Weird voltage drop with AC on. Advice?

Gotta strange problem I've been trying to track down for months now.
When I turn the AC on (GM A6 compressor) the alternator "quits charging". Voltage drops to 12.5 from the 14 volts otherwise.

Compressor clutch is new, takes 3 amps to run it (within specs)
Alternator is new, puts out 55 amps clean voltage.

Problem persists even if I disconnect the AC and jump the compressor right to battery.

Been suspecting slipping belt, yet no noise, and everything visually turns at full speed.

Any ideas? Anyone ever have a V-belt slip with no noise or obvious glazing?

Thanks!

 

Oh, if I turn on every accessory, light, etc.... the alt keeps up fine. Just the compressor does this. And the ac is cold, front and rear

 

Is this at all speeds and RPM? Maybe the RPM is dropping too much to charge when the compressor kicks on? I once had a charging problem that was caused by a bad brake booster. It caused a big vacuum leak, the rpm dropped, and the alt wouldn't charge.

 

Voltage drops form 14 to 12.5 measured where? Check the voltage with the common lead of the meter on the alternator case and the positive lead right on the alternator output lug. Does it still decrease from 14 to 12.5 with the compressor kicked in?

 

Youn need a higher amp Alt. Your blower motor for the a/c Might be pulling too much load. Also is this test @ driving speed? Even a 100amp Alt dosen't put out but about 60 or so amps @ idle

 

Check the fan for high amp draw. It should kick on when the A/C is turned on. I am talking about the evap fan but you might also have a fan on the rad that kicks in also.

 

The a/c evaporator fans draw a bunch of amps, even normally. The fuse is generally a 25/ 30 amp. A 55 Amp alternator is really kinda small for a car with any kind of accessories.

 

Where are you getting your primary source of voltage from to the compressor? Normally any car with A/C will have a 70 Amp or better alternator,any accessories like electric fuel pump,electric fan should have a relay in the circuit if you have not installed a relay you should.For a really good explanation on using relays visit the Watsons Streetworks website,they made it really EZ to understand and there are diagrams also.

 

A test is worth a thousand maybe's I would suggest you test the A/C clutch coil.

 

I've had a similar problem. Yes, a V-belt can slip without noise. I fixed the problem with a double belt/pulley setup. A serpentine belt will slip less too. A 55-60 amp alternator is minimal at best. Size does matter. Bigger is better.

 

Yeah, its at all RPMs.
Voltage is measured at the alternator to case, starter lug to block, you name it!
Clutch coil is good, draws only 3 amps. Brand new.
No electric fan on radiator.
Problem occurs even when I jump the compressor directly, eliminating the interior fans.


See? strange huh? Wish I had one of those point-n-shoot RPM gauges right about now.
I'm gonna recheck the belt after work. Wouldn't be easy to change to a dual belt system with the other belt there.

 

Its an internally regulated Nippondenso alternator, Toyota Corolla flavor. Its new.
I've got good belt wrap around the crank, alternator and AC.

 

What's a AC ??????

 

Troubleshooting stuff like this is hard to do through a keyboard. Wish I was there with my DMM and other instruments.

First the stuff I assume you have already done -

Battery fully charged, cables in good condition, cable connections clean on both battery and opposite ends?

Ground straps from block to frame and block to body with clean connections, negative lead of battery attached to block at same point as ground straps?

OK, alternator is new, but has it been tested out of car under full load to see if it's operating at spec?

have you tried another a/c clutch? Doesn't have to be on the car, just a similar compressor clutch outside of the car with the case grounded to the block and hot lead connected to clutch power in place of the one on the engine mounted compressor. Does it do the same thing with the second clutch activated? It's a longshot, but any inductive device (big coil) like a compressor clutch could be intermittent and generating voltage spikes on wiring that could confuse a voltage regulator, while still engaging.

If all the above has been checked -

Pull all fuses to everything in the car except those necessary to run the car and ac clutch to see if the problem goes away. If it does, add circuits back one at a time until you find one that seems to be causing the problem.

I'd have to do some research on exactly what kind of alternator you have, don't know that one. Is it 1-wire, 2-wire internally regulated, 3-wire internally regulated (charging light on dash), or 3-wire externally regulated (external regulator box)?

