Recently refurbished my console: Reinstated orginal shifter (removing the ***** the PO thought was cool), added SQR46 head unit, sorted PO's dodgy stereo wiring, glued down curling vinyl edges. After reinstalling my AC button wasn't working. Quick fix, hadn't pushed the connector on properly.

However now I don't seem to have cold air. AC light is on but doesn't seem to be cooling. I have previously sorted all the actuators, vacuum hoses, had the system regassed, and replaced the transistors in the flap motor. Was working perfectly and passed all the tests in the workshop manual.

So a bit of googling/rennlist thread reading and it appears the HVAC relay is the most likely culprit. Plenty of threads and good guidance out there.

I have tested voltage at the freeze switch and I'm getting 5.6 volts on the dual wire, and no voltage on the single wire. This doesn't vary with AC button on or off.

As I understand it, mainly from Dwayne's write up, I should be seeing 12v on the single wire with AC on, and I should see no voltage on single wire with AC off.

Can someone please confirm I am correct? And are there any other tests I should run before condemning the relay and starting the replacement process.

Also not sure whether the 5.6v on the dual wire is another problem...

 

I just went thru this with my GT. Tested the dual wire voltage per Dwayne’s write up with accessory on, AC on and had 0 V. Pulled HVAC and replaced relay, retested and still had 0 V. Pulled HVAC out and found broken/burnt trace on PCB. Fixed with jumper wire and retested at double wire at freeze switch, 12.8V. Started car and AC back to being cold. If you have only 5.6V at freeze switch, check make sure the rear connector to the HVAC unit is fully plugged in. Front connector is for blower, rear connector on pcb for AC power to freeze switch, compressor, etc.

 

Thanks re-read Dwayne's procedure after your reply and see I misunderstood, need 12v in the dual which is incoming. Not sure what you mean by front and rear connector on HVAC, I have 2 connectors both to the side of the main HVAC controller. The unit with the AC button has one connector to the rear and wires to a plug, also on the rear. Regardless all seem to be tight. It's as much as I can check tonight, after work tomorrow I will pull it out and clean the connector contacts just in case.

 

Sorry, yes connectors on the side, one toward the front ( blower connector) and edge card connector on rear side. I cleaned contacts as well before I put controller back together.

 

Thanks. I decided to pull the HVAC controller out before work and open it up. Having seen pictures of ones with melted relays I wondering if that might be quick way of checking. Discovered that it has clearly been rebuilt at some point. Not a massive surprise on my car, the flap motor is a replacement from 928 international. It looks like many things behind the dash have been touched before. I have a lot of receipts from the history of the car but this one is not in the pile. Anyway here's what it looks like. Is there a way to test the relay on the board?

 

Not brilliant work obviously. Looks destined for problems.

Based on your measurements there seems to be a problem with that relay - However the voltage should be the same on both pins of the freeze switch (it is a normally closed switch - opens only when it detects freezing).

Since you say it did not vary with AC switch on/off - measure the resistance between Pins 5 & 6 on the HVAC head. These should be open when unpowered but they are probably welded together with a high resistance

Alan

 

Thanks Alan, yes looks haphazard. 'Here could be problems' I thought when I opened it up. That said it's been working perfectly up to date.

Do you mean the pins circled? If so it's giving me a 1, overloaded reading. I guess that's what you were expecting? From up close I can't see anywhere the two are fused or joined, just seems to be some solder flow onto the contact points that looks messy. If not those ones please can you advise which you meant?

On the underside I did find one of the black wires disconnected, looks like this might just be not powering a bulb in the blower ****? Possible I dislodged this opening the case as I don't recall the illumination having a fault.

 

Those are the right connections. 'OL' would imply over limit = infinite resistance (if you are on ohms range) - so would imply the contacts are in fact open. Your meter may do different - compare it with the reading when not connected to anything? It may be that the relay is sticky and sometimes opens...or is functional for switching but with high resistance. You can engage the relay with 12v between Pin 10 (+) and Pin 4 (Gnd) with bottom slider all the way left. Then measure Pin5 to Pin 6 resistance again.

Alan

 

My meter reads 1 when not connected to anything.

Updated: Have applied power (I have a spare battery and test wires with alligator clips), +ve to pin 10 and -ve to pin 4, bottom slider on 0, and get 1 reading again on pins 5 & 6. Out of curiosity, I get zero voltage at the alligator clips when they are connected to these pins. Unconnected 12.8v so power is getting there.

 

Meter is as I expected switch terminal are Open Circuit.. Sorry I was looking at the later GTS head unit - for '90 Apply the 12v to Pin 7 (+) and Pin 4 (Gnd) to turn the relay on. (Pin 10 doesn't do anything useful on a '90 for bench test). Whatever voltage you apply you should see across those pins? if yes - and the relay clicks measure resistance between 5 to 6. the relay must click...

Alan

 

Very definite click when power applied, I guess that says the relay is working? Still a reading of 1 on pins 5 & 6. Also FYI, back of the circuit board says 14 Dec 1984. Can't assume it's the same pcb model as the car given it's been replaced.

 

OK so in this case the relay coil still moves the armature inside but the switch connection is not made. There is a second terminal set - between pins 8 & 9 you can test those if you like - though the conclusion is still that the main AC terminals don't work any more.

Alan

 

Between pins 8 & 9 getting reading of 1.5 on the 200 setting.

 

OK that is a good additional test. 1.5 ohms is quite high for this - but probably still works for that low current usage contact set. The relay is still toast! But this mainly validates your measurement method to confirm the initial conclusion.

So you need a new relay (with better installation). If the relay footprint doesn't work gluing the relay down and wiring from the pins is possible to do well - but this certainly wasn't that. In the other thread I mentioned other ways you can do this - its more work - but for a more reliable outcome. You will need to decide what you want to do to fix this. Whatever you do - you need a higher rated relay to avoid this issue recurring.

Alan

 

Have your unit properly repaired by sending it to 928 International, a Rennlist sponsor. They stock the units; http://www.928intl.com/