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Discussion Starter · #1 · (Edited)
So I have this truck I got about a week ago and it has a couple "differed maintenance " issues that need addressing. The first one I decided to tackle was the AC not working thinking it would be quick and easy.
Start the truck and turn the AC to max, compressor dose not kick on. My first thought was that is was low or even empty. I put the gauges on it and to my surprise if anything it's a little high. This is with the truck off.

My next step was to start testing the circuits and checking fuses. Both of the 10amp fuses are good. And with the AC "on" I have 5v at the pressure switch.

Unfortunately I don't have leads small enough to get into the back side and test the output.
My next step was to test the compressor clutch. 12v to the lead and it clicks.
Next with the truck running, AC on, test the output on the compressor lead. Nothing. At this point I was thinking dead relay so I jumpered 12v+ to the compressor lead and the clutch closed and in a couple seconds I had ice cold air.

I removed my jumper and To my surprise it stayed on and cycled as it should, pressure rises, fan kicks on, pressure drops, etc. pressure with it running. A little low given the ambient temp near 90deg. ?


Next I turned the AC off and let the pressure equalize back out and it will still turn back on and function properly. But if I turn the truck off and let it sit more than about 15-30 minutes it won't come back on without jumpering the compressor.
So dose anyone have any ideas??
Thanks.




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Discussion Starter · #3 ·
Gottogo forgot to mention that I tried unplugging the pressure switch connector and jumpered the red to the brown and it didn't kick on. But couldn't find anything on how that circuit works. Like it shows 0volts empty and 5v is over filled so it's really looking for a reading in between? Also if the pressure switch was bad would the system still cycle correctly after I'd jumpered the compressor to start? I was thinking that after I pull my jumper to start the compressor if they system saw no pressure or too high pressure it would kick off?


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As for the switch, i think it works like this.
When the pressure is normal the switch is Closed, so voltage is sent to the compressor to run (5Volts??).
When pressure is high the switch opens, interrupting the circuit. So no voltage shuts the compressor off.
However, 5V to the switch seems wrong. I would think it should be 12V, but I could be very wrong on that (relay maybe?).

As for jumpering the compressor, I would say yes. You're providing a direct power source so I would have expected the compressor to run. It's like jumpering the starter without turning the key. It's still going to start cranking.

But to be honest, I'm no expert and only offering suggestions (CYA).

Hell, I wouldn't have had the balls to jumper the compressor. :)
 

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According to my manual for a 98, the 5volts is correct.

It comes from the thermal control circuit.
 

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Discussion Starter · #6 ·
I did some more testing today and I think I have a bad evaporator temperature sensor. The chart on Alldata shows what the resistance should be based on the temperature.

But if I test mine I get nothing.

If I put it in the freezer for about an hour and pull it out and test it I get about 30 ohms.

If I pull it out of the freezer and rush plugging it in the truck the AC will work as it should for about a minute and the the compressor clutch disengages.
Any ideas on how to prove this before I spend $60 on one of these??



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I did some more testing today and I think I have a bad evaporator temperature sensor. The chart on Alldata shows what the resistance should be based on the temperature.
But if I test mine I get nothing.

If I put it in the freezer for about an hour and pull it out and test it I get about 30 ohms.

If I pull it out of the freezer and rush plugging it in the truck the AC will work as it should for about a minute and the the compressor clutch disengages.
Any ideas on how to prove this before I spend $60 on one of these??
I think you ALREADY proved it: you have 0 ohms resistance at room temperature, chart says there should be some. And the ohms after being in the freezer don't match the chart either. And when you rush from freezer to truck the AC works until the temperature increases and the resistance decreases. So I say good job troubleshooting.

Recommendation: if you can, take your ohmmeter to the parts place, and test the resistance of the new part at room temperature before you purchase it !!!
 

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Discussion Starter · #9 ·
To confirm my test I unplugged the sensor, took a 30k ohm resistor and jumpered the terminals and the AC worked perfectly. So I could have AC this week while I wait for the new one to come I cut the old sensor apart, soldered the resistor to the old sensor connector, and covered it with shrink tube. Now on to the next problem. Spongey brakes.



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