348 A/C problem

Hi Ben.

It is not normal, but it is not uncommon. My car does this as well, and yes, it is a problem with some portion of the A/C unit.

I have disassembled mine and cleaned the contacts, checked for worn components, but the issue still persists intermittently.

I have found that by "bumping" the passenger side of the unit the A/C will sometimes begin to operate properly. This can also occur if I go over a bump in the road. In addition, certain temperature ranges seem to cause this issue to crop up. It can work fine all winter, but as soon as the temps hit around 70f, the issue comes back.

In light of my observations, I'm almost certain it is a loose wire or a cracked /failed solder joint on the board somewhere in the blower motor circuit. Tearing this down and isolating the issue in on my to-do list, and I will certainly update the team if I find a root cause.

If you search this forum, you'll find others that had similar issues as well, but to my knowledge there has been no universal fix found yet.

 

I have a 1990 348 ts and it does the same thing. Often when first started the fan will come on strong and only a firm slap to the rh side of the console will it then stop. I have had the main control out and carefully cleaned and checked every pin on the boards and still have the same issue. Anybody have any success with this problem ?

 

You'll be pleased to know its not faulty but doing exactly as it should - check out the section "Air conditioning > Description > Starting the engine" in the owner's manual where it describes the behaviour where external temperature is under 20C
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That's interesting Jeff, thanks. I will add, however, that when this occurs it NEVER reaches a point where you can control the system. Perhaps it starts properly but gets into an unusual state.

Great info though.

 

IIRC you can always override the fan speed whether its in this special mode or not. If you cant do that it sounds like you do have a fault :-(

TBH The instructions for the aircon are quite confusing - Ive read them a few times now and still end up pressing random buttons! LOL

 

It is great info though, regardless, because it suggests that there is a logic issue on the controller, rather than a simple loose wire / power problem. That may help explain why it seems to only occur during certain temperatures. I can say with confidence, however, that I've had the unit engage when temps were well north of 90 F, so it is possible there are multiple issues in play.

I found a schematic at one point, and if I recall correctly, the blower has a variable speed controller attached to it. If so, I'll backprobe that one of these days and try to see if I can figure out if it is the blower itself defaulting to a high speed due to lack of ECU communication at power up, or if it is the A/C ECU sending the wrong signal. Could be as simple as a power drop when the starter engages.

Either way, I'll wager that the issue is related to:

- Cracked solder joints (board logic issues, flakiness)
- Oxidized pins on connectors (dauhterboard, or even power going to the unit being low)
- Common failure components on the PCB (capacitors, cracked traces, etc.)

Unfortunately a large portion of the A/C ECU is sealed with a protective coating, and troubleshooting anything in that is well beyond my pay grade.

If I do ever find a conclusive root cause, and by chance I am actually going to working on my A/C this weekend during a new stereo install, I will absolutely share the findings! It is definitely a common problem, so I'm hopeful it will be a single point of failure.

 

Yep, I have the exact same problem in my 1990 TS, although it was extremely intermittent, where at times it would work just fine, then it would suddenly become uncontrollable and behave as described above. Sometimes it would even flop back again and become operational again... all in the same drive.

I was eventually able to narrow it down to a loose connection at the big yellow multi-pin connector that must be unplugged from the back to remove the unit. When the connection is fine, all works as it should... but even a minor bump would move something just enough to cause a bad connection, and it would fail.

As a cheap/temp fix, I put a small plastic shim in with the connector to hold it tighter in place. After I did this it worked reliably for about two years, but started this behavior again a few weeks back. I haven't pulled the unit out again yet to see if the shim has simply fallen out.

I wish I had pictures. I'll try and take some when I pull it out to check on that shim...

But yeah... I'm absolutely convinced that this behavior is caused by a bad connection.

 

Just curious but have you checked the fuse net to the green relays to see what condition its in?

I replaced my fuse with a circuit breaker (which pops on a regular basis but that's another story) and cleaned the contacts on the relays.

348 DIY "Restoration" - Page 127

 

The connecter pins pins on the back of the control module are hilariously tiny, about 1 Angstrom in diameter and about 1mm apart (what engineer thought that was a good idea?) Too close to try to solder a wire onto each to hardwire the thing in. A replacement plug is impossible to find. My system worked fine 97% off the time, but seemed to go AWOL when most needed.
I finally scrapped the whole thing and went with a simpler set up.