Aux fan not coming on HELP

[ninetyseven1]

hey guys 97' 540i auto. Trying to diagnose aux fan not coming on. I have verified the fan indeed works by unplugging the pressure switch and jumping the brown(ground) against the other two wires. High and low speed. Big red fuse 75 behind the glove box is good also. i tried bypassing one of the resistors at the fan but the aux fan still wouldnt come on when i turned on the AC. Perhaps a bad low speed relay in the relay box under the hood? or maybe i bypassed the wrong resistor?

[jmc1590]

Funny you should ask, I've been fighting an aux fan on an X5 all weekend! Different system, but some core items may help...

Sounds like you've verified the fan itself is good. The early E39 has an aux fan relays (K21, K22 & K201) that provide power. State 1 turns on the fan when temp exceed 91 C. Stage 2 enables based on AC pressures. Stage 3 enables when temp exceed 104 C.

By bypassing the pressure switch, you triggered the relay and the fan ran in Stage 2. Tells me that relay, and the fan is ok. I'd look at the pressure switch. I'd also want to check the temp sensor to see if I could trigger stages 1 & 3.

Lots of good info at newtis. Includes wiring diagrams.

Sent from my SAMSUNG-SM-G900A using Tapatalk

[ninetyseven1]

does anyone know the color of the relays for the aux fan?? car is a 1997 540i

[jmc1590]

According to Realoem, all three are that light green color.

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[edjack]

Low speed relay (K21) is in the Ebox, at the lower corner near the fender.

Medium speed (K22) is in the back of the glovebox.

High speed (K201) is in the E-box.

[rdl]

The switch with 3 leads is the radiator outlet temperature switch. The A/C pressure switch has only two leads.

Your test has confirmed that stage 1 and stage 3 are working properly. When the HVAC panel energizes the A/C compressor clutch it simultaneously activates stage 1. Given the symptoms and your test results, it makes more sense that the HVAC panel isn't trying to turn on the auxiliary fan for some reason or there is a circuit break between the panel and connector X1022. X1022 is located in the tangle of wires above the glovebox fuse block to the left of the relays.

Does the A/C clutch engage and do you get cold air?
Perhaps the refrigerant pressure is low and A/C is inhibited. See the attached schematic.

See the attached fan staging schematics, which indicate that fan speeds are engaged as follows:
Stage 1 - A/C engaged OR rad outlet over 91C but below 104C
Stage 2 - A/C pressure elevated (indicating A/C is working hard to cool air)
Stage 3 - rad outlet over 104C

FWIW, I think the stage 2 schematic has an error. It makes more sense if 5 & 6 are reversed. I.e. energize stage 2 when pressure reaches 20 bar and turns off stage 2 (revert to stage 1) when pressure drops to 17.5 bar.

[ninetyseven1]

Low speed relay (K21) is in the Ebox, at the lower corner near the fender.

Medium speed (K22) is in the back of the glovebox.

High speed (K201) is in the E-box.

Ok I checked big red fuse under dash and it's fine , bypassed both resistors on the front of fan, swapped out the green relay in the ebox, still no go. I
Can jump
The pressure switch wires with a paper clip and only the high speed works so the fan does work, maybe u should have tried a brand new relay in the ebox?

[ninetyseven1]

Ok I checked big red fuse under dash and it's fine , bypassed both resistors on the front of fan, swapped out the green relay in the ebox, still no go. I
Can jump
The pressure switch wires with a paper clip and only the high speed works so the fan does work, maybe u should have tried a brand new relay in the ebox?

UPDATE: It was a bad aux fan low speed relay in the ebox along with a bad resistor at the fan.

[ninetyseven1]

ok, so i drove the car a couple of days using the a/c with the new relay in the ebox but i didnt yet install the new resistor near the fan, so i finally replaced the reisistor and now the relay is blown? NOW , if i bypass the bad reisistor, i can jump terminals 87 and 30 if i remove the relay and the fan then works , so is this a bad relay again?? i tried several spare relays i had that were the same specs and none of them worked either. Could be an electrical issue? im gonna buy another new relay today and try to find out. Can anyone shed some light on this?