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Thursday, February 29, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Sunday, January 24, 2010

Replaced Aux Fan Relays

(Image: Normal and high speed aux fan relays disassembled)The parts required to fix the aux fan came in this week and finally managed to get to the dealer to pick them up. This morning just before I ran an errand I installed both the normal and high speed relays with no issues.

To test the system I reinstalled the 30A aux fan fuse and verified that the fan was not running with the key off. Then I started the engine, turned the HVAC temperature controls to full cold (60F), turned on the compressor (snowflake button) and verified that the fan was running (meaning, I had reconnected power to it successfully and the fan wasn't merely dead all the time now). I noticed the fan and compressor stayed off most of the time but that was likely because the 45F outside air temperature was below the HVAC setting. I could have pulled the coolant sensor connector and shorted the center pin to each of the other pins to prove the fan ran in both speeds but I figured that this was sufficient since I knew the fan worked -- I just needed to verify it no longer ran continuously.

As usual I conducted a post mortem on both relays (shown in the picture). I first inspected the high speed relay (purple body) and found the contacts in remarkably good shape, although they did have a bit of oxidation and one contact (used in the depowered state) showed a small amount of pitting. Overall, it looked like it was in relatively good shape so the reason for the failure is not obvious to me.

The normal speed relay was another story, however. With a mere 56K miles in service the contacts were in a condition similar to the last time I replaced it. I have always known that the normal speed relay takes most of the abuse and that explains why I specified a replacement interval for it in my maintenance schedule worksheet, but based on the condition of this relay I have decided to reduce the replacement interval from 72K to 54K to better reflect the wear pattern of this critical part. I don't like throwing money at the car even in the context of preventative maintenance but given the failure mode in this case I think it makes sense to be proactive.

Mileage: 187826