Big voltage drop when the AC compressor cycles on.

[L3V3L1]

Like the tile says, the voltage drops from 14.3 down to 11 when the compressor cycles on. I'm checking the voltage at the jump terminal. If I press both window down buttons, when the windows are already down, I get the same drop. Where should I begin to troubleshoot this?
TY

[Timeless]

What is the condition of the grounds? Dirty grounds can cause a decent amount of voltage drop.

[JBaker4981]

Does it stay down at ~11V or is it dropping and then bouncing back to normal?

[L3V3L1]

Does it stay down at ~11V or is it dropping and then bouncing back to normal?
It bounces while the compressor is engaged, from 11.4 to 12.5. When it cycles, off, back to 14 +.

[L3V3L1]

What is the condition of the grounds? Dirty grounds can cause a decent amount of voltage drop.
I'll have to research ground locations and check them. Ty

[David T]

If you are checking this while the engine is idling, the alternator cannot output it's full rated current. Try your checks with the engine running about 1,000 RPM's.

[DavidProehl]

Have you verified all your fuses are fresh? I had a similar issue years ago when I was using fuses that were 10-15 years old. Swapped out the fuses and all my strange voltage drops disappeared.

[TheFover]

Mine also drops this low with the compressor is on. Even lower with the headlights. Brand new alternator with super tight belt too. I'll have to try the double window thing tonight.

[painterdave72]

Check the main wire from the post to the alternator also.. mine was a replacement from about 20 yrs ago and eventually it corroded enough to cause a massive voltage drop like you are describing.. replaced it and voltages stay up at 14 with ac and my sub and lights etc etc..i do have a larger amp alternator though but you get the idea..

[L3V3L1]

Could it be the alternator?

[JBaker4981]

Could it be the alternator?

Good Lord... that's an old 90a Motorola.

[FABombjoy]

Could it be the alternator?
Could possibly be related to the regulator melting and leaking down the side.

[L3V3L1]

Good Lord... that's an old 90a Motorola.
What replacement are people going with? DMCH, DELOREANGO? I prefer the least modification necessary.

[Rich]

Yeah, my 90A factory Motorola alternator has some brownish stains like that, too, but when we're driving the voltage is OK with the compressor running. Lower but OK.
As others mention alternator output drops at idle.

Before you pull the trigger on a replacing the alternator try checking the voltage with A/C on and idle at 1000-1200. If the system stays above 12.6V then you're not discharging the battery in that mode. You're not charging it, either, and it doesn't account for other loads like headlights. So >12.6V is better at high idle with A/C on.

Do the test with the blower speed set wherever you normally have it, too. Speed 4 is highest load, of course.

[glockworks21]

This thread got me thinking that ive never really checked my alternator. And it appears to be motorola 90? It has not failed me. I just checked it and it putting out 14.3 at idle. I would like less drag when fans, compressor, ect kick on. the 120 amp dgo one sounds good on paper. Any other suggestions for high amp alternator without doing any mods to make it work? If not I will order this one today..

I would like it to be more stable at idle when everything is running..

Thanks,

DT

[glockworks21]

What replacement are people going with? DMCH, DELOREANGO? I prefer the least modification necessary.


did you replace yours yet? if so which one and are you happy with result?

DT

[Bitsyncmaster]

did you replace yours yet? if so which one and are you happy with result?

DT

I have an old DMCH alternator. That is the one where I found you have to attach a small wire to the sense terminal to the battery terminal on the alternator to prevent an 18 second instantaneous drop in voltage. Not sure if the DMCH alternators still do that but I will run a voltage test for you to compare.

[Bitsyncmaster]

I ran the voltage check on my car. Note that I charge my battery every few months to 100% charge. I use a bench power supply set to 14.50 volts and it current limits at 5.0 amps. It normally takes two days to get the charge current down below 100 ma. I just did that a few days ago and hence my battery voltage today is 12.97 volts.

With engine idling at 775 RPM.

No loads = 14.61 volts
Headlight on = 14.60 volts. I have LED headlights
Blower on #4 and AC on = 14.50 volts. I have Toby fans.

With engine idling at 850 RPM which is where I keep it. It just seems driving the 5 speed at that higher idle seems much better

No loads = 14.64 volts
Headlight on = 14.61 volts.
Blower on #4 and AC on = 14.55 volts.

So not much different with a little higher idle RPM. But I did have one customer rave about his voltage using my idle ECU at 850 RPM. I guess he had a different alternator.

I haven't done any long drives for about 5 years now which is why I charge the battery every two months. It really does take a long drive to bring your charge up close to 100%. So some of your voltage loading is going to charging the battery.

Voltage readings were very stable.

I don't see any part info on my alternator. But that is what I would expect from a vendor.

[L3V3L1]

did you replace yours yet? if so which one and are you happy with result?

