OK.

I notices last year that my main cooling fan has not been coming on with the ECMs control (specs I put into the chip.). I did not have any cooling problems at all with the car so it has not been a real pressing issue, and I do have a toggle switch that I can put the main fan on with when I deem necessary ( in the pits between rounds etc).

However, I would like to get this sorted out and figure out why it is not coming on.

I have not had a chance to look at the wiring diagrams and know that the ECM from past memory grounds out the fans at their designated settings.

I also understand that the AC plays a role in both temp and high pressure with switching on the main fan. Having that in mind, I am wondering if because I removed the AC last year, part of the cirtuit needed might not be present, hence my no main fan situation?

I am wondering if one of the more electrical inclined people could run this down and see if the no AC is my reason for the problems.

To complicate matters even moreso, I have done the 165 to 730 ECM swap - which could also be the reason why its not working. Is there an easy way to check out these circuits to see if the ECM is sending the correct signal at the right time?

Thanks for the help...I hate this electrical stuff.

 

Jesse,
What years run the 730 ECM?? If I have a 730 in my 92' I should be able to run down the circuit in my CSM.
Denver.

 

90/91

I have what pin it is on the ECM. I would imagine I could monitor that pins line vs ground and when the ECM sends the signal to ground that line, my volt meter should go to zero resistance correct?

If that works then I could check to see if the relay is working off that signal, if that is working then it might be the line to the fan...or am I missing something?

 

correct. the circuit should stay open when the fan is off.

if you have a switch that already turns the fan on/off successfully, it would tell me that the relay, relay wiring, and the line going from the relay to the fan is good.

the A/C switch and pressure can trigger the fan control independent of coolant temp, but it only turns one of the fans on (i can't remember which). even with an A/C malfunction or even A/C delete, the fans should come on at high temps so i do not think that is the problem.

on my 92 when you put the ECM into test mode via the ALDL A&B pins, both fans turn on - i would think even the 730 ECM has that function. this would make it easier to check to see if the fan circuit inside of the ECM is at fault or if your harness pinout is messed up.

good luck.

 

I also understand that the AC plays a role in both temp and high pressure with switching on the main fan. Having that in mind, I am wondering if because I removed the AC last year, part of the cirtuit needed might not be present, hence my no main fan situation?

I have a 90 and the ecm provides power to the A/C coolant fan switch (high pressure AC switch opens at 190 psi) which is normally connected to ground. When pressure drops too low (OR you remove the switch and ac like you have done), this ground is gone. From the simple diagram in the electrical manual deduction indicates to me that one input is now at a high voltage (logic 1) instead of ground (logic 0) inside the ecm, preventing an internal gate from driving the output transistor for fan enable. For my car I would track down which wire (#603 in my case) connects from the ecm (connection B21 in my case) to the A/C coolant fan switch and ground B21 input of the ecm. Now the ecm thinks there is >190 psi. Interesting to me that both the main and aux fans use this A/C switch according to my book! SOME ONE ELSE DOUBLE CHECK MY INFO PLEASE.

 

I would start by checking the fan relay (use a "short" between the +12v constant red wire and the black/red fan pwr wire), if it runs the fan then the relay is shot or not getting a 12v signal from the ECM. that should narrow things down a bit. If the fan doesn't run, then just run a test wire straight from the battery to the fan in question (with fan grounded) and make sure the puppy spins. if it does, then you probably have a short in the +12v constant feed.

I just found out my relay was bad and the car nearly boiled over!



EDIT the A/C lead just provides another +12v switched source to turn on the fans regardless of whether the ECM has switched on (they both activate the relay)

 

Yes, Jesse your on target.

This is true, unless the manual switch was wired in a manor that bypassed the relay. In that case the relay could be bad yet the fan would still work on the manual switch.

 

so when you turn on the A/C and it builds enough pressure it turns both fans on? yet another L98/LT1 difference i overlooked?

