Yes after verifying that there is 12v then reconnect the 12v wire and check to make sure it goes through the switch to the other terminal.

 

The circuit is simple: headunit relay ---> freeze switch ---> low pressure switch --->pin 9 of 14-pin connector ---> compressor clutch wire.

That other switch on the top of the receiver drier is a high pressire switch that is not involved in controlling the compressor.

There are many ways you can test this. You've gotten that advice above. If you see 12V at one of the freeze switch leads, then that rules out the headunit relay. Then continue down the circuit. You might want to check that the compressor clutch is functional by applying 12V to its wire with a jump wire from the jump post.

 

Can the low pressure switch be replaced without loosing freon?

 

no, its sealed with an o ring to the drier

 

So is that where you are losing 12V? Did you measure each connector to ground with everything connected as specified above?

Dan
'91 928GT S/C 475hp/460lb.ft

 

Taken from last page - yes everything was measured as specified.

Freeze Plug - Back connection closest to the window I get 12v front connection closest to the engine 0V.

From 928's Specialists website I should be getting 12v's on both terminals which i'm not. Does this mean a bad freeze switch? If so how the heck do you remove it? There's a yellow wire on the bottom that looks to be soldered to it.

Low Pressure switch: One side I get 12v other side I get 0v

High Pressure Switch: On the side with the brown/blue wire I get 2.3v other side I get 0v

 

There are two telltales in your pictures and readings. The 7 volts at the freeze switch means the head ubit relay is probably toast. The 12V at one side of the pressure switch but zero at the other means you are low on freon, IF THE CONNECTOR WAS ON THE SWITCH WHEN YOU TESTED. Looks like it wasn't, so go after the control head relay first.

The little cannister pointing up, the device you refer to as the "high pressure switch", is a pressure transducer that helps control the cooling fans. It plays no part in the compressor operation.

The pressure switch is directly below that, and is in fact a high-pressure switch. It can be changed without losing gas, but doesn't need to be changed. If the switch is actually open, you are low on freon. Test that switch in place with the plug removed, using your ohm meter. Continuity through the switch (zero or a fraction of an ohm) says switch is closed and gas is OK.

 

That can't be right. If you are getting 0V on one side of the freeze switch, you have to be seeing 0V on both sides of the low-pressure switch. Were both freeze switch connectors plugged in to the switch when you took the voltage readings to ground for each terminal?

Dan
'91 928GT S/C 475hp/460lb.ft

 

Ok guys I went out there morning and I do have 12v on both sides of the terminals didn't realize they had to be still connected to the freeze switch. I kept understanding the test procedure as testing it at the wire terminals that go into the freeze switch. Next on to the low pressure switch

 

You won't get 12v on the other lead, you have to test the voltage ON the freeze switch, with the 12v power lead connected to it.

 

Ok, listen:

- Plug both wires into the terminals on the freeze switch
- Leave them both plugged in
- Pull back the rubber insulation on one wire and measure to ground
- DO NOT UNPLUG IT
- Repeat with the other connector
- Oh, and DO NOT UNPLUG IT!

The current flow is in-line. If you unplug anything and measure, you will break the circuit.

Dan
'91 928GT S/C 475hp/460lb.ft

 

On the low pressure switch I removed the connector and I do get 12v on one terminal but when I jumper the terminal the AC compressor does not engage. I test for power at the AC compressor power wire and I'm not getting 12v there either. If I use a test light on that same connector i'm getting 12v on, it won't even light up so I brought it to the jump post and it lights up of course.

 

Did you also check Pin 9 of the 14-pin connector? Do you get 12V there, because that's where the power goes next from the low pressure switch.

One thing to keep in mind is that these switches can pass 12V through just fine with no load, but as soon as you add load they all to almost no voltage at all. This is especially true with the freeze switch.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Dan I will check pin 9. Thanks

So is that normal for my test light not to light up?

 

Pin 9 on the harness shows 0 volts. Do you know what wire these leads back to on the freeze switch. I would like to do a continuity test to if there is a break somewhere. Thanks