160Swift

Has anyone had any issues or experience with their a/c cycling on and off at approximately 2-4 second intervals? The problem is intermittent when crusing but always happens at idle, and each cycle is accompanied by a 'knock' sound from behind the glovebox (blower motor area) and a 500-800 rpm drop which can be felt during cruise and causes the car to stall at idle. The cycling occurs even if you hold the rpms up while the car is at rest; it seems to me that something is telling the compressor to kick on and off, and it must be an electrical issue because the a/c will work perfectly during extended freeway cruises.

Car is a '93 MT with a just installed PowerFC from SR Motorsports (problem started months ago with stock ECU and did not change with the PowerFC).

Right now I'm thinking it could be a bad ground, faulty relay or malfunctioning sensor within the a/c system (high/low pressure or temperature/thermo sensor), but I thought I would enlist the collective knowledge of the forum prior to tearing half the car apart and buying/swapping out a bunch of sensors. Any suggestions or help would be greatly appreciated!

Adrian
Rochester, MI

mad_7tist

most likely a pressure issue. to little or to much freon

DaveW

I tend to agree with this - mine did that (short-cycling) when it was new in 1992/1993. At that time, the Mazda dealer could not find a problem. As the car has gotten older, this has been happening less and less, and it no longer does this at all. I believe it was cutting out on high pressure (there is a high pressure cut-out switch that safeguards the compressor), and now that a bit of Freon has been lost over the years (the AC is still all original, including the Freon charge), it no longer has that problem.

The "knock" sound cited by the OP is probably just the compressor clutch repeatedly engaging/disengaging.

160Swift

Definitely a good suggestion, but I would think that a true low or high pressure scenario would cause the cycling ALL the time, and the a/c will often work fine during cruise. I also had the system checked for leaks & pressure a few weeks ago, and everything checked out OK.

DaveW

Not true - mine also worked perfectly at speeds over 25 mph. The increased airflow thru the AC condenser apparently kept the peak refrigerant pressures down at higher speeds.

Dave

gracer7-rx7

iTrader: (16)

Happened to me once also after recharging the A/C after a rebuild. There are a few different compressors and each one has a different max pressure. The person recharging the a/c probably couldn't figure out which one.

160Swift

Thank you for all of your suggestions! I'll try having a small amount of freon evacuated to lower the pressure in the system and see if it helps.

Dave W - here's what I can't get my head around in your situation: I can't see how your system would lose just a little bit of pressure as it got older and then stabilize; with the a/c system completely sealed, the only way you could lose ANY pressure would be through a leak which would continue to leak until your system was empty (or you have an INCREDIBLY slow leak). Don't get me wrong though, I'm glad your problem fixed itself and I hope the lower pressure is the solution to mine also!

moconnor

iTrader: (14)

It sounds like a pressure issue to me too. However, a faulty thermoswitch is also a possibility. It is pretty easy to test that that it is at fault by jumping it (it is easily accessible under the passenger dash). You will need to look at the wiring manual.

mad_7tist

it is not a hard problem. if you do not have a manifold gague set for the ac take it to someone who does. obviously if the ac worked fine before and the high side pressure is to high, the system did not gain any freon but the condensor may be externally dirty, fans not pulling enough air, debris restricting flow in condensor.

DaveW

Any gas will diffuse (at much different rates, depending on the rubber/gas combination) through any rubber/polymer/plastic component like hoses and o-rings. For instance, air (or even pure nitrogen) diffuses through tires and causes you to have to reinflate them periodically. Cola will slowly go flat in plastic bottles, even if it has never been opened. Freon is no different, even though the AC system's rubber components' makeup is such that it happens VERY slowly.

So - my AC system is surely still very slowly losing Freon, but it has not yet lost enough to compromise the AC performance.

Dave

160Swift

Does anyone know where I can find an online wiring diagram for the a/c system? Pursuant to all of your suggestions and another thread I found (https://www.rx7club.com/showthread.php?t=324519), it seems like I should probably check the thermo & pressure sensors before going through the trouble of evacuating freon.

Thank you!

JM1FD

iTrader: (1)

Your description of a knock behind the glove compartment leads me to believe you have an issue with the thermoswitch on the evaporator. As stated previously, bypass it and see what happens.

you can get wiring diagrams via the sticky at the top of the forum.

moconnor

iTrader: (14)

Here is a picture of the thermoswitch. You need to unplug the white connector and jump the two lines on the opposing connector that correspond to the green wires going to the thermoswitch.

160Swift

moconnor - that's awesome, a picture is worth a thousand words. I'll try bypassing the thermoswitch and see if the problem persists!

Thanks - Adrian