A/C not working... narrowed it down.

[Spittybug]

A/C stopped blowing cold the other day so I'm in the midst of solving it.

Blower fans work on all speeds
Compressor clutch will engage if supplied 12v directly
Both high and low pressure switches show continuity across them (closed switch)
Compressor does not engage when commanded by mode switch
All fuses checked and good
No voltage on compressor feed wire
Pressure on both high and low side = ~ 70 psi (equalized since compressor is not running)


Looking at schematic and mini schematic on N:10:13, and of course they are materially different. I can't find a high pressure switch on the big one, but I assume it to be labeled either #7 or #8 on the mini. The other is the low pressure switch. If so, then I think since I found them to be continuous, I can move "upstream" which gets me to the mode switch.

Before I pull out the panel and check, I thought I'd see if anyone had any other suggestion. I see a couple of small capacitors between this circuit and ground along the way. Does anyone know where these are physically located? In addition, Dave M. also had me put another one (diode and capacitor if I recall) right at the compressor to eliminate some minor battery light flicker (it worked). If I find no voltage on the KO wire from the mode switch I've found my problem. If I do....then the wiring tracing fun begins.

Based on the above, I think I can safely rule out anything else, but I'm double checking! Thanks.

[Farrar]

Have you checked the diodes in the circuit? Maybe one of them has lost continuity.

[DMCMW Dave]

Power to the mode switch from Fuse 10 on the LG/W wire. Power out of the mode switch to the low and high pressure switches (in series) via the Pink/Orange wire. The diodes feed other circuits (radiator fans) but cannot disrupt this circuit.

The high pressure switch was missing on early cars.

[Farrar]

The diodes feed other circuits (radiator fans) but cannot disrupt this circuit.

Oops! My memory is unreliable. Sorry!

[David T]

Jumper the low pressure switch with a clip lead or a paper clip and try again. If that doesn't get you going my guess is the mode switch is not wired up correctly. You have power to it or the blower fan wouldn't work. It is also possible the wire from the low pressure switch to the compressor clutch is not continuous.
David Teitelbaum

[Ron]

$.02

Forget the capacitors.
If the blower fan works at all, you have power to the Mode Switch. Then, if there is no power coming to the first pressure switch, the K/O wire is bad or it's time for a Mode Switch (Wiggle it!).

[Spittybug]

Thanks guys..... I knew I was on the right track and you've confirmed it.

I'm going to do as David suggests and jump the switches just in case. If that doesn't work, I'll test the wires to those switches since they come from the mode switch. Anyone know the "order" of the switches in series between the mode switch and the compressor?

[DMCMW Dave]

Thanks guys..... I knew I was on the right track and you've confirmed it.

I'm going to do as David suggests and jump the switches just in case. If that doesn't work, I'll test the wires to those switches since they come from the mode switch. Anyone know the "order" of the switches in series between the mode switch and the compressor?

Not sure, but you should be able to follow the wiring, the two switches are very close together in mid-run cars, in late cars the high pressure switch is on the driver side of the condenser (it is missing on the very early cars). Look for a couple of 2-wire connectors with pink wires coming through the grommet near the heater hoses.

[Ron]

Look for a couple of 2-wire connectors with pink wires coming through the grommet near the heater hoses.
http://dmctalk.org/showthread.php?3928-A-C-Compressor-not-Engaging&p=56006&viewfull=1#post56006

Don't waste time and paperclips, just poke an ice-pick tester in the 4 terminals! ;)

[Spittybug]

While mine is an early VIN, I have the high pressure switch up front by the driver's side of the radiator. The low is of course by the accumulator. Since they are shown to be in series with one another between the switch and the compressor, I was just curious to know the relative order just in case I found myself having to check wire runs for continuity. Shorting their sockets for testing purposes is a 30 sec proposition. I really want to rule out as much as I can before digging into the mode switch however. Thanks.

[Bitsyncmaster]

While mine is an early VIN, I have the high pressure switch up front by the driver's side of the radiator. The low is of course by the accumulator. Since they are shown to be in series with one another between the switch and the compressor, I was just curious to know the relative order just in case I found myself having to check wire runs for continuity. Shorting their sockets for testing purposes is a 30 sec proposition. I really want to rule out as much as I can before digging into the mode switch however. Thanks.

It would not matter which switch is wired first or which wire goes to which switch pin.

[Spittybug]

It does if my issue lies in the run of wire itself Dave..... Since it runs in inaccessible places I simply wanted to know the physical pathing so that I could test known segments for continuity.

[Spittybug]

Hummm... checked and have volts at both switches, yet when shorted, still no compressor volts. I think I smell a bad connection in the bulkhead box.......



****EDIT****** Yup, sure enough.. Half of my bulkhead connections were done away with when I got rid of my rear wiring harness and all the K-Jet stuff. The few remaining ones are female bullets that slide over the male bullets in the bulkhead. The one to the a/c compressor needed tightening a bit. Must of happened when I was cleaning up the wiring to my alarm and engine bay lighting. Duhh, check the obvious first Owen!!!!!

[Bitsyncmaster]

I would think the wires would run over the tank for the high pressure switch and then return through the opening where the evaporator lines run. I think that is how the wires run but my car has the high pressure switch near the accumulator.

[David T]

Jumper the low pressure switch with a clip lead or a paper clip and try again. If that doesn't get you going my guess is the mode switch is not wired up correctly. You have power to it or the blower fan wouldn't work. It is also possible the wire from the low pressure switch to the compressor clutch is not continuous.
David Teitelbaum

Looks like I nailed it. A simple enough circuit, just a lot of connections.
David Teitelbaum

[Spittybug]

I like the easy ones... just wish I was smart enough to always check the simple things first. Glad I didn't dive right into the mode switch!

[Ron]

I like the easy ones... just wish I was smart enough to always check the simple things first. Glad I didn't dive right into the mode switch!

Ya still wasted the paper clips....