I have a 85 z28 with the 305 tpi. I let it get to 3 quarters then cut it off and start it back up then the fan kicks on. I replaced the coolant temp sensor but didn't change anything

 

Wonder if your resetting the ecm with the key on and back off again.
Could be a bad ecm like your other post about high idle.

 

Yah this is the other reason I was thinking something is up with the computer

 

Any other ideas ?

 

Any wiring issues, fan switch inside the car ? Someone tap the relay to the ignition switch.
On most years the ecm controls the fan relay ( might be located next to the radiator( with a green/white wire.

 

His ECM flashes 12 codes fine and even stored trouble codes for stuff we know is wrong (EGR delete), and grounding the green/white wire on relay by rad kicks fans on no problem. Just FYI

We think there was a fan switch hooked up in the car but flipping it does nothing lol

 

There is a relay wired to the neutral safety switch for some reason. There is also a switch in the center console it is ran to the relay on the firewall, its spliced in between the thick orange wire running to that relay.

 

That sounds like the Orange +12V constant line for the fuel pump relay, so now the question is what was he trying to power with the switch

EDIT: might be red neck kill switch

 

Yup kill switch to keep the pump from priming

 

I looked over the fan relay it appears to be fine. The connections had a bunch of old grease on them so I cleaned them up

 

Bump, tried a new ecm and the fan is still doing the same thing

 

Have you replaced the fan relay. Maybe the contacts are sticking. Not sure what else to tell you to try.

 

Actually just picked up one today but haven't tested it yet. Will do tomorrow

 

Welp tested it with new relay and still doing the same thing of only coming on for a few seconds when I let it get to about 3 quarters and cut the car off and start it again. I'm stumped.

 

How long are you keying off for? Maybe heat soak?

RBob.

 

I just cut it off for a second then start it back up and it comes on for a few seconds then cuts back off.
I just checked the relay wiring today and all seems good and fan fuse is good too

 

I think d2 of the ecm is fan control to the relay. This should be a green and white stripe wire. I’m wondering if it’s a bad coolant temp switch triggering the relay.Did you replace the one in front of the intake next to the cold start switch ?

 

Yes that's the one I replaced. Maybe the new one was bad ?

 

I wouldn’t think the cts is bad in the same manor as the first one. Is the wire from the ecm to the relay spliced . The other possibility might be a bad prom. You might have a code 41 or 51 if that’s the case. I can’t think of any other possibilities maybe rbob can suggest something.

 

[QUOTE=Tuned Performance;6194984]I wouldn’t think the cts is bad in the same manor as the first one. Is the wire from the ecm to the relay spliced . The other possibility might be a bad prom. You might have a code 41 or 51 if that’s the case. I can’t think of any other possibilities maybe rbob can suggest something.[/QUOTE

I couldn't find any splices in it when I looked it over and only code I get is for egr. Maybe it is the prom, I need to have a new one burned anyway to delete the emissions

 

The code 32 can be programmed out of the prom. The fan on should be set to 222f in the stock programming but can be changed too.
You might consider the 870 to 165 swap instead of investing in a prom.
It sounds like your prom is working if you can retrieve codes.

 

There should be a secondary fan switch in the cylinder head between cylinders 6@8.
I don’t see how this could cause the intermittent fan operation but the switch grounds the relays green/white wire like the ecm does for fan request.
Here is the schematic for the 870 ecm. I don’t see anything else that can trigger the fan other than the ecm and head switch.

 

I thought the 85 tpi didn't have the secondary fan switch

 

It says 5.7 only so it must apply to the corvette only.

 

So its only the ecm that grounds it. I can't think of anything else it can be

 

I found the coolant temp sensor connector was a bit broken and I think it wasn't making a good connection. I messed with it a bit and it fixed my high idle issue but the fan is still doing about the same thing. I did notice when I cut it off and back on the fan stayed on a little longer than before until it cut off