94 Z28 fan relay variance

As I previously learned, the 94 Z28 Lt1's had an earlier and later variance within the same 1994 production year for cooling fan control. The early version has two fan relays and the late has three fan relays. The early version turns on one fan (high speed) for low, then both fans (high speed) for high. The later version turns on both fans (low speed) for low, then both fans (high speed) for high.

My question is: Does the PCM have different programming functions for early and late production 94 fan control, or, is the different way the two versions operate controlled solely by the relay and wiring setup?

The differences I see is the wiring, later version uses 3 fan relays.

dave w

http://shbox.com/1/schematics_wiring.html


Attachment 16593

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Originally Posted by dave w

The differences I see is the wiring, later version uses 3 fan relays.

dave w

http://shbox.com/1/schematics_wiring.html


Attachment 16593

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I surely understand the difference in the relays and wiring. But, does the PCM programming have anything to do with it, or is it just the wiring?

I’m not too good at reading schematics. Does the three relay setup in the late production 94’s split the 12v current down to 6v for dual fan low speed, then allow 12v to both fans fir high speed? Is that how it works?

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Originally Posted by JD1964

I’m not too good at reading schematics. Does the three relay setup in the late production 94’s split the 12v current down to 6v for dual fan low speed, then allow 12v to both fans fir high speed? Is that how it works?

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on the 2 relay setup (top)
primary fan is top relay (turns on top fan)
secondary fan is bottom relay (turns on bottom fan)

the fans are supplied with 12 volts when ground is applied to their respective relays coil (the other side of the coil is wired to ignition.

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on the 3 relay setup, (bottom)

for low speed: ground cooling fan relay #1

relay 1 is the top relay. when ground is applied to that relays coil, it pulls in the normally open contact connecting d1 and d4 together.

this places power on terminal b of the left cooling fan.

terminal a of the left cooling fan is connected to j4 of relay 2

with no power on relay 2, j4 and j3 are connected to each other (normally closed contact)

j3 on relay 2 goes to b terminal on the right fan as well as f1 on relay 3. (relay 3 is not needed for the fans to work at low speed)

terminal a on the right cooling fan goes to ground.

basically this puts your fans in series with each other and connects them to power and ground, thus providing 6 volts to each fan






for high speed, fan relay 1 stays grounded, and fan relays 2 and 3 (their coils are wired in parallel) are grounded.

relay 1 still applies power to left fan terminal b (see above )
terminal a still connects to j4 on fan relay 2, but now the relay is energized and j4 and j1 are connected together, and j1 goes to ground. this places 12 volts across the left cooling fan for high speed.

with relay 2 energized terminal j3 is now disconnected j4


relay 3 is now energized and connects f4 to f1 supplying power to terminal b of the right fan which still has terminal a going to ground.

this places 12 volts across the right fan.

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Originally Posted by JD1964

I surely understand the difference in the relays and wiring. But, does the PCM programming have anything to do with it, or is it just the wiring?

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it's just the wiring. the signaling is the same so the ecm wont really know the difference

Interestingly enough the 94-97 PCM is actually set up to handle two fans running independently at either high or low speed, at least as far as the code goes. As far as I know, no car was actually wired to take advantage of this functionality.

@ grumbolt, That’s an awesome detailed reading on that schematic. Thanks for that. I understand clearly now.

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Originally Posted by NomakeWan

Interestingly enough the 94-97 PCM is actually set up to handle two fans running independently at either high or low speed, at least as far as the code goes. As far as I know, no car was actually wired to take advantage of this functionality.

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Care to elaborate, because you're not making sense? To get a low speed fan the way GM always did it requires using 2 fans connected in series to get the low speed.

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Originally Posted by NomakeWan

Interestingly enough the 94-97 PCM is actually set up to handle two fans running independently at either high or low speed, at least as far as the code goes. As far as I know, no car was actually wired to take advantage of this functionality.

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there are only two fan drivers. it would be mathematically impossible to control two devices independently, each with three possible states(off,low,high), with two switches. that'd need at least a third fan output giving 8 possible states (2^3 kinda thing) and a bunch more relays

Pardon the intrusion.
Think NomakeWan is stating that the LT1 pcms had the potential to operate one and/or two fans at low and/or high speed independently …
… yet instead chose to handle this with the wiring differences between B- & D- cars vs F-cars external to the pcm.

jthompson, dzidaV8, and kur4o (among others) have TunerPro $EE definition files with access to several potential abilities that GM never bothered to utilize.

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Originally Posted by lionelhutz

Care to elaborate, because you're not making sense? To get a low speed fan the way GM always did it requires using 2 fans connected in series to get the low speed.

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Originally Posted by steveo

there are only two fan drivers. it would be mathematically impossible to control two devices independently, each with three possible states(off,low,high), with two switches. that'd need at least a third fan output giving 8 possible states (2^3 kinda thing) and a bunch more relays

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Sorry about that, misunderstood the table layout at 264A. It's referring to vehicle speed hysteresis, not fan speed hysteresis.

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Originally Posted by JD1964

@ grumbolt, That’s an awesome detailed reading on that schematic. Thanks for that. I understand clearly now.

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im glad to help, and happy i can contribute!

im an industrial controls electrician by trade so i have a slight advantage P

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Originally Posted by grumbolt

im glad to help, and happy i can contribute!

im an industrial controls electrician by trade so i have a slight advantage P

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Cheater!!! :-)