Strange A/C Compressor cycling

[Bubba431]

I have a 1991 D21, and a friend of mine who knows all about air conditioning helped in evacuation, recharging, taking temps and pressures, and all looks totally normal. But after 7 to 10 minutes, it will cycle off for a few minutes while running at about 1500 rpm. The pressures look normal at that condition, at about 250 and 48 psi and cooling well. Even after jumpering the low/high pressure switch, it still will cycle at random. After researching the wiring, it appears that the ECU may play a part in the control of the compressor, as there is a control wire that goes to the ECU. We also saw in a service manual for the truck that there is or may be an overtemperature protector in the compressor. The truck is not here now so can't inspect to see if there is another device that the clutch wiring is wired through on the compressor. Any thoughts, anyone?.. thanks in advance.

[Bubba431]

By the way, if the clutch is jumpered direct to the battery, it will not cycle off.

[Bubba431]

Hasn't anybody had this problem? We are thinking that the ECU could be doing the cycling, as the throttle control is working, in that it raises the rpm as the compressor kicks in. I'm thinking that signal has to be coming from the ECU. Anybody ?

[longtooth]

Does it have a aux cooling fan in front of the condenser that is not coming on?

[Q451990]

I'm sorry I didn't notice this earlier. First of all to NICO!

The A/C system is controlled through the ECU. It's designed to cycle on and off under different conditions - one of them being that the evaporator coils get below a certain temperature. The goal being to keep them from freezing up... I believe thermal protection on the compressor is a one-time thing - as in it gets too hot and melts... kind of like a fuse. For the compressor clutch to engage, the thermal protection, thermo control amp, and dual pressure switch all have to be "happy."

The closest FSM (factory service manual) we have is for the 1990 Hardbody - but I suspect they're very similar if not the same. You can download it at http://www.nicoclub.com/FSM/Hardbody/ Be patient and give it a few minutes to open, it's a big file.

There's a schematic on HA-46 and a wiring diagram on HA-47 that give a lot of information on how everything works...

Is a/c not keeping the truck cool enough? Some cycling is normal... if the evaporator is freezing up I think that usually points to a low refrigerant charge - but I couldn't say that for sure...

Good luck!

Heath

[wedo43]

My 2001 Frontier 3.3L has same kind of problem, erratic compressor on. When it's on it works great but unfortunately, the hotter it gets outside the less likely the compressor is to turn on. Idea's???

[Q451990]

My 2001 Frontier 3.3L has same kind of problem, erratic compressor on. When it's on it works great but unfortunately, the hotter it gets outside the less likely the compressor is to turn on. Idea's???

There is a known issue on your series of Frontier with the thermo control amp sensor on the A/C evaporator. Apparently it fails in a way that makes ECU "see" a frozen evaporator core when it's actually fine.

I don't normally like to link to other forums, but this write-up is really good...

http://www.clubfrontier.org/forums/f46/ ... ier-72291/

Heath

[stevedacracka]

I have a 1990 d21 hard body and I just had a remanufactured AC Compressor installed then paid to have the extension valve replaced and when I tested it the air would come on for a while and then it would all of a sudden start getting warm inside the truck. At this point I was about $520 into it including parts and labor. I brought it back to my mechanic and after about an hour he decided it was one of the AC relays. There are two and they're identical. He told me to replace one and if that didn't fix it to switch out the other one with the new one and it still was doing the same thing afterward. I called and asked what the chances were that both relays were bad and he said he would just go ahead and replace the other one because that's about the cheapest fix if it is that. I called my local auto parts store and he said that the relays usually don't tend to burn out and suggested I switch out the AC cycle switch but was oblivious to where it was. Does anyone have any suggestions? It appears Bubba431 is having a similar problem. Did you ever get your problem resolved Bubba431? Any suggestions would be appreciated form anyone. Thanks.