While AC on, Engine Bogs Down

[Trstno1]

Start by checking that you have the "Relay Upgrade Kit" with the uprated circuit breakers. Remove the blue fan fail relay if you still have it and replace it with either a solid state controller or the fan fail bypass wires. Check all of the connections to the fans on the power side AND ground side. After you do all of that, find and fix all of the vacuum leaks. Vacuum leaks reduce the power (effect) of the idle motor, preventing it from increasing the idle to compensate for the additional load of the A/C. The idle motor can also be sticking making it slow to react. That can be fixed by cleaning it with some solvent. Then there are some "oddball" things to check like the decel springs. If they are deformed and not holding the valves closed on the throttle plates, that would mess up the idle. Do you really use the A/C in Alaska?

Yes.. I have all new relays circa 2018. I also have DM's solid state lambda/RPM/Fan Fail Relay(2x fused). I also have his front relay ground bus but I have not installed it yet. I'll need to look into the circuit breakers as I cannot remember. What am I looking for in regard to those? All connections to fans are solid. I'm also pretty sure I don't have any vacuum leaks since the motor runs pretty good w/o AC. I had a new ISM and actually took it out and lubed it up with WD40 and tested it, so that's working. (I just don't know if it's reacting quick enough. The Decel springs seem good, and yes I definitely use the AC in Alaska. The last couple years in AK have been really hot. My wife requires the AC to be on if I want to take her anywhere in the D.

[Trstno1]

It may not have anything to do with your electrical system and more to do with the condition of your AC system. My AC system would kill the engine if I let off the clutch coasting to a stop. Lack of oil or the wrong amount of refrigerant will cause the compressor to stress. I noticed David mentioned this but no follow up to it. You might hook up some gauges and confirm what your pressures are before replacing everything electrical.

This could be the issue for sure. I have swapped to 134A, but I do have a small refrigerant leak. Every summer I put in about 2 cans of 134A and that gets me through the summer. Up until now I had other things on the repair list that was much more pressing than the AC system. I honestly thought the electrical surge from the OEM fans kicking on was the culprit for causing the problem. However, now that they are swapped to DPI's and the problem albeit better, is still there, the root cause must be different. I was thinking I should just replace all hoses and AC compressor. That way I solve the leak and put my mind at ease about it possibly being an AC compressor issue.

I also just purchased DM's microprocessor controlled idle ecu, so I could just program it to bump the idle by 100 rpm when the AC is on. I know, I know.... another band-aid. I actually would really like to find the root cause of the issue.

[David T]

If the compressor was that bad you would see the clutch slip or the belt slip/squeal. It would get very hot, and you wouldn't have enough cooling. It can't hurt to check the pressures. If you have a leak and have been constantly "topping off" you could be out of oil and that would make the compressor work very hard and eventually fail. Especially if you have the wrong oil for -134.

[dn010]

If the compressor was that bad you would see the clutch slip or the belt slip/squeal. It would get very hot, and you wouldn't have enough cooling. It can't hurt to check the pressures. If you have a leak and have been constantly "topping off" you could be out of oil and that would make the compressor work very hard and eventually fail. Especially if you have the wrong oil for -134.

My guess would be too much refrigerant especially if you're adding cans and don't know how much is in there. Manifold sets are cheap, under $30 on Amazon and is a good starting place before you toss some serious money at it. If you're running 134 on original hoses, that may be a source of your leak. Do you have any compressor noise with the AC on?

+1 about the oil.

[Trstno1]

If the compressor was that bad you would see the clutch slip or the belt slip/squeal. It would get very hot, and you wouldn't have enough cooling. It can't hurt to check the pressures. If you have a leak and have been constantly "topping off" you could be out of oil and that would make the compressor work very hard and eventually fail. Especially if you have the wrong oil for -134.

I changed the oil when I did the 134A swap, but maybe its low. I'll check the pressures and get back.

[Trstno1]

My guess would be too much refrigerant especially if you're adding cans and don't know how much is in there. Manifold sets are cheap, under $30 on Amazon and is a good starting place before you toss some serious money at it. If you're running 134 on original hoses, that may be a source of your leak. Do you have any compressor noise with the AC on?

+1 about the oil.

I am running original hoses and completely understand that the 134A is probably just leaking out of it. Yet another reason I thought replacing the hoses would be a good idea. I do have a manifold set from when I originally did the work. I'll hook it back up and report back with the pressures. Though, even when I did my original 134A conversion my book values for pressures were pretty much spot on and the engine still bogged down when the AC compressor turned on. Let me get the pressures and I'll get back and we can go from there.

[Trstno1]

So, I took out the gauges and checked the static pressure at 67psi on the low and 63ish psi on the high. It is 69 degrees F, so about 20.5 degrees C.

