fan problem

[enjenjo]

95 LT1 out of a Caddy. Howell wiring harness. No fan with the AC, or with the coolant sensor. Both relays check ok. Where does the fan relay get it's ground?

[wayne petty]

gm ecm or cadillac BCMs have control of the ground side of  electric cooling fan relays.... with under the hood PCMs.. they have control of the cooling fan relays...

is this in a cad... with duel fans... and a fan control box up behind the grill or under the top of the core support?



i have had problems with that exact circuit...  had to swap out a few cad bcms with reman units to get them to work ..


how about a little more info...

i am taking that you are working on something other than a cad..



here is a link to the wiring diagram for some of the cads..

with a pusher and a puller fan....

with 3 relays...

if the diagram is right side up..    upper left corner...

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/25/ef/41/large/0900c1528025ef41.gif



this is the cooling fan diagram for a corvette with the ls1 motor... vin P motor...

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/08/fd/ae/large/0900c1528008fdae.gif


for some reason they show 2...

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/08/fd/ad/large/0900c1528008fdad.gif

[wayne petty]

i am looking at a corvette wiring book i happen to have..

there are 2 control circuits for the radiator cans control...


PCm pin  A 11   circuit #335  dark green wire...

this pulls relay number one to ground...  sending power to one of the radiator cooling fans...    only 4 wires on the relay...


PCM  pin A 10... circuit #473   dark blue wire...  this causes relay 2 and relay 3 to activate and turn on both cooling fans..




but this is complex...   it takes all three relays to make both fans operate..

as relay #3 makes and ground...

relay number 1 makes or brakes  positive...

relay number 2 works as normal fans are wired...



the second link above is how it shows

with the wires shown going into the right side of the pcm being dark blue.. to pin A10

the single wire on the left end of the drawing is dark green and to pin A11

[Fat Cat]

The motor is in a 57 Chevy Wagon

[enjenjo]

I printed out the diagrams, it looks more like the second Corvette, but the colors are dufferent, and I only have one relay. I won't be back on the car until the end of the week, but I have something to work with now.

[wayne petty]

going farther... looks like when the AC system pressure comes up..

the pressure switch closes and grounds the dark green with white stripe wire on circuit  C238 to PCM pin B1... A/c request signal is how it is tagged...

grounding that with a pressure switch on the high side..  should cause the PCM to turn on both fans through the PCM chain of command/programming..

~~~~~~~~~~~~

the PCM wants to monitor if the compressor clutch is activated..by being hooked to the clutch side  of the relay.. this happens on PCM B21...    


the PCM also controls the clutch relay.. via pin D8... circuit 459.. the pcm pulls this to ground to activate the ac clutch relay...    

want a scan of the diagram for the AC controls for a Corvette pcm...



cooling fans can also be commanded with a scan tool... sometimes...  this is just for testing...

[enjenjo]

want a scan of the diagram for the AC controls for a Corvette pcm

That might help too.

[wayne petty]

drop me a direct email.... i have a PDF file for you ...

[enjenjo]

Email sent

[enjenjo]

Well, I tracked it down to no ground out of the computer. So I am going to get it hooked up to a reader to see what the problem is.

[enjenjo]

It's "fixed". After careful checking, the fan would work with the engine heat sensor, but not with the AC. So rather than get another computer and get it reflashed for this setup, I rewired the AC around the computer so it turns on the fan directly now. All it took was 2 ft of wire, and a diode.

[jaybee]

Simple is always best!

The computers CAN be confounding.  I have a friend who was working on an S10 Blazer that would start and immediately die.  It acted like it wasn't getting fuel so among other things he checked out the fuel pump and power to same.

Well down the road and completely out of ideas he realized that someone had rewired the fuel pump for some unknown reason.  Because the computer had been removed from the circuit the PCM thought the fuel pump wasn't running and shut down the engine.

[kb426]

This thread didn't sound like any fun. The ecm is at fault?

[enjenjo]

This thread didn't sound like any fun. The ecm is at fault?

Yes. The ECM was reprogramed to use one input from the AC to turn on the fan. For whatever reason, it failed. So rather than replace the ECM, I just wired around it.