mkhargrove

A new problem with my '84. I recently purchased the car, and I'm pretty certain that most of the vacuum flaps aren't functional...I'll deal with that later, as I don't think it's related to the problem that's my priority now.

My AC compressor has been engaging as expected until a few days ago. Now it won't engage until well after the car is warmed up. I thought maybe it was a low pressure issue so I added a bit of refrigerant and the pressure seems to be in range. I've read multiple threads of AC issues that indicate problems with the relay in the controller, but I'm not sure that mine aligns with those, as most of those seem to be unable to get the compressor to engage at any time. Would a faulty freeze switch potentially cause my problem? I'm not sure what to check next...

SeanR

It could be a faulty freeze switch, easy way to test that is to disconnect it and put a jumper between the wires and run the car again and see if the issues continue. What pressures are you seeing?

mkhargrove

Thanks Sean! BTW, I'm the new guy that you met at the breakfast a couple weeks ago.
I appreciate your help...I'll give that a try and hopefully not have to proceed further down the checklist

Christopher Zach

Could also be the climate relay going out. Before it goes foom I'd recommend putting a booster relay by the freeze switch and feeding the switch 12v through the boost relay with +12v coming from the overtemp fan switch (with a nice 3-5a fuse in line of course since the overtemp fan runs at around 20-25a fuse). Feed the relay with the line that used to go to the freeze switch and ground to your nearest favorite screw.

Easier than replacing that relay with an outboard one.

mkhargrove

I've been trying to make heads or tails of the different threads regarding replacement of the relay inside the climate control unit (i.e. replacing the "undersized" stock relay with one from Radio Shack...I assume they're still functional enough to order the relay online...maybe not?). So, the "booster" relay...does that rely on juice from the relay in the climate control unit to close the switch to get a higher amperage from the overtemp fan switch to the compressor clutch? (my apologies...I'm not sure how that's supposed to work... if I'm already having trouble with the relay in the climate control unit, wouldn't the booster relay be unable to do it's intended job?)

FredR

It seems you understand the concept although I am confused as to what you mean by "the overtemp fan switch"- maybe it is a feature of your model year I am not aware of. There should be a feature to cut off the a/c if the coolant temperature rises too much but I would think that logic takes place outboard of the relay but that is probably not relevant to the discussion.

The stock relay built into the control unit is felt by many to be marginal at best and when it ages the current [circa 3 amps] flowing through it that operates the compressor clutch solenoid causes the relay to start misbehaving. I experienced this on the GTS unit, threw in a spare control unit from my late S4 and the problem disappeared. The theory is that a separate more beefy relay should be used to operate the a/c solenoid taking a fused [5 amp?] power stream from the hot post and using the original feed to the solenoid to trigger the newly installed relay that should only need a small current to trigger the new power supply.

Whether it works long term or indeed whether it works at all has to be a "try and test" type of approach. Remember the problem driver is a relay that is failing thus the original item may fail completely any time. The upside is a suitable relay and an in-line fuse.can be had for minimal bucks and probably about one hour to install the mods. The best approach is to fit a new relay in the head unit but it is quite an involved process and only for those with reasonable soldering skills working with printed circuit boards or so I would think.

Alan

The feed to the compressor goes from head unit relay to freeze switch to refrigerant pressure switch to compressor.

If you aren't sure what's at fault start dissecting the circuit. Discover which of the connections of the freeze switch feed to the refrigerant switch (DMM). Unplug the other one, then connect a fused feeder from the battery (5A) to that connection - run the car and see if the same thing happens - if so monitor the freeze switch other connection and the compressor - if the freeze switch drops out you will know, if the refrigerant switch drops out you will know...

If everything now works normally it points to the head unit

Alan

mkhargrove

Hey guys, I appreciate your help. I have a 10 amp 8 pin dpdt 12v relay, and I'm going to just start with the mod to replace the 3 amp relay from the stock climate control unit.. I have read a lot of threads talking about the amperage draw of the compressor clutch coil, and I think it's worth the effort to replace it now, as I know it's likely to fail.

One more question....I don't know what the external and cabin temperature sensors are supposed to accomplish. It sounds to me like they perhaps control "something" that can bring in warmer air or colder air, using an algorithm which considers the setting of the temperature slider on the control unit, as well as the internal and external temperatures? Do the temp sensors impact a properly functioning heater control valve? Do they impact impact the position of the fresh air flap? And (the real question), do the temperature sensors have any ability to determine whether or not the compressor will be getting power when the temperature slider is not on max?

soontobered84

MKH,
Before you replace that relay, might I suggest that you find out the cause of your issue before introducing another variable into the equation?

You might transition from a late starting compressor to no compressor at all. That would not be good in the oppressive heat wave that we now find ourselves at this point.

Just a thought....

dr bob

Always a good idea to upgrade the relay. Note that the load on the relay is DC and inductive, so the contact current rating needs to consider the "kick" from the clutch coil when it opens. The original relay is rated for ~2.5 to 3 Amps AC. Your replacement needs to be rated for DC, and the contact spacing will be a little bigger.

I recommend that you spend some time with the workshop manual on this. The temperature control "loop" has two thermisters with opposite coefficients, one in the dash with a fan, the other mounted in the alternator cooling duct under the left headlight. The middle of that resistor bridge is compared with the resistance at the temp slider in the head unit. There's a servo motor that operates the a blend door in the air box by your right tow when driving, moving as needed to try and keep the temp in the cabin the same as what you have selected at the temp slider. It mixes air that's come through the heater with air that's passed through the evaporator. The little servo has additional contacts that operate the fresh air flap over the CE panel, and also the heater control valve. Both via solenoids for the vacuum actuators. Failure modes with missing or failing thermisters is full-hot unless the temp slider is at the full-cold position. There's a microswitch at the left side end-of-travel that bypasses the thermsiter string and allows the servo to go to full-cold position. In effect the system becomes binary, either full-hot or full-cold (really "full not hot". You still operate the doors for floor and/or dash.

mkhargrove

I finally had time to test a bit...with key in run position, engine off, 8.4ish volts in/out of freeze switch (slight resistance in switch). I unplugged the controller and wire brushed the plug connections a bit, pulled the controller apart and wiped off the contacts, (I noticed that the max cold microswitch was a bit loose and i really had to push temp setting to get it to click even intermittently....I pulled the switch into more proper position and put a dab of epoxy in to hold it and that seems to work). I threw it back in and tested again at freeze switch with key in run position, engine off...9.5ish in and out, (small difference between in and out as expected). I started the car...12+ volts in and out. And, the compressor kicked in immediately. I didn't start the car before cleaning the contacts, so I don't know what voltage at freeze switch was with the engine running. I did notice that the plugs going to the fan switch and to the circuit board both seem to have a little wobble in them and broke the circuit when I was moving the controller. I'm starting to wonder if maybe the problem was just a bad contact...regardless, I can't reproduce the problem now to I'll head on to the next one...

soontobered84

Good job MKHargrove.