Once i started the car and turned on the AC, the compressor kicks On with low side at 47 psi and the high side at 85 Psi.

The low pressure gauge seems high
The high pressure gauge seems low

The online resources I have interprets such results as “AC needs service, possibly faulty compressor”.

Cycle time is 6.5 seconds off, 14.5 seconds on. What should the correct cycle time be?

Also, I have a 100RPM drop when the compressor kicks on. The ISM seems to compensate fairly quickly, but not before I feel the RPM drop and engine vibration. Note that this problem is only witnessed at idle. I usually purposefully turn off my AC if I am at a stop light so I don’t witness the engine bog down. With the current cycle rate I’ll get that bog down at least 2-4 times per light. Quite un-desirable when you are trying to look cool next to other cars.

What do you guys think?

Note: RPM drop prior to the new fans were more like 200-250 RPM, so I’m getting there, just not there quite yet.

[Ron]

So, I took out the gauges and checked the static pressure at 67psi on the low and 63ish psi on the high. It is 69 degrees F, so about 20.5 degrees C.

Once i started the car and turned on the AC, the compressor kicks On with low side at 47 psi and the high side at 85 Psi.

The low pressure gauge seems high
The high pressure gauge seems low

The online resources I have interprets such results as “AC needs service, possibly faulty compressor”.

Cycle time is 6.5 seconds off, 14.5 seconds on. What should the correct cycle time be?

Also, I have a 100RPM drop when the compressor kicks on. The ISM seems to compensate fairly quickly, but not before I feel the RPM drop and engine vibration. Note that this problem is only witnessed at idle. I usually purposefully turn off my AC if I am at a stop light so I don’t witness the engine bog down. With the current cycle rate I’ll get that bog down at least 2-4 times per light. Quite un-desirable when you are trying to look cool next to other cars.

What do you guys think?

Note: RPM drop prior to the new fans were more like 200-250 RPM, so I’m getting there, just not there quite yet.

The static pressure for R12 and 134a @ 70°F ambient is ~70psi. The reading must be taken after the system has been allowed to cool off completely (overnight). The high and low sides will always be the same in this state regardless of the ambient temp. (They will balance via the orifice tube. So your gauges are a tad off.) Static pressure will only tell you if there is 'some' refrigerant -- Over or undercharged systems will read the same.

You should take readings when the compressor cycles OUT/OFF and refer to the chart at N:09:01 in the WSM.
When the compressor kicks in at 70°F ambient, the high side should rise from 70 psi to 125-150 psi while the low side falls from 70 psi to 25-32 psi, causing the clutch to cycle off. (134a will have higher high side pressures.)

The cycle times are affected by everything from system condition to humidity. Theoretically, the system may not cycle at all under the right conditions -- Use the cut out pressures found using the procedure above for anything useful.

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Did you try plugging the main vacuum feed as suggested above?

[Bitsyncmaster]

A lot of people post "Bogs Down" with AC on and most of us think your engine has no power with AC running. In reality the AC only requires about 5 hp. to run so it seems most people say bogs down because the idle RPM drop. This is usually a problem with the idle system and not a problem with the AC.

Your 85 PSI high side does sound to low and would indicate a low charge or bad compressor but do as Ron suggested and take those readings when the compressor has just kicked off.

[Trstno1]

A lot of people post "Bogs Down" with AC on and most of us think your engine has no power with AC running. In reality the AC only requires about 5 hp. to run so it seems most people say bogs down because the idle RPM drop. This is usually a problem with the idle system and not a problem with the AC.

Your 85 PSI high side does sound to low and would indicate a low charge or bad compressor but do as Ron suggested and take those readings when the compressor has just kicked off.

The readings I got when the compressor kicks off was 25 for the low side and 120 for the high. So I suppose those readings just point towards adding more refrigerant. However, I know I have a leak simply due to the fact I need to keep topping it off occasionally. And the fact that I am using original hoses with 134A. But when I do have enough refrigerant in the system, the AC blows cold. Right now it is blowing cold.

My problem is the sudden RPM drop when the compressor kicks on. Before the new DPI fans I would idle at 775rpm and when the compressor would kick on it would drop to like 550rpm. Big dip! Now with the new fans I would say its only dropping about 100 rpm but still noticeable when at an idle since the engine labors and vibrates. I have tested the ISM, and it is compensating about a second after the rpm dip, and then the rpm returns to 775 like it should. So my idle system seems to be working as it should.

I guess I was hoping I could do something so that I wouldn’t even feel the initial 100 rpm drop at all. Does everyone else feel the engine vibrate the car when the AC compressor kicks on? Is it just the normal operation of the Delorean?