Aux Fan not working

[mcaden]

I recently got my a/c working on my 1987 325ic, but the aux fan hasn't been kicking on. I checked all the fuses and they were good... and getting power, so I decided to pull the 3 prong plug on the radiator. I used a jumper wire on the high and low speeds and they both operated the fan properly (so the fan works fine), so I decided to replace the switch along the driver's top side of the radiator.

Well, I just replaced the aux fan switch and bled the radiator... and the aux fan is still not kicking on with the a/c (blue snowflake illuminates and a/c blows cold). I assume I put in a good new switch (OEM from dealer), so what is left to troubleshoot... if jumping the switch operates both high/low fan.

Could I jump fan if the resistor wasn't working or can I eliminate the resistor? Other thoughts.

Thanks for your help.

-Marc

[jlevie]

If both speeds work when jumping the thermo switch wires, the fan, relays, and resistor are good. The fan not coming on at low speed when A/C is selected will then be a problem with the A/C select switch or wiring to the HVAC panel.

[mcaden]

Jim,

Thanks for your response. When I jump both sides of the switch e.g., hi and low... the fan turns on either side. What is not, however, clear to me is whether the fan is operating at different speeds or just one speed. If I can jump both sides of the switch... does that mean things are okay or is it possible to jump both sides, but only have it operating on the hi speed and NOT low speed (meaning I have problems behind the HVAC panel).

Can you let me know whether you think it is a problem behind the HVAC panel and, if so, what to look for? As fate would have it... I am doing a dash swap this weekend and will have that whole area exposed.

Many thanks.

Marc

[mcaden]

Could it be the diode under the fuse box? Also, since I can jump the fan to make it work - are there any potential work arounds (if I can't sort this out in the short term) - like wiring a manual switch to turn the fan on/off?