1227727 Cooling fans won't run

[Anti Venom]

I installed a Megasquirt in my 91 vette. It controls fuel and spark but I was hoping my original ECM would continue to control the fans. The original ECM still has coolant temp input. Does anyone know what the ECM is "missing" that keeps it from controlling the fans? I don't believe the AC is turning on either, but I'm not yet concerned with that. I just tried turning on the AC to see if the fans would come on.. Even if it was limp home mode I would think the fans would still be controlled. TIA

[Six_Shooter]

Why not use the MS to control the fans?

Anyway, The ECM MAY need to see a DRP signal (engine RPM) in order to trigger the fans. There may be some reference grounds that may also be needed. This is such an odd set-up that, I think you'll kinda be on your own. Using an ECM to simply control fans seems overkill.

[Anti Venom]

Well, the original ecm also send signals to my electronic gauge cluster. I was also hoping the fans would come on when I turned on my AC. Yes it's possible with MS but I'm gonna run out of inputs and outputs. The electronic Speedo works, things like that. The computer is functioning and I believe it is talking to the CCM. Just no fan control.

[Six_Shooter]

The only signal that should be sent to the cluster from a '7727 should be vehicle speed which could be done with va DRAC or SGI-5. I believe there also some economy parameters sent to the DIC, but with the MS controlling the engine, those values will be meaningless. You can expand inputs and outputs with some daughter boards that are available though jbperf. He has a couple that work on CAN BUS.

[RobertISaar]

the 91 vette has some weird stuff going on...

the CCM communicates fuel level and vehicle speed to the cluster over a data link, those are the only digital gauges in the cluster IIRC, so that makes sense. everything else is run via sensors.

going off of other code I play with, the engine speed signal is critical to fan operation.

[Anti Venom]

Thanks for the info guys. The vette does have some weird communications between the CCM and the ECM and not many people understand it and therefore info is hard to come by. I think I even read once that the CCM is the master. 84Elky broke down some code for the fans on the 727/730's before but I wasn't able to interpret it enough to see what all was needed for the "on command" for the fans. Probably needs to see an ignition event like was suggested. Not sure what I will do about that yet..

[lionelhutz]

I would put money on it requiring the ignition signal.

[Anti Venom]

Hopefully I can just split the signal between the two computers. I haven't looked that close at it yet, but I'll check on it soon. Thanks...

[RobertISaar]

the ICM just creates a 0-5V(or maybe 0-12V) square wave for engine speed signal, right? that can definitely be fed to two modules.

[JP86SS]

Pretty sure you need "Running" indication which would be the DRP.
DRP is a square wave.
Here is a variable one I built for my test bench but you could just make it constant RPM.
http://www.thirdgen.org/forums/diy-prom/397901-bench-calcs.html

The speed sensor input was most of the conversation was about because it needed a sine wave for magnetic sensor,
Square wave for the optical type but you don't need that.
Jp

[Anti Venom]

I haven't had time to work on the car lately. I do believe the DRP for me is a 0-5v square wave. I hope I can just split it. I just wanted you guys to know I appreciate the continued support. I haven't gone anywhere, just haven't been in the shop.