I'm not getting the signal to engage the radiator fan

ventilatorEnjoyer · Post by **ventilatorEnjoyer** » 22 Sep 2025, 07:59

Hey guys, I should have asked for help a long time ago. My radiator fan just isn't kicking in. The fan relay is getting the power and when I hardwire the fan with just the positive side of the battery and leave the ground as it is, it does kick in. It also kicks in when I ground one of the two rods (I'm not sure what to call them) in the middle of the relay. The ECU seems to be grounded properly. I've tested the ECT in boiling water and the temperature gauge showed that it's overheating. The AC doesn't turn the fan on either. I'm confused and I no longer know what to do. Would you have any idea what could it be?

Post by **Clemens** » 22 Sep 2025, 23:49

The coffee hasn't soaked my brain yet, but doesn't that simply imply that the fan relay is bad?

ventilatorEnjoyer · Post by **ventilatorEnjoyer** » 23 Sep 2025, 05:30

Clemens wrote: ↑22 Sep 2025, 23:49 The coffee hasn't soaked my brain yet, but doesn't that simply imply that the fan relay is bad?

It shouldn't be the source of the problem, since I've already replaced that. I have two of those here and the result is the same

ventilatorEnjoyer · Post by **ventilatorEnjoyer** » 23 Sep 2025, 11:17

Clemens wrote: ↑22 Sep 2025, 23:49 The coffee hasn't soaked my brain yet, but doesn't that simply imply that the fan relay is bad?

Alright, I checked again and I got the ground signal but it isn't strong enough to switch the fan relay on. When I do it with a thicker wire, it does the click and the ventilator kicks in. It's the same problem with either of the relays so I guess that it's rather the ground that's the problem... Could it be?

Post by **Clemens** » 23 Sep 2025, 14:36

Sure. The wires on these cars do rot occasionally, especially the larger ones to the starter and alternator.
Simple fix could be to run an extra ground wire to a non corroded bolt.

Post by **volvolugnut** » 23 Sep 2025, 14:54

Clemens wrote: ↑23 Sep 2025, 14:36 Sure. The wires on these cars do rot occasionally, especially the larger ones to the starter and alternator.
Simple fix could be to run an extra ground wire to a non corroded bolt.

I agree. Sounds like bad wire connections or partially broken conductor wire.
volvolugnut

Post by **abscate** » 23 Sep 2025, 17:34

Those two control wires are really thin, I’ve broken a few on p80s

Post by **jreed** » 24 Sep 2025, 05:25

The pins in the connector on my fan harness failed, and that prevented a good low-resistance connection. I replaced the wires+pins.... wrote up the problem and fix here: viewtopic.php?t=101867