1990 air conditioning

[drt0800]

Air is blowing hot. Compressor not turning on. Added freon, no help. Direct wired compressor to battery, works great. Yesterday was 90 degrees. Static pressure on gauges was 120 hi and low. Start the car, no change. Direct wire the compressor and lowside drops to 40 and hi side goes to 220. Remove wire to compressor, goes back to 120. Tried to jumper low pressure switch, no help. I cleared the hvac programmer. No codes. Replaced ac clutch relay..checked fuses. I have no ecm or ccm codes. Checked for power at ac compressor..it is good. Can someone please tell me what i'm missing. Any help would be appreciated.

 

[ecss]

When AC is called for, the HVAC Programmer sends a signal
to the ECM to enable the AC Clutch relay.

When the relay closes, 12 volts is placed on the Light Green
wire of the High pressure cut off switch. This switch should be
closed when the high side pressure is below 430 psi.
The connector has two wires a Light Green wire and a Black wire.

The 12 volts passes thru the switch to the Low pressure cycling
switch (Black wire) which should be closed when low side pressure is above 47 psi.

12 volts then goes to the Dark Blue wire to the AC compressor coil which engages the clutch.

Appears you are missing the 12 volts.

 

[drt0800]

Checked power

I have 12 volts all the way to the dark blue wire that plugs into the ac compresser. I have continuity at the high pressure switch but not at low pressure switch. I checked compressor again with direct power from battery. Works perfectly. I'm pretty sure it's the low pressure switch. Numbers all over the place on ohm scale

When AC is called for, the HVAC Programmer sends a signal
to the ECM to enable the AC Clutch relay.

When the relay closes, 12 volts is placed on the Light Green
wire of the High pressure cut off switch. This switch should be
closed when the high side pressure is below 430 psi.
The connector has two wires a Light Green wire and a Black wire.

The 12 volts passes thru the switch to the Low pressure cycling
switch (Black wire) which should be closed when low side pressure is above 47 psi.

12 volts then goes to the Dark Blue wire to the AC compressor coil which engages the clutch.

Appears you are missing the 12 volts.

 

[ecss]

I have 12 volts all the way to the dark blue wire that plugs into the ac compresser. I have continuity at the high pressure switch but not at low pressure switch.

If you have 12 volts on the Dark Blue wire at the compressor,
it should engage unless the voltage is breaking down from
the load of the clutch coil.


Here's the schematic. Follow the red line which is 12 volts
that goes to the compressor clutch coil.

 

[drt0800]

Thanks for the diagram and explanation. Here's what I've done. I tried a different ac clutch relay but
didn't check all those wires with a test light. I cleared codes from hvac programmer. Started car
and turned on max ac. With a meter i checked for voltage at high pressure and was at 13 volts.
Followed to low pressure..13 volts. Followed to dark blue on compressor..13 volts. Plugged into
compressor..nothing. Disconnected, checked again 13 volts. Ran wire from battery again to compressor.
Clutch engaged and was getting 46 degrees at ac center vent. Removed wire, reconnected dark blue and black to compressor. Nothing. I did try to jumper low pressure switch by inserting paper clip into connector. Could not engage clutch. How can i have 13 volts at the ac compressor pigtail and not have power to engage clutch but can do it running direct? If the low cutoff switch was bad, wouldn't power be interrupted?

If you have 12 volts on the Dark Blue wire at the compressor,
it should engage unless the voltage is breaking down from
the load of the clutch coil.


Here's the schematic. Follow the red line which is 12 volts
that goes to the compressor clutch coil.

 

[ecss]

Not sure I understand how you're running 12 volts to the compressor clutch coil.

The compressor clutch coil has two wires that
end in a connector. You disconnect the connector and apply 12 volts to one pin and ground to the other pin and the clutch engages.

You measure 12 volts from the clutch coil relay on the Dark Blue wire with the plug disconnected from the clutch coil but the clutch does not engage when you reconnect the plug.

That would indicate the clutch coil ground wire is open or something is wrong with the connector pin for the ground wire.

 

[drt0800]

Problem solved

Thanks for the help. Turned out to be the low pressure switch. Replaced and works great.
Next question..
ran the ac checked temp, blows about 47 degrees at the center vent. Turned off engine and heard a hissing coming from the evaporator. Seemed to be where the low pressure hose comes from. I saw that there was some type of sealant around the metal pipe where it comes out. The seal is dried out and falling off. Can someone tell me what the sealant is so i can replace and hopefully eliminate the hiss noise

Not sure I understand how you're running 12 volts to the compressor clutch coil.

The compressor clutch coil has two wires that
end in a connector. You disconnect the connector and apply 12 volts to one pin and ground to the other pin and the clutch engages.

You measure 12 volts from the clutch coil relay on the Dark Blue wire with the plug disconnected from the clutch coil but the clutch does not engage when you reconnect the plug.

That would indicate the clutch coil ground wire is open or something is wrong with the connector pin for the ground wire.

 

[ecss]

These guys are in your area. Call them up and maybe they have something in a smaller quantity. You could use a $2.00 can of plumbers putty from Home Depot but the color would be Grey.

http://www.eastwood.com/ew-flexible-strip-caulk-bk-2-lbs.html

 

[vigman]

Once the system shuts down.....

And it's been cooling for a bit.... As you noted the pressure changes 120psi ( ish ) static (not running)....
vs
40 on the low side 220+ on the high side ( running )

So when you shut it down.. you hear " hissing ".. it's the freon in the pipes " equalizing " internally.. and to me it's more of a squeal.. than a hiss.