99 Sierra electric fan conversion problem

[Brettworth]

The 1999 and 2000 Truck OS (and some 2001) need a “patch” for the fans to work correctly. The best course of action is to update to 2002 OS 12212156. If this OS update isn’t an option for whatever reason, swapping in a Camaro system segment will often (but not always) get the fans working correctly.

How do I learn to perform this? This is a very powerful tool, and lots to learn.

[2xLS1]

With factory wiring configuration using 3 relays they never operate separately. They either both run low speed (fan 1) or they both run high speed (fan 2).

[gofastwclass]

Good point 2xLS1. I always wire the two fans separately with separate relays, but I prefer that operation.

As for a fan patch, I'm not saying it doesn't exist but I've never heard of it. I've also never had a problem getting the fans to work on these PCM's. Are you 100% sure you have them wired properly?

[Brettworth]

Yes, they are wired up correctly. Using a Tech2 I can turn them on. The Fan 1 controls are PWM for some odd reason, and fan 2 does nothing with the engine running. I can manually activate the relays with either HP Scanner or the Tech2 so obviously the connections are fine. The software isn't controlling the outputs on their own.

[gofastwclass]

You mentioned the fan controls are PWM... What are your settings under the A/C tab? Look for the Hardware section and the Recirculation Fitted option should be set to Fan #2 and Type Fitted needs to be Analog for proper operation. If the Recirculation Fitted is AC Recirc, the second fan won't energize via the temp control logic.

[KFXGUY]

The 99-02's do not have provisions to turn the fans on w/ AC. there isn't a AC pressure sensor like on later trucks. We build a fan harness for these that has a mixing circuit for AC High/Low operation. Basically low and high work just like it would from the factory and depending on the setup (a jumper wire you can cut or leave connected) turning on the AC will either bump the fans to the next higher level or turn them on in the first place.

http://www.pcmofnc.com/product/1999-...version-kit-3/

Attachment 62662

Are you sure? I have a 99 silverado and mine works flawlessly. Comes on at 185 when commanded and the ABC fan comes on when the compressor kicks on. I had to tap into the ac clutch wire and when the compressor kicks on the fan kicks on. It's simple wiring it with two relays.

Btw I'm not using different speeds as my fans are one speed.

[buktooth]

This solved my fan problem. I had fan#1 hooked up to low speed and I used the a/c clutch wire and a relay to provide ground for high speed fans . My fans were on low speed all the time, the pcm was grounding pin #42 after I started my truck . I could only shut them off with the scanner. Just as gofastwclass stated a few posts above recirculation Fitted option should be set to Fan #2 and Type Fitted needs to be Analog for proper operation. I thought I would be ok with leaving the pcm opening the recirc door when compressor pressures got high because I was using the a/c circuit, not fan #2. But it has to be set to fan #2 and analog or fan #1 wont work either . Thanks soooooo much guys!

[scubasteve81]

I'm a little late to this thread but I had the same problem with my 01 ecm. I changed under system> ac > Type fitted to (analog cycling) and now my fans run off of temp. Hope this helps someone after all this time. ( I am running 2 cheap 12v fans off of the ls swap premium fuse panel by BP Automotive) I do not currently run AC so I am not sure if it will effect that. I will update in the future when I have the AC hooked up.