The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Electrical

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 04-10-2018, 09:57 PM   #1
GM man
Registered User
 
GM man's Avatar
 
Join Date: Aug 2011
Location: Gainesville, Texas
Posts: 127
Fans and AC don't wanna mix

This is driving me crazy. I have a Vintage Air system and I'm trying to hook up the trinary switch to my fan relays. My PCM sends a negative signal to the fan relays to turn them on at the set temperature. I've spliced into those wires to send a negative signal when the AC gets to the set head pressure but the truck doesn't like that at all. It's actually killed the engine. As it turns out, I've got 12 volts on that leg (PCM to fan relay) until the PCM signals the fans to come on and then the leg becomes a ground. The PCM isn't putting out 12v on that leg, I'm sure it's coming from the relay, but I can't figure why it will take the ground signal from the PCM but not from the trinary switch. Any ideas? Here's the fan hook up diagram, with the PCM taking the place of temp sensor:
Attached Images
 
__________________

1972 GMC Jimmy 6.0/4L65
1972 Chevrolet long bed 1/2 ton
GM man is offline   Reply With Quote
Old 04-11-2018, 12:11 AM   #2
VetteVet
Msgt USAF Ret

 
VetteVet's Avatar
 
Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,703
Re: Fans and AC don't wanna mix

Are you running the PCM control to 85 or 86 of the relays? if it's 86 and the relays are diode protected you may be getting the 12 volts from the relays through the diodes.

Just a thought.
__________________
VetteVet

metallic green 67 stepside
74 corvette convertible
1965 Harley sportster
1995 Harley wide glide

Growing old is hell, but it beats the alternative.
VetteVet is offline   Reply With Quote
Old 04-11-2018, 08:50 PM   #3
GM man
Registered User
 
GM man's Avatar
 
Join Date: Aug 2011
Location: Gainesville, Texas
Posts: 127
Re: Fans and AC don't wanna mix

Quote:
Originally Posted by VetteVet View Post
Are you running the PCM control to 85 or 86 of the relays? if it's 86 and the relays are diode protected you may be getting the 12 volts from the relays through the diodes.

Just a thought.
Well.. The PCM is on 86. I reversed 85 & 86 and had the same result. Tried a couple of other relays and the same thing. I'm getting 12v on that leg which doesn't make sense to me, but I hooked it up and it's working now. Before I soldiered it together, I was just touching the wires together to ground it and it sparked and killed the engine, which really confused me, but it seems to be working like it should now 🤯
__________________

1972 GMC Jimmy 6.0/4L65
1972 Chevrolet long bed 1/2 ton
GM man is offline   Reply With Quote
Old 04-12-2018, 11:26 AM   #4
VetteVet
Msgt USAF Ret

 
VetteVet's Avatar
 
Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,703
Re: Fans and AC don't wanna mix

Quote:
Originally Posted by GM man View Post
Well.. The PCM is on 86. I reversed 85 & 86 and had the same result. Tried a couple of other relays and the same thing. I'm getting 12v on that leg which doesn't make sense to me, but I hooked it up and it's working now. Before I soldiered it together, I was just touching the wires together to ground it and it sparked and killed the engine, which really confused me, but it seems to be working like it should now 🤯
The inner workings of the vehicle computer is a little above my pay grade so I would have to say that the polarity change you mentioned is hard to comprehend unless they use gated diodes or logic circuits. The bottom line is that it is now working for you and congratulations on that.
__________________
VetteVet

metallic green 67 stepside
74 corvette convertible
1965 Harley sportster
1995 Harley wide glide

Growing old is hell, but it beats the alternative.
VetteVet is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 11:18 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com