Weird IHKR Problem

[franksierra]

Hello all, Im new to posting but I've been reading a lot here.

I've a weird problem with my AC system since I bought the car. I've probably read all the topics here about it.
I'm going to describe the problem and all the steps I've gone through.

So, it's a 1991 316i with the analog AC controls, the compressor is active all the time.

(TLDR: I don't really have an idea on why the DME is pulling the relay to engage the compressor clutch with or without the AC signal)


The first things I checked:

• Clutch is not stuck (Engages after a couple of seconds of the engine getting started)
• Relay K19 (compressor is good)
• Fuse 16 disengages k19 and the clutch disengages

So far on the electrical...

I suspected a bad IHKR,

• None of the buttons would work at all, sometimes they all would come up, work (just the lights ie; pressing the AC switch would turn off the led, but that's about it) and then after some time stop working, compressor still engages always.
• Pulled it out, and replaced the caps (some seem to have leaked)
• No luck with that, got a new IHKR, same problem.

I discovered that if I disconnect the IHKR completely from the 3 connectors, after some time, when reconnecting it and putting the key on ACC, I would hear all the flaps cycling and all the switch's LEDs would work again, this time if I pressed the air circulation switch for example I would hear the flaps close.
Same for the rear defogger, I would hear the relay click, so my conclusion is that both IHKRs are "working".

After a couple of engine cycles, both Air circulation and AC switches would stop responding all together

Starting suspecting something between the DME and the IHKR

Pin 48 from DME goes to Pin 4 on X20 and then K19 and enables the compressor, thats working DME is pulling that relay

Pin 18 on X18156(black) from IHKR is the to be AC request that goes to Pin 6 on X20 and pin 86 on the DME
I don't have a pin puller atm, so I tested continuity between them and open to ground, all good.
I wanted to know if the IHKR was sending the AC request there is a terminal in between the IHKR and DME under te steering column, I tested it when pressing the AC switch on the control module it sends 10volts

Decided to check on the DME and discovered some corrosion on the left side of the PCB right around where pins 85, 86, 87 and 88 are. Pinout
85 A/C pressure switch input
86 A/C switch
87 Data Link Connector RxD
88 Data Link Connector TxD
WhatsApp Image 2023-10-01 at 9.45.24 PM.jpeg
Being that the data communication pins where there it made me suspicious that that might be causing the IHKR to "deactivate" after some time.
Clean them up, resoldered the header but the problem still persist

At this point, I don't really have an idea on why the DME is pulling the relay to engage the compressor clutch with or without the AC signal

If anyone has any pointers I would really appreciate them.


Some things on my mind:
Is is possible that the DME is programmed to always turn of the AC?
Does the IHKR has some kind of self test on all the components inside the heater box and that's why it disables itself?
I'm assuming the pressure switch is not important at this time because it should tell the DME what to do with the compressor after its being requested ON?
WhatsApp Image 2023-10-01 at 10.14.25 PM.jpeg

[Eric93se]

I would suggest taking another look at the DME for cracked solder joints and oxide. After reflowing with some fresh solder with flux any suspicious joints then clean both sides of the board with Deoxit 5% and an old toothbrush, spray everything liberally and scrub, repeat. Deoxit is what you want don't use basic contact cleaner.

[JumpingJacques]

following this thread coz I too have aircon problems. Mine is now modded to turn on a/c with the fan. But I'd like to bring it back to original so that I can have recirc, heat, rear defogger and select flow to face, floor or windshield