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Tuesday, April 23, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Saturday, January 16, 2010

Aux Fan Runs Continuously (High Speed Relay Failure)

With the drive home completed for yet another day I stopped at the end of my driveway to pick up the mail. As I got out of the car I realized that the aux fan was running -- at high RPM, no less. Given that the outside air temperature was a balmy 26F, the coolant temperature gauge was pointing at the usual 12 o'clock position and I didn't have the air conditioning on (not out of the question in winter given that it is sometimes necessary for defogging the windshield, by the way), my instinct diagnosed the problem in an instant. One of the relays (likely the one that switches the fan to high speed) was fused closed. I'd read of this happening to other E36's and recalled that this failure mode would cause the fan to continue to run even with the ignition off (position 0), a condition decidedly unfriendly to the battery. Time to test that theory.

I pulled up to the house, turned off the key and opened the door with an odd sense of anticipation. Sure enough, the fan was still running. I got out, closed the door, and walked around to the front of the now dark vehicle with the fan eerily running with a mind of it's own. I stood there for a few moments contemplating the ghost in the machine before I set about to come up with a temporary workaround so I could sleep peacefully knowing I wouldn't come out to a dead battery the next morning.

I opened the hood and then pulled the cover off the fuse box. I began by tapping on what I believed was the low and high speed fan relays in an effort to free the contacts in case they needed a little persuasion. It had been a while since I'd messed with the low speed relay and while the fuses are labeled on the inside of the fuse box cover the relays are not so I wasn't sure I was hitting the right relays. When that didn't have any effect I decided to pull the 30A fuse for the aux fan (#41). Not surprisingly, with its supply of electrons cut off, the fan spun down to a halt. Problem solved -- for now.

While I waited for the wheat crust for my world-famous deep dish pizza recipe to rise I spent a half hour reading the schematics in the Bentley manual in an effort to further troubleshoot the problem. It didn't take me long to confirm that the aux fan receives power from the "hot at all times" bus through the high speed relay which in its depowered position delivers power to the normal speed relay. The normal speed relay contacts are normally open and break the path to ground (and thus keep the fan turned off) until the relay's coil is energized. And as it turns out, the coil of each relay is powered from the "hot in run and start" bus, so I quickly concluded that given how the fan was running in high speed with no power to either coil the high speed relay was faulty. The schematic also indicated I could have pulled the connectors associated with the refrigerant pressure switch or engine coolant temperature switch to rule those out but based on the symptoms I remained convinced that the relay was the cause.

The next morning I went out to the car and it started without any problems. I drove to the dealer to get some information from my technician and he was gracious enough to pull the wiring and fuse box layout diagrams for my specific model year from his archives and confirm the locations of the low and high speed relays. I pulled each relay and wrote down two part numbers to give to the parts department:

Normal Speed Relay (Yellow Case) - 61361389105
High Speed Relay (Purple Case) - 61361388911

While BMW is famous for supporting their cars far longer than most manufacturers, the one down side to maintaining a BMW older than 10 years or so is that the dealers tend to not stock parts for it and so it was in this case: I had to special order both relays. Cost for the high speed relay was $20 and the normal speed a mere $10 so given the critical nature of these parts, particularly in the upcoming summer months, I decided to replace both. They should be here in a few days. In the meantime, the car will run just fine with the aux fan fuse removed. All I have to do is treat this as an aux fan failure and keep the car moving if I decide to use the A/C to defog the windshield. I've never seen the aux fan run to address a coolant temperature problem in the winter so an overheat is of little concern.

Mileage: 187275, Parts: $40