the amount of voltage lost in that section of wire. Hopefully it’s a couple tenths or less. If it’s more, either there are some bad connections, or the wire is undersized and the voltage lost is actually being used to heat the wire.


If that’s good, someone with an inductive clamp style amp meter, find out what the alternator is actually capable of putting out. These trucks are decades old, who knows what alternator is on there. A 63 amp alternator might test good on a bench, but 63 amps will be pretty much maxed out with the lights, heater and other accessories on.

If the alternator os undersized, a 94 amp alternator from a late 80s/early 90s small block Camaro or Cadillac will bolt on. Just make sure you install a minimum of 8 gauge wire from the charge stud to the battery.

 

[cdillon12]

Thanks guys I really appreciate the great feedback! I will check everything out tomorrow afternoon when I get home from work.

 

[Snoots]

Wouldn't hurt to check the tension on the alt belt either.

 

[Redfish]

Thanks guys I really appreciate the great feedback! I will check everything out tomorrow afternoon when I get home from work.

Lots of great advice here, I agree with pretty much all of it so no sense repeating it. I will say that I didn't realize just how bad my original alternator was until after I replaced it.

About where in Mississippi are you located? Do you take that truck to Cruisin the Coast?

 

[cdillon12]

Lots of great advice here, I agree with pretty much all of it so no sense repeating it. I will say that I didn't realize just how bad my original alternator was until after I replaced it.

About where in Mississippi are you located? Do you take that truck to Cruisin the Coast?

I’ve only had the truck for a month. I would love to take it. I’m in Tylertown, so only about 1.5 hours away.

I found the small ground wire that came off the main battery ground cable had been spliced with a butt splice and was barely holding together. I made a good splice and that seems to have solved my problem. Thanks to all!

 

[RanchWelder]

Word to the wise... everything these guys asked you to check and verify could now be partially damaged, as a result of the broken wire you found... especially your ground wires having gasses and corrosion being generated. When your system undergoes stress from poor grounds, it can empty/dry out the end cells on your battery of acid/water, or all of them... verify your battery water levels, use Distilled... and clean all your ground connections, especially to the frame, firewall and alternator.

I had to do this yesterday for a guy who refused to verify his grounds, as asked, 12 months ago... his car was my ride home, so he got his car serviced and repaired in return for some Corned Beef and Cabbage...

Once the brown out starts, the effects can kill batteries, alternators, ignition modules and starters and they can all go in order or individually for months on end.
Clean your connections with baking soda, carefully, (don't spill baking soda water anywhere near your battery or BOOM) and sand and polish your terminal connections.
Use dielectric grease on everything... after you have cleaned and inspected every connection or connector.

...Or you MAY be posting here again in a few months.

Your problems may have only just started, unless you clean everything to spec, now you have found a poor ground and repaired it.
Especially at your starter connections, where the corrosion can and will accumulate where you may not see it.

Nice Rig!