If it's 1-wire and only putting out 12.5 after the internal regulator has been triggered on by the engine rpm raised above 2500rpm, belt could be slipping, alternator could be faulty, load on it could be too high, or again voltage spikes generated somewhere in the car travelling back to the alternator on the charging wire and confusing the internal regulator.

If it's a 2 or 3-wire internally regulated alternator, the voltage sense wire from the fuseblock tells the alternator when to increase or decrease system voltage. Anything that could put voltage spikes on this sense wire or raise its voltage would confuse the internal regulator. Again, some faulty inductive devices like that clutch could put voltage spikes into a car's electrical system, and any other battery or source of voltage in the car like a miswired battery backup for a radio or a stereo system battery could conceivably raise the sense wire voltage, decreasing the alternator's output voltage. Measure the sense wire voltage at the fuseblock when the problem occurs - is it the same, lower, or higher than the alternator output voltage?

Hope this helps.

 

How are you measuring the Amp draw of the compressor clutch? To do this correctly you will need either a low amp probe or put a meter inline between the power source to the compressor and the battery. It will be alot more than just 3 amps.

 

Check your ground on both the alternator, and compressor. Sound like a bad ground.

 

I guess the best advice is to take the car to a professional that can lay hands on it and find what seems to be a difficult and confusing problem.

Not knowing very much of anything about the poster's car, I suggested the simplest and most likely steps to solve the problem first. Hopefully all those simple things have been done or will be done.

Secondary voltage sources and inductive kickback problems are very unlikely, but I attempted to cover all the bases. I'm guessing that if his problem was an easy one to solve, it might have been solved by now.

But I've personally seen solenoids, clutches and other big electromagnetic devices with an intermittent in the wire in the coil that would generate not one voltage spike, but many very rapidly, a spark coil if you will, with enough energy to stay engaged. Only an oscilloscope would show if that kind of thing is actually happening. As I suggested, substituting another clutch would be the easiest way to rule that out.

Many cars these days have a large sound system with a big battery of its own, a secondary voltage source. Sometimes I see the secondary battery wired directly to the B+ buss on the fuseblock through large gauge, low-resistance cable typical with large sound systems. This is the wrong way to wire such a system, as it can temporarily hold the fuseblock (sense point) at a higher voltage than a simple charging system would under a load. This would tell the car's alternator to charge at a lower than necessary rate, and is possible because there is more resistance in the wiring from the charging system to the fuseblock than there is from the fuseblock to the secondary battery. Of course, the secondary battery voltage will eventually sink to the charging system voltage over time, but that may be a fairly long time depending on the size of the secondary battery. Meanwhile, the problem the poster is describing would occur. Any secondary voltage source in a car should rightly be wired as close to the charging source as possible, not to the fuseblock.

 

AC is what you install when you build a car for your wife and 2 daughters

Grounds and hots check good, no voltage drops. Battery cables are welding cables, oversize with soldered/crimped on heavy lugs. All new.
Ground strap is same cable design. Heavy from bellhousing bolt to frame, and frame to battery

Speaking of battery, I have same symptoms with either the new Optima red, or the new Deka lead-acid.

Compressor clutch is measured with my Fluke123 labscope and a clamp-on ammeter. It really is 3 amps, which is spec. Coil ohms out around 4 ohms, which is spec.

I've got a GM connector at the comp with a good diode across it.

The belt routing goes from the crank, pulls on the ac, then up to the alt. In other words, the alt is on the slack side of the belt from the ac.

I'd be happy to find a serp belt style pulley for the 250 six, but I've never seen one. Or one for a A6 compressor either...

 

There's 3 wires on the alt. Output, ignition and a sense wire. They all stay on when running, like they're supposed to.

I'm still leaning to it being a belt issue. Hope to get time to look into it more this week before Friday.

Thanks for all the replies so far!

 

We might have a winner here. I think in most every factory installation that I've seen, the alternator and A/C compressor are driven by separate belts. With a common belt driving both, you've got a couple of things working against you with the alternator on the slack side of the belt. For one, when the compressor clutch is engaged, you are drawing a little more current from the system, plus I assume your blower motor is running as well, causing additional draw. When that compressor clutch kicks on, you now have the load of compressing your refrigerant, which will cause the slack side of the belt to be "looser". Your alternator is being driven by this "looser" side of the belt, so even though the alternator is spinning, you probably do have some slippage around the alternator pulley when the compressor is running. You might want to simply overtighten the belt for a very short time (couple minutes) and see if that is the problem. Belt dressing (ask at your local Tractor Supply or similar farm store) might help. It makes the belts "sticky".....