DT
I have not, and with a/c season being over, this is a job I will tackle next year. 😬

[mhanch]

did you replace yours yet? if so which one and are you happy with result?

DT

WHen i did mine I used this one, https://www.deloreango.com/us/alternator.html

I've been really happy with it and it was an easy install.

[glockworks21]

WHen i did mine I used this one, https://www.deloreango.com/us/alternator.html

I've been really happy with it and it was an easy install.



Yeah i saw you posted that...I ordered the same one sunday. It arrived today.. I may run a few tests on the old 90amp before swapping it out this weekend just to see the differences.. I can report out any results if interested..


DT

[TheFover]

I would love to know if anyone has solved this problem. This is driving me mad. I've upgraded to the new DMCH cooling fans, new DMCH alternator, new cabin fan, cleaned grounds, new battery, and still have a massive voltage drop when the compressor cycles on. All those things increased idle voltage by a bit, but didn't solve the main issue which idling in hot summer nights with the A/C on and headlights ends up killing the battery as the voltage eventually drops too low.

[Bitsyncmaster]

I would love to know if anyone has solved this problem. This is driving me mad. I've upgraded to the new DMCH cooling fans, new DMCH alternator, new cabin fan, cleaned grounds, new battery, and still have a massive voltage drop when the compressor cycles on. All those things increased idle voltage by a bit, but didn't solve the main issue which idling in hot summer nights with the A/C on and headlights ends up killing the battery as the voltage eventually drops too low.

Is it just a short drop or does it say low? What is your voltage reading with an without the AC on?

[TheFover]

Is it just a short drop or does it say low? What is your voltage reading with an without the AC on?

With all the accessories on including the cooling fans with no compressor, I'm in the low 13s volt range. Very good especially with the new fans and alternator. I barely see the volt gauge move when I add accessories. HOWEVER, when that compressor kicks in I see a significant drop, to like 12.5V and then it starts slowly dropping down toward 12.0V over a minute or so. And I'm talking about the compressor kicking on when the fans are already running - so just the compressor is doing this. This is only at idle and it bounces around a little as the car idle hunts a bit. It goes back up when I rev the engine or drive. But why would the compressor be drawing so much current is the head-scratcher. :huh:

[Helirich]

I would look into a better alternator.

[TheFover]

I would look into a better alternator.

That was my first thought, so I replaced with a new one alternator from DMCH, but no joy.

[Bitsyncmaster]

With all the accessories on including the cooling fans with no compressor, I'm in the low 13s volt range. Very good especially with the new fans and alternator. I barely see the volt gauge move when I add accessories. HOWEVER, when that compressor kicks in I see a significant drop, to like 12.5V and then it starts slowly dropping down toward 12.0V over a minute or so. And I'm talking about the compressor kicking on when the fans are already running - so just the compressor is doing this. This is only at idle and it bounces around a little as the car idle hunts a bit. It goes back up when I rev the engine or drive. But why would the compressor be drawing so much current is the head-scratcher. :huh:

The compressor clutch current is about 5 amps. Is your idle RPM dropping when the compressor kicks on?

[TheFover]

The compressor clutch current is about 5 amps. Is your idle RPM dropping when the compressor kicks on?

Yes, there is typically an idle drop associated with the voltage drop when the compressor kicks on. The idle regulates itself back up to around 775 afterwards, but the voltage generally does not come back up until the compressor kicks off.

[Bitsyncmaster]

Yes, there is typically an idle drop associated with the voltage drop when the compressor kicks on. The idle regulates itself back up to around 775 afterwards, but the voltage generally does not come back up until the compressor kicks off.

Sounds like your alternator can't put out enough power at 775 RPM. It's strange that only the compressor load would show that up. You can get my idle ECU and set your idle speed higher but it also bumps it up an additional 100 RPM when the compressor is on. I run my car at 850 RPM idle speed so when the AC goes on it bumps to 950. Really makes a big AC and voltage improvement.

[TheFover]

Sounds like your alternator can't put out enough power at 775 RPM. It's strange that only the compressor load would show that up. You can get my idle ECU and set your idle speed higher but it also bumps it up an additional 100 RPM when the compressor is on. I run my car at 850 RPM idle speed so when the AC goes on it bumps to 950. Really makes a big AC and voltage improvement.

Yeah, I'm considering your idle ECU, because I do think that might fix it, but I hate not knowing what the problem and having an issue down the road. Can I bump up the idle using the curb idle screws? The process sounds a little tricky and its not clear to me if that actually raises the idle RPM.

[Bitsyncmaster]

Yeah, I'm considering your idle ECU, because I do think that might fix it, but I hate not knowing what the problem and having an issue down the road. Can I bump up the idle using the curb idle screws? The process sounds a little tricky and its not clear to me if that actually raises the idle RPM.

Yes. Only problem with it is your idle RPM will change depending on the loading at idle.