LS PCM not turning on the fans

[shep]

I am working on a LS swap and I cannot get the cooling fans to work. The PCM is sending a ground signal to the relay but the relay is not activating. When I measure voltage at the relay I get 12v on both sides of the coil. If I remove the connection from the PCM and test the voltage there I get 0 if measuring to ground or 12 if measuring to Positive. If I ground the relay terminal that the PCM connects to the fans come on. The fans are using a two relay setup and both both relays work when the proper terminal is grounded just not when the PCM is signaling for them to work. I did not do the initial wiring on this I just get to try to fix it. Thanks for in advance for any help with this.

[andrewb70]

I moved this here thinking you might get more play, since it sounds like a electrical issues and not really a LSX issue...

Andrew

[ccmc]

It may be that you need three relays or just that it's not hot enough. The stock on temp is about 225 or so

[EFI69Cam]

does your relay have a diode across the coil? If so and you don't have the polarity right you are just shorting through the diode.

[shep]

I let the temp get all the way to 240 then grounded the relay directly to the battery and both fans came on until it cooled down enough the PCM shut one fan off within a min or so. No diode on the relay, grounding the relay pin the PCM is connected to turns on the fan. CCMC can you tell me why or how using 3 relays would work when the PCM wont turn on one?
Thanks for moving this Andrew.

[EFI69Cam]

Do you have access to a tech 2 or VCM Scanner? With that you can force the ECM turn the fans on? What vehicle is the PCM set up for? The fan type must be set to "Two fans" to get proper fan control. Has your PCM been programmed at all?

The three relay setup mentioned is the stock GM setup the uses three relays to switch the fans from a series wired slow speed to a parallel wired high speed setting. This should not be used with aftermarket fans like the Spal since they are not rated to run at 6.5 volts in the series arrangement.

[H2Ogbodies]

Yep...another reason why aftermarket fans suck IMO.

And no pun intended.

[ccmc]

Check at ls1 dot com for pcm fan wiring and verify how you are connecting the fans

[shep]

OK Got some more info. I was told the PCM was originally from an escalade, I don't know what year. The fans are aftermarket units they came with the radiator, that is all I know about them. The fans are wired with 2 relays,
driver side fan is connected to pin 87 on one relay, battery power is on pin 30 and pin 86, pin 85 is connected to the PCM blue pin 42 I think.
Passenger side fan is wired the same way except pin 85 is connected to the AC pressure switch and Red 43 I think.
I have not verified the pins on the PCM.
I have access to a scanner not sure if it is one of those mentioned or not.
The PCM was reprogrammed not sure by who or exactly what was done. Trying to get this info just have to find the right person to talk to.

[H2Ogbodies]

A couple things-first I would suggest using a scanner to view the tables programmed for what temp the PCM triggers a ground signal. Even though GM wired the Escalade fans in series, I'm not sure why it would matter from the PCM standpoint other than possibly a low voltage ground signal....seems to me your issue may lie with the way the relays are trying to run the fans and jot the PCM-its merely sending out a ground signal-however if its a 6v signal, that could be problematic-have your verified that? How are your fans grounded?

[terrydmorgan]

Since the fans come on when you manually ground pin 85 of the relays, you know the fans/relays work if a ground is applied to that pin. Thus, it sounds like the PCM isn't grounding pin 85 on the drivers or the passenger side fans. Have you tried turning on the AC to see if the passenger side fan comes on? At least you'll be able to verify that the AC pressure switch is wired properly, and that will allow you to run one fan. You could wire the drivers side fan pin 85 through a manual toggle switch to ground at least temporarily to allow you to operate the drivers side fan.

In the mean time, disconnect the PCM lines to pin 85 and put a scope on them to see if they are in fact providing any ground when the temperature is high enough. If you don't have a scope, connect a tester between +battery, such as at pin 30, and the blue pin of the PCM (disconnected from the relay). If it lights up, PCM is giving you a ground on that line. On the other hand, if no light, then no ground and that's your problem and you have to figure out why PCM isn't providing a ground. The problem may be the temp sender or the PCM itself. Do you have temp sensor connected to PCM? If so, verify that the temp sender is working properly. For example, are you using PCM to drive temp gauge? If so, and the temp gauge works properly, then the temp sender is working and your problem is likely in the PCM and may need reprogramming.

[terrydmorgan]

Shep,
Schematic I saw says high speed fan output is red 33 (not 43). Double check your connection.

[EFI69Cam]

You need to verify what year your PCM came out of. The full size GM trucks did not get electric fans until 2005. If the PCM is out of a truck and has blue and red connectors( 12200411) and has not been programmed for electric fans, it will not work. Most conversions use the 411 since they have IAC drivers for DBC throttles.
If your engine is DBC and either a t56 or 4L60e I'd recommend getting the ECM flashed with a 2002 Camaro base cal.

[shep]

OK got more info. The PCM is Escalade because the trans is 4l80 and the engine is LS1 with BDW. PCM was programmed by Street and Performance, Using a test light the PCM is not signaling the fans. Reading through the street and performance documentation a specific temp sensor is called for, We have ordered it and hopefully that will fix it. The AC is tied into drive side relay and does operate the fan. Verified the ECM connections green connector is pin 33 and Blue connector is pin 42, I had not pulled the connections off the PCM so didn't know the one connection was green not red.

[terrydmorgan]

So, are you saying that the PCM is programmed to turn on the fans, but it's not doing it because it's not properly sensing the temp due to an incorrect sensor?

[shep]

That is the current thought. We have the new sensor just haven't got it installed yet. I read that unplugging the sensor will trigger the fans anybody know if that is true or not?

[terrydmorgan]

Shep
Did you get this worked out? I've got a similar situation.
Terry

[shep]

Unfortunately no the sensor that the instructions call for is three pin, and the harness is set up with a two pin plug. The guy that originally sent off the PCM is calling street and performance to find out what is going on, or what the next step is.

[12sec ss]

I know this thread is old, but depending what tune is in the PCM the fans could be turned off. Trucks that didn't have electric fans will need them to be turned on in the PCM, and temperatures set. Temperatures will be based off of the 2pin sensor in the front of drivers side head.

[H2Ogbodies]

Id just go stand alone for the fan(s).