the flyin' scotsman

Following Dwaynes excellent write up on testing the HVAC compressor relay theres a test involving the 14 pin connector voltages.

Its stated that the female side of pin 9 should have a potential of batt. voltage but I'm unclear if thats with the prior tests being completed and the disconnections remade (freeze switch, low pressure switch and compressor wiring).

My car shows all symptoms of the the HVAC head relay being bad until I got to this test and dont see batt voltage on female pin 9.......its small milli voltage but I reconnected the compressor.

A good test would be disconnect the compressor and with the load off of the relay, retest?

The compressor engages with batt voltage applied to male pin 9

Yes the key is on, ac button pressed and sliders in correct position.

the flyin' scotsman

ok update.....with the compressor disconnected still no batt voltage on female pin 9

My car is an '88.....theres batt voltage on pin 10

edit: wsm wiring diagram confirms pin 9

the flyin' scotsman

"Power passes to the compressor through pin #9 in the connector. The female side is bringing voltage from the Low Pressure switch while the male side of the connector carries voltage directly to the compressor.".....Dwaynes epic words

This statement confirms the Low Pressure switch has to be reconnected.........thats what I did and presto theres my batt. voltage on female pin 9 of the 14 pin connector.

All tests complete and have confirmed the relay is bad.

Hope this helps the next tshooter.....thx Dwayne again

Dwayne

Hello Malcolm,
Just to confirm....the test at the 14-pin connector is with all the other tests run and everything reconnected (after you've tested/eliminated the possibility of faulty freeze switch or low pressure switch). I would start at the freeze switch and see if you are getting battery voltage into the switch following the procedure in the write-up and let us know what the results are. If you have already run the test, what were the results?

the flyin' scotsman

thx so much for replying Dwayne.....see my last post....I figured it out that of course the test of the LPS is for continuity so it has to be reconnected and in circuit for the batt voltage to appear at pin 9.

I followed your testing sequence and had proven the freeze switch to be good.

thx again....I'll be following your complete write up to replace the relay with a Radio shack unit.

Dwayne

...we were writing at the same time - just saw your last post....so you are getting battery voltage at female pin 9 but when you connect the compressor, the clutch doe not engage - correct? If so, that was the same symptoms mine had - the A/C relay failed and couldn't carry the amps required to engage the clutch.

the flyin' scotsman

correct Dwayne............my tshoot test results are exactly the same as yours.

Stereo typing

the flyin' scotsman

Wanted to tie this one up.

Symptoms are no ac compressor clutch engagment with ignition on, ac button depressed, heat slider set to max cold, vent slider set to anywhere but off and the fan on.

Following Dwaynes tshooting I confirmed no 12v to compressor, compressor clutch engages with direct 12v feed, freeze switch tested good for continuity and confirmed 12v input, ac low pressure switch tested good for continuity and no difference to system with connector bridged, 14 pin connector tested good for 12v on female pin 9 and compressor clutch enagaged with direst 12v feed to male pin 9.

Conclusion: HVAC head relay has failed. Will follow Dwaynes/Dr Bobs methods of removal and replacement with more robust Radio shack relay.

Lesson learned: reconnect components (freeze switch, low pressure switch) after testing and found good before moving down the line.