Jfrahm

Nope, sorry. I did not take any pictures of this install. There is no real need for a wiring diagram, if you can wire up an amp or some fog lights you should be able to do this.

The relay was a relay from a fog light wiring kit, probably 30a. Standard inline fuse holder, probably a 10 amp fuse. All you need to do is use the signal to the AC clutch to energize the relay, then power the clutch off the jump post via the relay contacts.

-Joel.

86'928S MeteorGrey

My head relay is toast, so maybe I'll do a socket install replacement and the under hood relay too... Overkill?

jthwan22

I just did this job plus a relay socket. Thanks Dwayne for the write up.

Hilton

I didn't like the idea of an external relay.. so I followed this post from 2004 and put a small relay inside the AC head unit's casing.

https://rennlist.com/forums/928-foru...freashing.html

Done it to two of my S4 control heads and it worked a charm.

WallyP

On the earlier cars, using a double-pole underhood relay to drive both the compressor clutch and the aux fan will improve HVAC performance.

Cole

This thread was helpful. Maybe someone can give me a little more guidance.


1987 car.

I get 4.5 volts at the freeze switch.
0 vols at the 14 pin connector.
0 ot the pressure switch.
All fuses are good.

So is this relay my likely culprit? Or is there somewhere else to look?

This was all tested with the car in the " on" positions, AC "on" but car not running.

Thanks.

WallyP

Check both terminals at the freeze switch - terminal to ground. Should be battery voltage in each case. If you do not have battery voltage at either terminal, the relay is the usual suspect.

Cole

Thanks. I did check both. One is dead, the other gives 4.5v.

jwillman

How are we defining early cars? Any value of doing this on a 79? My HVAC has a simple on off switch for the compressor on the controlhead and seperate fan relays on the CE panel.

StratfordShark

These are odd voltages - the freezer switch shouod have same voltage at both terminals unless it's been tripped by iced evaporator.

If there is corrosion on freeze switch there is voltage drop across it which shows up as low voltage on outout under load. I had this problem and fixed it with new freeze switch. If you bypass freeze switch do you get 12V at the pressure switch with aircon engaged?

Don't ubderstand your reading of 0V at 14 pin plug. The power for compressor clutch goes from hvac relay to freeze switch to low pressure switch then to clutch. Confirm where you are losing power then it's possible to identify if the hvac relay is faioed or not.

Cole

How can I confirm where the voltage loss is when all but the one freeze lead show 0v?

I didn't try bypassing the freeze leads since they were not providing the right voltage to begin with.

I tried both sides of the 14 pin and just got 0v.

Not sure what else to look at.

WallyP

1) Ignition on, HVAC on A/C, Pull both connectors from the freeze switch and check voltage from each to ground. Report what the voltages are.

2) Hook them together and tape them (very temporary!), then check voltage at the violet/yellow wire at H15. (Plug H, top wire on left side) Report voltage.

3) Check voltage at the black wire on M12 (Plug M, 2nd wire up on the left side). Report voltage.

4) Disconnect both black wires for the low pressure switch on the receiver/dryer. Check voltage to ground on each. Report voltages. Reconnect both.

5) Find the connector on the black wire to the compressor clutch. The connector is near the oil dipstick tube on the front of the belt cover - a single black wire. Check voltage to ground on each side. Report voltages.

Cole

Did that. Those are the numbers posted above. Tried it several times with the same result.



What is this H15 and M12 you speak of?

MainePorsche

Cole,
H15 refers to the pin 15 on plug socket H. The plug sockets are all those wire looms at the bottom row of the Central Electrics panel where the fuses and the relays reside. Go to 928-electrics.com and choose the wiring primer - it will be helpful.

Cole

Thanks.

No time today, maybe tomorrow.