 

I forgot about belt dressing! I'll get some to use as a test. If I have to, I think I can fab up a bracket to have a small idler pulley on the backside of the belt. Depends on if there's room or not.

 

He lives in AZ, you live in PA. I bet he doesn't know what a heater is

 

did it work fine before or is this a new installation?

 

All new installation. About 1500 miles on the Burb so far.

 

When I engage the A.C. on my '91 S10 Blazer SUV the output voltage from the original GM 85A alternator drops from 14V to 12.5V. The A.C. is the only accessory that does that. I can turn on all other acessories, except the A.C., and the output voltage remains at 14V. That may be a sufficient reason to replace my 85A alternator with a 100-105A alternator but not to exceed a 140A output alternator or a power wire adapter harness must be installed. An alternator rated over 140A output is required only if you have the normal acessories plus running lights, amps and boom boxes. An 85A or 105A alternator is fine for use with accessories such as radio, heater and A.C.

My 1963 Pontiac Catalina with a 421 HO (now 455 CI) engine has no radio and no A.C. and it still has the original 46 year old Delco-Remy 47A, one wire alternator and it charges a Interstate 1100 CCA battery just fine. The 1100 CCA battery is only necessary in order to start the 455 engine with 12:1 compression ratio.

An alternator goes bad in stages. To test the voltage output, put a cheap $10 VOM on the DC scale and connect it across the battery terminals (VOM red lead to the + terminal, black lead to - terminal) with the engine running at a fast idle with all accessories OFF. The VOM will measure 14V but if it reads less than 14V, you have at least one failed diode in the alternator. If your VOM reads 14V, turn on all the acessories. If it drops below 13V, you have a alternator that is not up to snuff.

 

12.5 volts sounds like it is still charging, providing your volt meter is accurate. Does your battery go dead?

 

Not only does it go dead, I've had the alarm go off while driving. Caused by low voltage.

If I run the engine with all accessories on, except for the AC compressor, voltage stays right at 14 volts.

 

Get a Deltran Battery Tender (1500 ma or 800 ma) and connect it up to the battery terminals. If it is still charging (yellow light on) after an hour, you have a parasitic battery drain somewhere. Disconnect the battery terminals and reconnect the battery charger to the battery and if the green light come on within one hour, that proves it.

A continually running accessories such as a clock, sensors or the ECU draws from 8 to 50 ma when the engine is not running. Some late model computer driven Fords have a parasitic battery drain up to 50 ma. Also a shorted solenoid, a shorted relay or a wiring short can cause a major parasitic battery drain. If one of the computer driven cars sits and is not driven for a few days or a few months, the battery may be dead, depending on the amount of parasitic battery drain and the condition of the battery. I had a shorted power door lock relay that killed a 18 month old battery in two days. It was an Autolite 24 month battery which is not among the best batteries. Normally a battery is only good for about one half of the warranty period. That trick keeps you coming back for another of the same brand of battery. Anyway, I replaced the power door lock relay and installed a new AC Delco 75-60 battery and then hooked up a Deltran 800 ma Battery Tender for 24 hours and got a full charge. I have no more problems. I don't need the battery tender unless I plan on not driving the car for a few months. That is because the normal parasitic battery drain is less than 10 ma for the sensors, the ECU and digital clock on my car.

It seems to me a battery that is continually low in voltage due to a continous parasitic battery drain when the engine is not running, will show up as a low alternator output when the engine is started and a major accessory is turn on.

 

How do you come up with that conclusion? 12.5 is Battery voltage.

I would be looking for a differant alternator.

 

12.5 is almost full charge battery voltage. 12.65V is full charge. If it is dropping below, it is discharging. An alternator putting out 12.5V under heavy load is probably working. I would suggest checking to insure the battery is grounded to the block, and the block grounded to the frame. Bad grounds and bad crimps cause the vast majority of auto electrical problems.

Do you know Ohm's law without the google?

To 1950ChevySuburban: what battery are you running, how old is it, and has it ever been below 11Volts or worse in its life? I generally start with a load test of the battery and then a bench test of the alternator/generator/regulator. Establish a baseline with known good components and then go to the wiring and grounds.