Hey there folks, I got an 85. Ive looked on several sites and postings for various fan issues trying to chase this bug. My fan doesn't operate untill engine reaches op temp,BUT , it does not shut off after reaching the low temp settings. It will run until the engine is shut down. Ive replaced the coolant sensor on the intake base, and the ecm override switch between cyl 6 & 8. Still no dice. I cant figure out if its an ecm prob, wiring fault, or relay. The b4p fan does not function at all. This is the only one not replaced aside from the dash sender in the base. Its not related to this thread, but the dash cluster is fried. No info no nothing. I included that bit for those who would want to ask what the display reads. The fan draws quite a bit of power, and I know it gives the batt and alt a fit being its running at all times. Any advice or points saying "right here" would be greatly appreciated -billy

 

Hi Billy

The auxillary/booster fan operates from 240F, you can simply ground the wire that goes into the boster fan temp switch to test the booster fan works.

The cooling fan has no connection to your instrument cluster, is all done via the ECM (computer)

The ECM controls the cooling fan via the thermostat in the front of the inlet manifold base that is sending the temperature to the ECM.

See the images below of the ECM temp sender (with arrow pointing to it) the one to the left is the cold start temp switch for the 9th injector.

Have added the wiring diagram for the dual fan option, that "overide switch" as you called it is only for the booster fan. Only the single cooling fan without booster fan uses that temp switch as an overide.

Note the a/c pressure switch with the small round connector (next to the high pressure cut out that has two spade terminals) will request the ecm to run the cooling fan (on open circuit) close cooling fan request off. So if you unplug that small round plug the cooling fan will run continually (from ignition on till ignition has been turned off) is a great way rto get home if the ecm is fried and not grounding the green/white wire on the cooling fan relay.Or the relay contacts weld together, Remove the cooling fan relay when the engine is back below 200F and refit if it stays off it may be a bad relay OR the ecm that grounds the relay coil stays grounded.

You can only check that the ECM temp sender is acurate by using a scanner or tuner program like Tunerpro (your dash display is via a temp sender in the cylinder head, if the sender is not the right one or is damaged it may give the ecm a false temperature reading

Good luck

 

This model has rpo code b4p, auxiliary cooling fan. Ecm override swich (closes at 239 deg) is in right head, aux fan switch is in left head, ecm temp sensor is in the intake base just under the t-stat housing. Ecm override switch completes ground through the same wire coming from ecm via a junction. The temp sending unit for the ecm and the main fan override switch have been replaced with new components. The fans do not run until the engine reaches operating temperature, the problem is the ecm is not turning off the fan, it is still sending ground to the relay. The relay contact points aren't fused or stuck together.

 

That is not correct. If your car is connected that way then it has been modified.

The temp switch in the right head controls the auxiliary fan directly. There is no connection to the ECM. If you ground the green wire the auxiliary fan should run. The connector for this switch is the same as the knock sensor connector.

The sensor in the driver's side head is for the dash coolant temperature display. There is no connection to the ECM. This sensor has a single spade lug on it.

The water temperature sensor in the front of the intake manifold is for the ECM. The ECM turns on the fan when the coolant reaches 228°.

The temperature sensor and the auxiliary fan switch were swapped from side to side in 1986. It's possible that somebody got confused about what should be where and rewired them incorrectly. It's also possible that somebody has put in a lower temperature auxiliary fan switch and wired it into the main fan directly, bypassing the ECM. Some people do this and connect the main fan and the auxiliary fan together so they both run off the auxiliary fan switch.

 

Hmmm.... Ok. I possibly could have had the wrong information. Thx for the replies, The component ( prt# sw502) between cylinder 6 & 8 has the female mushroom connection, coming from the main harness, the component between cylinder 1 & 3 has a single male spade, I haven't been able to locate this one new. ecm sensor (prt# tx3) has been replaced. The diagram I have shows the cooling fan relay ground leaves ecm to the relay for main fan with a junction tie-in somewhere for the override switch on the ecm ground wire. The car was badly neglected poorly maintained. The stock engine block was pulled and rebuilt before I bought the car and when a piston busted I pulled it( whom that built that engine should never be allowed to touch a wrench ever again!) and rebuilt a 1 PC block, so things could have been moved by that person. I cant seem to figure out why the ecm isn't shutting off the fan, if its been modded, I cant find where. It comes on at op temp but doesn't shut off till engine is switched off.

 

This is a shot in the dark, but I had a similar problem on my 1990. I had my thermostatic switch set at 185 degrees on but had a 195 degree thermostat in it. Once the fans cam on, they never shut off. So I had to change the fans thermostatic switch or run a cooler thermostat to fix it.

 

Disconnect the two sensors in the heads and see how it works after that.

 

Here you go to end confusion of correct gauges temp sender location on 85 Corvettes.

my 85 shop manual showing the standard location for the temp gauge sender on the driver side head between cylinders 1 and 3 with the fan override switch (for single fan) or auxillary booster fan on the dual fan models. (V and N5) and D the coolant temp sender to ECM.