Electric fan problem. Any ideas or help-please!

slocrow · April 15, 2004, 10:41:46 PM

It's my 91 Corvette with a TPI 350 (L98). I've been chasing this and am looking for some knowledge and help.
Sorry about this being so long and I hope not too confusing.
It's a two fan system.
OK. Here's what I know. Both fans work, though the passenger side, the secondary according to the service manual, was a little slow starting up and squawk's at start up and low revs. It ran (sound wise) OK at speed.
I checked both at the relays by jumpering them both at the relays. I jumped both, one at a time, using the thick red from the battery in the relay harness, too their appropriate fan run wire. This worked with the left primary fan but not with the right, secondary. To get the right (noisy) secondary to run I needed a hot 12 directly from the battery. The 12 volt wire in the relay harness was dead.
Why would this be dead assuming the wire is solid?
Earlier I pulled the plug off the temp sensor under the throttle body (it's the only temp sensor I'm aware of and none are in the block, right 6&8 or left 1&3 locations) and jumpered the plug to see if the right fan would run with the key in the run position. Nothing. I also jumpered the A&B on the ALDL which ran only the left fan. Also when engaging the A/C only the left fan will run.
This leaves me thinking that maybe it's the temp sensor though I don't know why the ECM didn't allow 12 volts to get to the right fan when I jumpered it or else the ECM itself is bad (hope not).
Any help, direction or answers out there? Thanks, Frank
Now with all this said

Bruce Dorsi · April 15, 2004, 11:11:19 PM

Quote from: "slocrow"Both fans work, though the passenger side, the secondary according to the service manual, was a little slow starting up and squawk's at start up and low revs. It ran (sound wise) OK at speed.

I checked both at the relays by jumpering them both at the relays. I jumped both, one at a time, using the thick red from the battery in the relay harness, too their appropriate fan run wire. This worked with the left primary fan but not with the right, secondary. To get the right (noisy) secondary to run I needed a hot 12 directly from the battery. The 12 volt wire in the relay harness was dead.

Why would this be dead assuming the wire is solid?

Frank, I'm not familiar with the Vette's wiring, but is it possible the fans are protected by separate fuses, and that the fuse for the "secondary" fan is blown?

The slow/noisy start-up of the non-functioning fan may indicate that too many amps are being drawn, and the fuse blew. .....This would also explain why the fan worked when you hooked it directly to battery voltage.

I hope it's something this simple!

slocrow · April 15, 2004, 11:19:59 PM

Frank, I'm not familiar with the Vette's wiring, but is it possible the fans are protected by separate fuses, and that the fuse for the "secondary" fan is blown?

The slow/noisy start-up of the non-functioning fan may indicate that too many amps are being drawn, and the fuse blew. .....This would also explain why the fan worked when you hooked it directly to battery voltage.

I hope it's something this simple![/quote]

Thanks Bruce; I can't find a fuse for either fan. I believe that's because they are both controlled by the ECM. I can over ride the ECM on the prime fan by turning on the A?C. I realize not having the circuit fused seems incorrect but I think they're not or at least I can't find any in the service manual or any reference.....

enjenjo · April 15, 2004, 11:32:02 PM

Frank, I think you will find that the relays have power all the time, and are actuated but the ECM switching the ground on.

Try disconnecting the secondary fan from the relay, and seeing if you get any output there. If the fan is shorted, it can eat up enough power that you won't be able to light a test lamp.

As I recall there is a forest of fusable links at the powr distribution box, you may have burns the one for the secondary fan.

Bruce Dorsi · April 15, 2004, 11:36:38 PM

Hopefully, someone with more experience can jump in and help you.

I am not up to speed with late-model technology, but I find it difficult to believe the fans are not fused or protected by a circuit breaker or fusible link.

Although the fans are controlled by the computer, I don't think that the high-amperage of the fans passes through the computer. ....The computer probably switches the power to the operating COIL of the relay. ....The CONTACTS of the relay then switch the power to the fan.

Do you have a wiring diagram for your 'Vette?

Does your 'Vette have multiple fuse panels, like some other late-model GM's?

slocrow · April 15, 2004, 11:41:41 PM

Quote from: "enjenjo"Frank, I think you will find that the relays have power all the time, and are actuated but the ECM switching the ground on.

Try disconnecting the secondary fan from the relay, and seeing if you get any output there. If the fan is shorted, it can eat up enough power that you won't be able to light a test lamp.

As I recall there is a forest of fusable links at the powr distribution box, you may have burns the one for the secondary fan.

You're correct Frank. The ECM only controls the ground path for the relays.
By disconnecting #2 from the relay do you mean pull the harness plug from the relay?
What do you mean, "power dis box"? Behind the battery?

slocrow · April 15, 2004, 11:48:00 PM

Quote from: "Bruce Dorsi"Hopefully, someone with more experience can jump in and help you.

I am not up to speed with late-model technology, but I find it difficult to believe the fans are not fused or protected by a circuit breaker or fusible link.

Although the fans are controlled by the computer, I don't think that the high-amperage of the fans passes through the computer. ....The computer probably switches the power to the operating COIL of the relay. ....The CONTACTS of the relay then switch the power to the fan.

Do you have a wiring diagram for your 'Vette?

Does your 'Vette have multiple fuse panels, like some other late-model GM's?

I've only got the electric cooling fan control circuit that's in the service manual.
Yes, multiple fuse panels but no slot for the fan circuit. Like Frank stated it's more then likely a fuseable link for protection. Maybe with the apparent fan drag I've gone through one but everything else works yet it probably is it's own.......

Bruce Dorsi · April 15, 2004, 11:49:33 PM

Quote from: "enjenjo"Frank, I think you will find that the relays have power all the time, and are actuated but the ECM switching the ground on.

You type faster than me, Enjenjo!

I have to believe you are correct about the ECM completing the GROUND circuit, not the POWER side of relays. ....You also THINK faster than me!

Would the fan relay be switching the POWER lead for the fan, or the GROUND for the fan?

slocrow · April 16, 2004, 12:01:36 AM

Quote from: "Bruce Dorsi"
Quote from: "enjenjo"Frank, I think you will find that the relays have power all the time, and are actuated but the ECM switching the ground on.

You type faster than me, Enjenjo!

I have to believe you are correct about the ECM completing the GROUND circuit, not the POWER side of relays. ....You also THINK faster than me!

Would the fan relay be switching the POWER lead for the fan, or the GROUND for the fan?

What do you mean by "switching the power lead for the fan"?
Each fan has it's own relay and if I understand correctly that the ECM only grounds the relay then the dead hot wire in the aux fan relay is just that, daed. Why it's dead is what Frank is suggesting re the fuseable link or Bruce's fuse suggestion. Hummmmmm....

enjenjo · April 16, 2004, 12:18:38 AM

Somewhere near the battery will be a box about 4 by 6 with a lot of red wires connected to it. the cover comes off, some with clips, some with screws, there will be power terminals in it, with fueable links connected to it. More than likely one of them is bad.

on the relays, there should be two hot terminals for each one, one for fan power, and the other for the relay coil.