87 300SDL oscillating cabin temperature, new climate control unit
 

I just replaced the climate control unit last night. The old one was just not working right, but when I had it on auto and ~22C, it worked OK'ish. Defrost was sketchy and the other fan control buttons were really bad. The economy button did not work, etc. So, I replaced it.

The new one behaves OK, as far as the air flow is concerned, but it seems to oscillate wildly between AC and the heater. It was ~75 last night (7 PM'ish) and when I took it for a test drive, the AC came on as expected and then the air flow switched to the defrost vents and the heat came on. Hot air started blowing, and then the AC came on again. This repeated several time. The good news is that the AC really works well and the various vent vacuum actuators are working as well (I was worried about these). With the old unit, I was not sure.

Other things that I have changed in the last year are the following: mono-valve (torn membrane), aspirator motor (sounded like a dying animal) and the blower motor (sounded like a dying animal).

Did a get a bad unit or is there something else likely broken that was masked by my bad climate control unit.

It sounds like your antiwarmcooljustrightoscillation relay needs replacing. $6000.

Ha! My heart skipped a beat. When the message came up, the "needs replacing. $6000." was in the middle of the screen and is all I saw.

Oh no! Sorry :D.

Try these 3 things in order of likely cause:

1) Make sure F7 fuse is good. Pull it and look for a hairline crack in the element. If the fuse contacts in the fuse box are tarnished use a dab of metal polish on a q-tip and clean them. No abrasives! Also make sure it's a copper and ceramic fuse.

2) Look near the brake booster, there should be a green vacuum hose coming from the cabin into the engine compartment. Follow that hose, it should end up on the vacuum splitter atop the vacuum pump. Make sure the hose is not split or cracked. New hose available from Pelican.

3) Start the car and cut or tear a 1" square of paper. Put it on the grill opening on the upper control panel adjacent to the dome light. The paper should suck to the grill and stay in place. If not the aspirator fan is bad, this draws ambient air over the cabin air temp sensor. This is the feedback for regulation of the temperature.

I saw you replaced the aspirator fan but let's do that check anyway to make sure it is drawing good across the sensor.

I cleaned fuse #7 with contact cleaner (there was a little corrossion, but I try to keep thinks clean, so it was not too bad) and inspected it. It looked OK. I could not see the green hose, probably because I am looking in the wron place (sorry I am complete novice about some of these things). I looked on the driver side, around the brake fluid reservoir. I could not make out a green vacum hose, but the vacum hoses that were there, seemed in OK shape. At some point I know I need to change them. Some are as old as the car, but look OK.

Lastly, checked the aspirator and it is working (I could hear it), but it couild hardly hold a 1" Kleenex sqare. That does not seem right.

When I changed the aspirator many months back, the hose had a kink right by the aspirator that I tweaked to open up, but who knows. What is the best way to clean this rubber hose? Should I us a vacum cleaner or something like that? I'll dig wound the forums. I know I have seen this written about.

I called the CCU re-builder and they think that there is a problem with my "hot water circulating pump". It may be drawing too much current and that I should first disconnect it and possibly replace it. Where is this thing? Is this a likely issue?

It's possible. The auxilary pump is operated by the PBU to help circulate coolant through the heat core under low flow conditions (like at idle). The 124 PBUs have an overcurrent limiter in them to prevent the board from getting fried if the pump shorts out. 123s had no protection and many PBUs got fried from a bad pump.

If you remove the windshield washer tank (suck out most of the fluid with a baster, unclip the pumps, lift straight up) you will see 2 heater hoses going down below where the tank was. There is a small pump about the size of your fist. Follow the electrical lead to a 2-pin connector and unplug it. Since it's summer you don't need it right now and it will remove the load from the controller to see if that's the problem.

Use copier or notebook paper, not tissue paper. Tissue paper is porous.

I confirmed that the aspirator is fine and there is no obstruction in the hose, Replaced with my old/bad CCU and it works better than the new one. I think I am just going to return this new one and stay with the "bad" one. I'll start restoring another area and try to find a cheaper used unit. Thanks for all the help. I think I will revisit this next year.