I just installed a rebuilt compressor on my 87 pu. it wont turn on unless i hook a jumper wire from the wire coming off the compressor to the battery. any ideas whats causing it not to come on?

 

AC is kicked on when the pressure switch reads higher than a certain value (usually ~50psi) but lower than the max value (usually ~400psi).

You can try jumping the pressure switch. That'd show if it's truly electrical or are you just low on freon.

 

where is the pressure switch and do you jump it the same way as the compressor? what does it look like?

 

Pressure switch is on the high pressure line from the compressor (the smaller of the two). Looks like a knock sensor. If it's a two-wire switch, just unplug it and put a piece of wire between the two pins.

Up here @ the shop, a diagnosis is only $40... and I tell you how much refridgerant you have in the system vs. what is "full". Perhaps that's an option where you are?

 

Hey Noltz

Would it be this one called the "low pressure switch"?

 

well, the blue and black wire that goes to the compressor in the engine bay is not hot. but the same wire is hot under the glove compartment. and all the other wires are too. Does anybody have a good wire diagram for the a/c on an 87 pu efi? the one in the haynes manual is a piss poor diagram.

 

What year truck? Theres a downloadable FSM for the 85 p/u and runners.
should work for you with a 2nd gen pickemup.
heres the link...

http://www.ncttora.com/fsm/

 

really? is that with any refridgerent? cuz my old origional compressor starting hissing, squeaking, when it came to life with ~50 PSI, scared the stuff out of me, the system takes up to ~400 pSI?

 

yep... up to 400 psi can be expected. That is high, but not unheard of. 250-300 is more normal.

but if the compressor was just replaced, it's likely that it won't turn on until you've got a decent amount of refrigerant in the system... which isn't possible without pressurized filling- something the average can of refrigerant doesn't give enough of.
I found that after fixing a few whatswrongs with mine, I had to jumper the clutch on in order to run the compressor long enough for the compressor to draw enough refrigerant in to cause the low pressure cutoff switch to reset.
A mechanic would have the equipment to force the freon in and override the low-pressure cutoff but the garage mech won't.

 

we put two cans of freeze 12 in it, and jumped the compressor off for at least 15 minutes. there are no leaks which the gauges indicated. but the wire that turns the compressor on is not hot, so i think that is the problem.

 

IIRC, (and don't quote me) but I seem to remember I had to run the compressor long enough to draw in 2 full 12oz cans of hfc134a before it had enough pressure to run on its own.
I didn't connect one can and let it fill itself, and then another can... I had to jumper the clutch on and fill with both freon cans inverted.

 

we put in one can at a time while the compressor was jumped off. but, is that wire not supposed to be hot until the the low pressure switch cuts on?

 

you're confusing me....

if you jumped the compressor clutch, it should be 'on' meaning the compressor is running... and you're saying 'off'.
the cutoff switches are in series to that both have to be closed in order for the compressor to work. so if the pressure is too low, the low pressure switch breaks the circuit, and if the pressure is too high, the high pressure switch breaks the circut.

the compressor would be off, until you jumped it on.

get my confusion?

 

yeah i mean it was on when we hooked a jumper wire from it to the battery. im sorry about the confusion

 

so, if you got it to turn on and suck in freon by jumpering the clutch- the clutch and compressor are working.
But if it don't turn on with a can or two of refrigerant, then ...
there's a leak...
... either the low pressure or high pressure switch is stuck open
or...
... the a/c switch on the dash and associated fan and temp controls, or the a/c amplifier aren't working correctly.

 

the gauges showed there were no leaks. we ran dye through it and found no traces of it. the ac switch on the dash is kinda not working right. it wont go all the way to the cold side but it almost will. the amplifier was changed with another one it but the compressor still wont cut on. in might be the switch on the dash but, when we had the jumper wire hooked up it started cooling. but still my question is, is the blue and black wire under the hood leading to the compressor not supposed to be hot until the switch is cut on?

 

try jumping the relay, if the A/C works & the dash switch will turn it on & off then the problem is either the pressure switch, line pressure or a thermal switch on the engine - I just went through this on my '95; if the systems are the same that'll lead you to the culprit.

 

ok, i tried jumping the low pressure switch under the dash and the compressor still doesnt come on. does this mean the low pressure switch is not working or i have an electrical problem? also, we put two can of freeze 12 in the system is this enough refrigerant or do we need to add more?

 

we used two different amplifiers. the original one, and another one from the evaporator i got from a junk yard.

 

the evaporator i got from the junk yard was off a 85 pu. all the plugs on it matched up to my exsisting plugs except for one. so i used the original wiring harness with the amplifier off the 85. could the amplifiers be different since one plug was different?