AC Compressor only engages when Low pressure switch is jumped Topic is solved

dj_v70 · Post by **dj_v70** » 04 Jun 2021, 09:27

I just read that post. I don't think it is applicable. You could safely bypass that relay but don't think it would change your overall outcome.

You have found that you can bypass the low pressure switch to make your faulty system work. That means either pressure is too low in system, switch is faulty and providing wrong ohms. You have measured your pressure and it is not low. You have measured your switch and it is providing correct ohms. Your switch has the same ohms as the jumper that are using that does works. So, you don't appear to have a problem that you are bypassing.

You could just leave the jumper that you are using in place. The potential issue is that if your pressure becomes low, you could damage the compressor without the LP switch there to disable system.

LOB · Post by **LOB** » 04 Jun 2021, 11:53

Just to clarify. I've got 1 ohms between the pins of the low pressure switch (close to the firewall) both with the compressor running (jumping the switch harness) and with the compressor not running.

dj_v70 · Post by **dj_v70** » 04 Jun 2021, 16:37

Understood, that is a good number. If the pressure of your system gets low or your switch was broken, that number will get very high.

LOB · Post by **LOB** » 05 Jun 2021, 00:49

Ok, given that I've got pressure (lubricant) in the system, for how long can I keep the switch jumped? On a hot day, is the compressor running all the time?

So you don't think bypassing the relay on the compressor will help. Can't the AC relay be broken? What about the sensors? Thanks in advance!

dj_v70 · Post by **dj_v70** » 05 Jun 2021, 07:07

The only purpose for the low pressure switch is to disable compressor when pressure is low. Usually it is wired in line with compressor, meaning it cuts off 12v to compressor when pressure is low ( high ohms vs low ohms) So, as long as pressure isn't low, jumper and switch are doing exactly the same thing. Changing any other sensor or relay isn't going to change this. Other sensors will still cycle compressor on/off due to high pressure, temperature, etc.

The real question is why does jumper fix fault if switch is working. Either:
a) operator error
b)I don't know what I am talking about
c) pins/socket are corroded or damaged in some way such that jumper gets better connection than switch.
d) switch is bad in such a way (possible but rare) in that it gets right answer for low current (ohm meter) but wrong answer for high current (compressor)

Want to try to verify it is c or d? If so, you can do this by measuring voltage at compressor when connected normally vs with jumper and ac is running. Using jumper, you should get 12v at compressor. When connected normally, I'd expect some weird voltage at compressor between say 1v to 9v if compressor isn't engaging.

LOB · Post by **LOB** » 05 Jun 2021, 16:57

Did you mean that I should peel of insulation in order to measure voltage from the cables going in to the compressor while the compressor is running? I tried to measure the voltage on the wiring that was accessible. The reading was 0 volt on all the cables but since I had disconnected the cables from the compressor the compressor wasn't running.

LOB · Post by **LOB** » 06 Jun 2021, 02:25

Shouldn't the AC switch cut off the electric current if the pressure is to low? The resistance in the switch should go from 0 to 1 ohm when the pressure is to low. I'm reading 1 ohm (broken circuit or 100% resistance) all the time on my switch even if the pressure is high enough. I can't see any other reason than a faculty pressure switch.

Post by **abscate** » 06 Jun 2021, 04:47

It should go to infinite ohms at low pressure, not 1 ohm. You have to remove the wires to check the resistance, too.

Post by **Chuck W** » 06 Jun 2021, 07:29

LOB wrote: ↑06 Jun 2021, 02:25 Shouldn't the AC switch cut off the electric current if the pressure is to low? The resistance in the switch should go from 0 to 1 ohm when the pressure is to low. I'm reading 1 ohm (broken circuit or 100% resistance) all the time on my switch even if the pressure is high enough. I can't see any other reason than a faculty pressure switch.

Yes, it breaks the circuit when the pressure is low.

Unplug it and check the resistance across the pins of the sensor. (while the AC is running)(Jumper the plug so the compressor runs).
If it's 1 Ohm (or very low resistance) it should be "good". If it's very high, or infinite (broken circuit), it's bad.

Do you have another car handy to pull one from? We had the issue on my wife's car. The fan would only run when the car got to temp, but not when the AC was switched on. Traced it to the switch and swapped the one from my car to verify.

LOB · Post by **LOB** » 06 Jun 2021, 08:49

I think I've confused you Chuck with the term "1 ohm". My multimeter says "1" ohm when there's 100% resistance, a broken circuit and 0.00 ohm with no resistance, connecting the two multimeter probes. My pressure switch has infinite resistance between the two pins all the time, ac running or not. I will try to replace the pressure switch (will it leak refrigerant for a few seconds while I'm unscrewing it? I'd better be fast? ). Thank you Chuck and everyone else! Sorry for fooling you with my "1 ohm" statement. I thought 1 ohm ment infinite resistance as that's what my multimeter says....

AC Compressor only engages when Low pressure switch is jumped Topic is solved

Re: AC Compressor only engages when Low pressure switch is jumped