 

I can mail you the fan relay wiring diagrams if you do not have them already. What ECM are you running?

The diagram for a '89 states that the A/C switch opens at 190 psi and closes at 240 psi. If you ground pin d11 of ecm connect C1, that will tell the computer a/c is requesting fan and should turn it on... then you could see by running the engine, if the ecm commands main fan on. It appears that the aux. fan is only controlled by the coolant fan temp switch and not the ecm. That is not how my 7727 ecm is.

 

Guys thanks for all the help. I was reading the posts, just did not have a lot of time to respond.

I worked on the fans finally last night.

Jumpered the aldl - and just the small fan came on.

Checked the relay out, the MAIN RED wire had no power, must have blown the fusable link, where ever that is. So I tied in a new 12v power source with a 20amp fuse. Instantly blew the fuse when I would hit the key, but only with the switch hooked up that I use when I want to manually turn the fans on. I have that tied into the black and red wire before the main fan.

I could not figure out what the hell was going on for about 1 hour. I knew it had to be shorting out, but why? Both ways worked fine by themselves.

Finally I pulled the earth ground off the switch (that is used to light the small LED to let you know its on.) Then both ways worked together with no problems.

My question is: Why would that earth ground on the switch mess up the way I was doing this? I am leaning towards a faulty switch and was going to get another and try it. ????

Worst case I can go without the LED - not really necessary. But moreover I would like to understand why it shorts out when that is hooked up if its working ok.??

Did I mention I hate electrical work?

 

Yep, sounds like the LED is shorting out. You would think a guy could get a decent switch I'm gonna wire in a manual switch, best looking one I've found so far is a push-pull type, still looking though. I would like to find a weather proof one, need to stop at NAPA.


PS. Jesse did you get my e-mail sent 3-1-05?
Denver.

 

I have the manual fan switch from MidAmerica on my 86 and both sides are wired into the green/white wire. Only three wires on this switch with one to each green/white for main and aux fan and the other to ground. The assembly also had a diode so that one side didn't backfeed the other, if you chose only to turn one fan on. Putting switch on black/red is switching some pretty hefty amperage, while the green/white is lower. Hope this helps.

 

Yeah with the green and white your just completing the ground side of the setup. Only problem with it that way is the key needs to be on. Which in the pits I want to keep as much stuff off as possible, meaning I had to send the power to the fan.

I think I have a faulty switch which is causing the bulk of the problems.

Going to go out tonight to get it finalized and will try a new switch, if it still blows the fuse, I will leave the earth ground on the switch off for the LED>

Thanks

 

hey Jesse i think the problem is that your car is making so much power that's its blowing up everything that comes close to it , thinking it the competition. its a bad machine

 

Watch it and let the engine get to 228º. If it kicks in, it's ok. If not, or if the secondary one kicks in, but never the primary, could be a faulty ECM driver. These drive directly from their relays, and they are run by the ECM. But there's more. They get power via separate fusible links. And it could be that when you removed the a/c "too much" was removed. There is a refrigerant pressure switch that lived on a freon line back by and below the coolant tank. That may ned some kind of "dummy" connector to work. Not sure of that part.

I've just been chasing my primary fan today. But it gets power, it has a bad bearing. It just refuses to be removed from the fan shroud. No clearance behind, so I'm stumped on that one.

GeorgeC
92 Red/Red LT-1

 

Jesse,
I've been through this with my 90.The fusible links for fans, along with several others, are located at power distribution junction down below/behind battery. If you find one that looks like a snake that just ate a rat, thats likely the one. If you cant locate any fusible links at an auto parts store you can replace it with an inline 20 amp fuse. Next problem is, why did it blow in the first place? So try replacing it and see if it holds. If not it's time to check the relays/ fans ect..

I wrote a tech tip on bench testing relays that may be helpful to you.

Here it is.

https://www.corvetteforum.com/techti...=282&TopicID=2

Good luck