AC troubleshooting: check my thinking please

Elkins45

Elkins45

Joined
May 7, 2018
Messages
2,677
Location
Northern KY
1998 Chevy K1500, 5.7L Vortec. I went to start it this week for the first time in 2-3 weeks and the AC doesn't work at all. The push button switch on the dash lights up but checking under the hood the compressor clutch isn't engaged. I changed the 10A AC fuse and swapped the relay with another identical one that happened to be the starter relay. The truck still started but no AC, so my first assumption is that this proves the AC relay is still good, since it wouldn't have started if it were bad. OK so far? My second assumption is that the starter relay should have fixed the AC if it were a relay problem. Right?

Next I disconnected the wires to the compressor and tested them with a voltmeter. With the AC switched on it had 0V to the compressor clutch. Based on this my conclusion is that the problem is likely the AC switch in the dash or a wiring issue somewhere between. What else might I be missing?
 
Looks like you are on the right track, provided the system is charged fully. If you had a hose or seal blowout in those 2-3 weeks and all the refrigerant leaked out, you would not be getting power to the clutch. You can try bypassing (jumping) the switch to see if that part of it is faulty.
 
When I owned 95 suburban, the Low Pressure switch connected to the dryer is the weak point.
Most of the time when AC is acting, that switch was bad.
Unless, the compressor is bad.
It can be replaced without evacuating the system.

Worth checking out if it is still the same as in 98 model year.
I am not sure since the 98 uses a different compressor.
 
Last edited:
Failed low pressure switch, or actual low pressure (leak).

Low pressure in the system will prevent the compressor from getting power. It's an interlock/check to prevent the system running when out of freon.
 
Elkins45 said:
1998 Chevy K1500, 5.7L Vortec. I went to start it this week for the first time in 2-3 weeks and the AC doesn't work at all. The push button switch on the dash lights up but checking under the hood the compressor clutch isn't engaged. I changed the 10A AC fuse and swapped the relay with another identical one that happened to be the starter relay. The truck still started but no AC, so my first assumption is that this proves the AC relay is still good, since it wouldn't have started if it were bad. OK so far? My second assumption is that the starter relay should have fixed the AC if it were a relay problem. Right?

Next I disconnected the wires to the compressor and tested them with a voltmeter. With the AC switched on it had 0V to the compressor clutch. Based on this my conclusion is that the problem is likely the AC switch in the dash or a wiring issue somewhere between. What else might I be missing?
Click to expand...

you're missing that the system might be preventing clutch engagement purposely.
 
Look at the bottom front of the condenser. If it's is original, and somewhat coroded it's likley where all the refrigerant leaked out. And of course the lowpressure limit switch would then prevent it from working.

For a 1998 Chevy in N. KY I would expect it's about time for condenser replacement. You'll also need some other new parts. Orifice and screen, ( dryer and accumulator ) if it has one. Also, sometimes condensers on old Chevy's rub near the top somewhere and wear through.
 
Update: I jumpered the clutch cycle switch and the clutch engaged but there was no cooling. I jumpered the low pressure cutoff switch and the clutch didn’t engage.

Can you measure the pressure from the low pressure side when the compressor isn’t turning? I have one of those gauges on a can of R134a but I don’t know if it will give a reading if the system isn’t running. If there’s no gas then messing around with electrical stuff is just wasting time.
 
Elkins45 said:
Update: I jumpered the clutch cycle switch and the clutch engaged but there was no cooling. I jumpered the low pressure cutoff switch and the clutch didn’t engage.

Can you measure the pressure from the low pressure side when the compressor isn’t turning? I have one of those gauges on a can of R134a but I don’t know if it will give a reading if the system isn’t running. If there’s no gas then messing around with electrical stuff is just wasting time.
Click to expand...
Yes, if the compressor is not running, you can still get a reading on the low (suction) side. However, the pressure will be equalized so the high pressure (discharge) side will be nearly identical. Based on my (fading) memory, the static ambient pressure will be in the ~70 psi range, but don't hold me to it. If the pressure is way below that (e.g., <20 psi), you probably have a leak.
 
Well, I put the gauge on it and it didn’t even register. Fortunately the windows still roll down. I don’t see myself paying to replace whatever part leaked out the refrigerant on a vehicle that only gets driven 1-2K miles per year.
 
Elkins45 said:
Well, I put the gauge on it and it didn’t even register. Fortunately the windows still roll down. I don’t see myself paying to replace whatever part leaked out the refrigerant on a vehicle that only gets driven 1-2K miles per year.
Click to expand...
If it has the multiple piece compressor those always leak where the seams are. You can buy a better 1 piece body compressor that's not as prone to leaks.
 
Chris142 said:
Unplug the low pressure switch on the accumulator and jump The wires . Does the clutch engage? If it does is the low side getting cold?
Click to expand...
If that’s the one in the picture below I jumped it and the clutch didn’t engage.

IMG_7764.webp
 
If the system pressures are reading flat on the gauges then it leaked out.

R134a on a cold vehicle should have a static system pressure of roughly the temperature in F* so of it's 70* outside you should have roughly 70 psi, 90* should have close go 90psi. A pressure temperature chart will have the exact correlation but it's a close enough estimate for working with vehicles.

You need to find the leak though. The ideal thing is to charge the system with nitrogen but DON'T RUN IT WITH NITROGEN IN IT the refrigerant carries the oil around whereas nitrogen does not. It would be like running your engine without oil. With a nitrogen charge of 100psi maybe a little more, just spray down everything with soapy water and find your leak.

Your next best option is grab a pair of uv glasses and it's light and look for the dye that marks the leak. If no luck then charge it up, drive around the block and look again until you find it.

Third option which is not ideal but works is to charge the system with air and spray with soapy water. Same as nitrogen, don't run it and not ideal as you're just ramming and compressing moisture into the system which is not good for a number of reasons but it is an option.

If you look now you may even just see dust collecting around the leak from the oil that leaked with the refrigerant.

It's a good ideal to replace your accumulator/dryer when you find and fix the leak but it's not compulsory
 
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