Today I was going to AZ for more parts for my dd, noticed the coolant temp climbing while I was sitting at a light. It got to 230, and I didn't hear the fan. It was up to 248 by the time I pulled itno the parking lot. At that point, I heard the boost fan in front of the radiator, but the one behind was not. I eventually made it home, and did some tests. All fuses still good. Grounded A and B on ALDL, both fans come on. This means the relay is still good, right?

One strange thing happened that morning, I turned the key to the on position, and both fans come on, and CEL is flashing code 12. What the heck, did I leave the code reader in the ALDL? No, what's going on? Eventually it went out of diagnostic mode and I started it up and everything is fine, apart from the fan issue later. So I think these two problems are related. It only used to enter diagnostic mode like that intermitantly, and from what I could tell, A and B were not grounded at the connector. So there's a wiring issue? The wiring under the dash is very messy, wires cut up everywhere for an aftermarket alarm system. But if there was a short somewhere in that mess, wouldn't it be blowing fuses? It does blow the courtesy light fuse when the module for the AC and heater controls is plugged up, but that is disconected right now. So I guess I only have 2 options. Get a new wiring harness for the interior, and fix both those issues, or get a stand-alone unit to run the fans. I really need some help here I am very confused.

 

Do You have a FSM for your 86 Vette ?

It will have troubleshooting flow charts for the main electric cooling fan system.

Code 12 means everything is A- OK.

You can always take a picture & show us how bad the under dash wiring is.

Brian

 

I misread my book, it says Jump A to B on the fan relay connector, not on the ALDL connector. I need to read things more carefully! I will do this test tomorrow. BTW, I have also grounded the connector for the coolant temp switch, but that didn't make the fans come on.

 

I don't have a FSM but I have a POS Haynes that Im using. I know what the code 12 is, I was only saying that it's entering diagnostic mode for no reason.

 

Likely a bad main electric fan relay.

You are close to solving the problem.

Good luck tomorrow.

Brian

 

POS Haynes manual is better than nothing.......

I have used them on non- Corvette vehicles & even Mercede Benz's that some doctor's owned & talked me into repairing for them.

POS Haynes service manual saved the day for me more than once.

BR

 

Brian, the Haynes is better than nothing, you are right. My dad was replacing the head gaskets on his truck a few weeks ago, he had everything back together and couldnt get it to start. The plug wire diagram was wrong in the manual, he put it back the way it was supposed to be and it started right up. I do have to say though, it's helped me quite alot on this issue. I am close to figuring this out, and my bet is on the relay as well.

 

I have found printing mistakes in my GM 1987 Corvette FSM too.

Even old Pontiac GM service manuals printed back in 1963.

1-5-4-2-6-3-7-8.

That's the firing order for Big block 460 ci Ford & 302 ford except for the H.O. 302 & Ford Cleveland.

351 Winsdor had a different firing order.
If i think hard enough for a few minutes longer I will remember it too.
Same as Ford 302 H.O. that was Holley carburated in 1985.

Brian

 

A lot of people think Code 12 means "All OK". That is NOT correct. The correct meaning of Code 12 is "No reference pulses from ignition module". You get this because the engine is not running when you do the "short terminals A & B on the ALDL connector" thing. Pin A is connected to ground, so if pin B somehow gets shorted to ground you'll go into diagnostic mode.

 

If that was happening ( unless engine over 238F temp of Aux fan switch ) ;you have non stock wiring so FSM may be of little use.
Stock , the boost fan as you call it has no connection to the ECM or the main fan and is independently controlled by a standalone temp switch to a seperate relay.
Does not run with main fan and does not run in diagnostic mode

 

Today I jumped A to E on relay connector, nothing. Took the relay apart, I can close the switch and the fan comes on. There is no wire going to the B terminal on the connector for the fan relay. I even tried jumping the wire to the fan from the battery with the multimeter, nothing. I am so confused right now. I think my best ption is to get an aftermarket stand-alone kit for the main fan until I get the wiring sorted out. Right now, what can I do to get the fan to come on so I can go to the store and get something like that?

 

I got a similiar problem with my 85 fan, comes on cold then turns off when hot,
read the FSM, tried a new fan relay, same situation, but with the relay out the fan goes off when its cold, the FSM says if it does that it could be the sender from head or the wire attached to it (possibly bad earth etc)
I bought another sender and wire but havent tried it yet (top of engine apart)
Could it be your sender unit in the head or the wire attached to it ?

 

Blackozvet I have tested that by grounding that wire and the fan still did not come on.

 

Im thinking at this point I should do with a stand-alone kit. Any ideas fo that?

 

Not to be smart assed...but did you try a new relay? If you close the relay manually and the fan works...well? The only thing besides the relay itself would be the wire between there and the switch that completes the ground. Relays fail all the time. Its a $12 gamble with very high odds in your favor.....

The fan system is very straight forward....a switch to a relay where power is supplied to the fan. There is interface with the a/c thru a pressure switch in the a/c system.
Remember that everything on a Vette is managed by grounds...not the power supply.

 

I know it's probably the relay, but the wiring on this car is so screwed that I'll just buy a stand-alone kit for it until I get it all sorted out. I'll see if i can get all new harnesses from painless or get them from a parts car.

 

Did something wrong.
Manually closing the relay puts power A - E

You have a problem
How does the ECM operate the relay then?

 

Vetteoz, thats what I'm so confused about. It was working just fine the other day. It doesn't even make sense for the main fan to come on in diagnostic mode when there's no wire going to it from the ecm. The PO really screwed this car.

 

I bought an aftermarket fan switch kit, it works fine, but I think I have a seperate problem now. When going down the highway at 60mph, the temp will not go below 190. When I got home and let it idle in the drive, the temp went up to 240, both fans were running, and it only went down to 237 and no lower. Coolant was boiling in the radiator and into the overflow tank. I pulled all the plugs to do a compression test to check for a blown head gasket, all were over 150 psi. The #5 and 2 plugs had yellowish corrosion built up on them which look alot like it had blown a head gasket.#7 had a bit of oil on it. I wish I could upload some pics of them, but I can't right now. I really have no idea what to do now.

 

No one has any advice?? Can a headgasket be blown even though the compression is fine?