Hi,



The PCM circuit/internal micro relay is too weak is what my tests have revealed from what I can tell why my pusher fan will not come on.

I mean:



With my original or an eBay reprogrammed PCM, my pusher fan will not come on with either installed.



-Even though:

The engine radiator pull fan comes on from the engine temp getting up there or from turning on my AC(compressor runs, blows cold).

The PCM knows the AC is on since it controls the compressor relay from what the manual says.



When I idle the engine till the pull fan comes and then I immediately turn on the AC, still no pusher fan.

I believe the PCM is programmed to turn on the pusher fan whenever either the AC is turned on or the Engine is too too hot.





-Pusher fan Relay tested:

A fan 2 Trouble code throws if I unplug the pusher fan relay and run the car. (not sure how the PCM knows the relay is unplugged, sensitive electronics high tech design I guess).

The pusher relay has been swapped with the working Radiator identical relay, still no pusher fan.

Power is at the relay electromagnet plug terminal #85 when the ignition is on.

Main power is at that relay plug terminal #30-I can jump from that terminal with a wire right to the fan motor feed terminal #87 and the fan comes on.



-PCM wiring tested:



Ground for PCM:

While attached, tested metal case of PCM for ground presence, no ohms of resistance. Attached alligator wire to metal case and to ground, still no pusher fan turns on.



Removed both large connectors to PCM and with a needle and ohm meter, checked the 6 or so wires that the manuals says provides the PCM with ground. None showed any resistance to ground.



I've removed and wire brushed grounding wires that are on the radiator cowl and back of engine. What I could find.





PCM wiring to the Relay tested:

I've also disconnected the PCM large blue connector and found the correct terminal #31 per service manual and placed a needle pin in it and alligator clip and wire to the AC pusher fan relay ground terminal #86 and checked for resistance with an ohm meter. There is no resistance in the wiring from the relay plug end to the PCM plug end. Also tested the wire for a short to ground.



The electro magnet terminal #86 from the PCM that provides ground comes on but is too weak to close the relay magnet.

If I manually provide ground to terminal #86, the relay and fan comes on.



Everything else works just great on the car except for the Cruise Control(haven't found the problem yet with that), I don't see a correlation.



Seems any used PCM internal relay that provides ground will be too weak too. I'm guessing this is common.



-

I can't see how I could boost the ground signal from the PCM...

=



So I guess I need to manually bypass the PCM(without trigger a Trouble Code), so both fans come on at the same time.

Wouldn't you say?



-It seems I only have 1 simple option to wire the pusher fan to come on when the radiator fan comes on:



I can get another relay with plug with wiring cut short from the salvage yard.

Then I can cut both my PCM wire that goes to relay plug terminal #86 and the electromagnet pos terminal #85.

Wire them to this salvage yard relay that will serve as a dummy so not to throw a TC.



THEN wire ground directly to the original pusher fan relay terminal #86 and wire electromagnet pos terminal #85 to the Radiator motor terminal #87.

-

Can anyone think of anything better, am I missing something?



Thank you very much for your time!

You need to load test that control circuit. Resistance tests will not find most problems.
This guy shows how to do a load test.

Ok Great thank you that helped increase my knowledge!

Ok so what I've now done last night and today is:

Double checked my service manual, shows there is No connection or splice in the wire between the ends that connect the PCM C1 31 and the fan relay trigger.

Carefully I removed the relay socket terminal end for the PCM negative signal under the hood. By removing the relay then the green plastic bottom cover from the plug and using a small pic tool to release the plastic catch on top and then the metal terminal end slid right out the bottom.
Inspected it. No corrosion, multi crimp is strong and the spring metal tongue that touches the relay spade is closed and strong, even holds a small pin when inserted.

Next I disconnected the car battery and then the PCM '* 2 large connectors.
Again found the pin 31 on the blue PCM connector for the 3.1L and inserted a small pin into the hole and alligator clip to that.

Reconnected battery and then connected/fed 12vdc pos to the relay socket terminal under the hood I had removed.
Then connected a wire from the PCM 31 connector pin to my removed glove box light bulb(194?) connector and another alligator clip from the other bulb connector and touched a ground screw.
The light bulb light just fine/bright and hot and for a minute till I stopped.

This evidence and the fact that if I manually feed ground to the disconnected PCM 31 connector, with the pusher fan relay installed, it will close and run the fan....
Seems to show the problem is the PCM not providing an adequate ground signal.
To test any further, I don't have a replacement wire and special female pull to try wiring directly from the PCM to the relay with new wire and connectors and I don't think the results would change.

So... I think I'm going to the salvage yard to connect the fans together using the method I talked about, unless someone has a better idea?

your PCM should be be able to command the fans, did your car have any mix-year parts come at it in the past? and whats your vin?

This Buick hasn’t had many owners and fairly untouched:1996 Buick Century 3.1 V6

VIN 1G4AG55MXT6414148

I actually saw your vin on your other thread lol I posted a chart with factory updates available

Thank you for sharing, interesting.

Where did you find the updates online?

How are updates applied? I suppose sending the PCM into a programmer. Seems like a lot of work unless it'* for a fix that is really needed.

https://tis2web.service.gm.com/tis2web/ is made available by gm and it covers a fair range of the EFI vehicles up to a certain year quite reliably (since updates to old cars dont really happen much)

The dealer, a shop who can program or has something like a tech 2 or gm update tools(You yourself can purchase access to calibration for a vehicle for reasonable and so can shops, the right tools would be the biggest thing at that point)