Good friend of mine picked up his Lime Rock Green Convertible this week, he noticed on the drive home the A/C didn't appear to be working so we took a look at it tonight. We first verified that the compressor clutch was not engaging. We did a quick fuse check in the under hood fuse block, fuse 17 and 35 were good. We then swapped relay number 57 kr2 headlamp/ washer pump relay with the A/C clutch relay number 58 kr29 because they are the same relay and the compressor engaged and the A/C started cooling. We decided to shut the car off and try it again to make sure it would engage and it would not. I then wiggled the compressor clutch relay and the clutch engaged again, wiggling and pushing down with slight pressure on the compressor clutch relay will get the compressor to engage but it will not stay on consistently. We examined the pins in the box where the 4 relay pins go in and it looks like there is good engagement into the female terminals, but we feel like there is an issue in this electrical box possibly down where the terminals are connected to the wires. We do not know which of the 4 terminals (30, 86, 87,85) have the issue. Car is going back to the dealer tomorrow. It's looking like this car may need an entire underhood fuse box unless they try to repair it. I will post up more info as it becomes available. I want to thank Theta for his tireless efforts helping me locate diagrams etc. All Data is finally starting to get some diagrams.

 

Update; the dealer agreed with our diagnosis that the underhood fuse/relay box had a faulty connection below the A/C relay. It took several days for them to acquire the part. This cars A/C is fixed..

 

Glad to hear that they got it fixed!

 

Glad to hear the dealership took care of it. The UBEC (underhood bussed electrical center) is what I refer to as complex low tech. It performs a very important role of providing power distribution along with fuse protection and I can see where a connection problem could occur.

Last year while troubleshooting an intermittent issue with my 2006 GMC Sierra diesel powered pickup where the engine always started perfectly but intermittently wouldn't shut off with the key I got a good inside view of this under hood fuse center. At some point a mouse had set up house keeping in the upper section where the fuses and relays live and designated one area as his bathroom. The mouse urine seeped into the sandwich in middle where the staked ends of the sockets for the fuses and relays makes contact with bare copper bus wiring that takes care of power distribution. Corrosion created a conductive path between the constant and switched 12 volt bus which had sufficiently low resistance in humid weather to cause the ECM to remain active even with the ignition off. The UBEC itself was cheap and easy to replace but I spent a good 10 hours with the service manual and a meter running down the problem.

The attached photos show how the connecting wires fit into slots in the terminals and this is probably where the poor connection occurred. With high humidity the resistance between the two buses should have been near infinity but dropped to 268 ohms which allowed enough voltage leakage to keep the ECM in active mode. Since that experience I have multiple layers of mouse defense in place.

 

Eww.... Now there's just one of those 'too weird to be true unless you see it' stories.

I can only imagine your face when you started seeing the ohms dropping toward continuity.

 

Probably the weirdest part was the first time I got in the pickup and the check engine light was on before I even put the key in the ignition. It started fine but when I got to the post office it kept running, I had to pull the ECM fuse to kill it when I got home. The only DTC it ever set was loss of communications with the TCM, with the Allison automatic there is a separate controller for the transmission and that controller went to sleep as it should when the key was off. The DTC wasn't very helpful in the troubleshooting process and my first set of factory manuals went awol somewhere in Missouri after tracking showed them bouncing back and forth between two postal sorting centers, Helms sent a new set via FedEx and the post office later sent a terse message that the remains of the package were at a postal reclamation center in Atlanta proving mice and the USPS can both be quite destructive..

The inside of the UBEC looks like a cross between the wiring from a 1920s era radio and something made in a developing country.

 

Thank you, he came by today and was chilling in the A/C.

 

Great find! Some of this stuff is very challenging.
Thanks for the great pictures too.
I heard tonight another C7 went into this same dealer with no A/C, I am going to see if if I can find out what the failure was. I hope this not a pattern starting.

 

I was going to say... it appeared to be modern tech combined with a rudimentary/3rd world connectivity solution. Quite odd just having bare copper like that...

 

Animals can cause lots of problems. We have repaired this truck 3 times because of chewed up wiring, the last time the culprit was still there between the rear of the engine and the firewall lol.

 

One of my favorites from the MINI community.... Guy lost boost and had a lugging engine on a turbo car... this was in his intake: Chock full 'o' nuts - damn squirrels!

 

While I have heard of using crushed nut shells as part of the intake valve cleaning procedure I believe walnut, not oak, shells are used and I don't think this is the recommended procedure. Next time the MINI driver needs quit pinching pennies and hire an ASE certified squirrel instead of this obvious trade school dropout.

 

that's a good one.

 

That made my day.

I don't want to meet the sandblaster that could fire off grains this size at something. Whatever you'd need to take off with that much firepower doesn't need taking off.