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Old 03-19-2018, 01:51 PM   #1
jrpontiac
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PCM is not grounding fans properly

I posted this in the electrical forum as well.

I am working of the relay setup for my engine fans on my LS conversion. I got a dual fan setup with each fan triggered by a relay that is activated by the engine PCM. Each fan has it's own relay. The PCM activates the fan relay by triggering the ground to the relay. My problem is that when the pcm triggers the fan (about 190degrees) that fan only runs 1/2 speed, maybe less. The relay is also rapidly clicking, both relays do this. When I bypass the PCM ground with a wire clipped to a solid ground the fans run at 100% and the relays don't click.

So I assuming I have a grounding issue, my problem is I don't know how to improve the ground since it terminates somewhere in the PCM.

The fans have their own ground, which I believe is adequate because when I activate the relay by grounding it to the neg battery post the fans work great. When I let the relay be activated by the pcm ground switch the fans don't work so great.


Any suggestions?
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Old 03-19-2018, 08:54 PM   #2
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Re: PCM is not grounding fans properly

Does the case ground on these?
Maybe check resistance from the grounds and negative cable. Might be a bad connector or junction. If I remember there are more than one at the ecu.
Also, if you have another brand of relay you could try, it might help.
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Old 03-19-2018, 09:13 PM   #3
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Re: PCM is not grounding fans properly

Case is supposed to internally ground, but its a known issue when addressing grounding problems. Run a hard ground off the ecm case (just hold a wire to it and try) and see if it helps.

I'd also make sure you have the little mesh ground strap off the head, as they get lost in swaps sometimes and have been known to cause issues with things like injectors.
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Old 03-19-2018, 09:57 PM   #4
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Re: PCM is not grounding fans properly

Had the exact same problem with the high speed fan output on my EEC. Low speed works perfectly. I ended up triggering the high speed relay via a switch that overrides the low speed relay (and I have yet to need to use it; the fan cools the engine adequately using just the low speed). I would be interested to hear if you find a solution, as I'd love to have the EEC properly control both speeds.
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Old 03-19-2018, 11:00 PM   #5
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Re: PCM is not grounding fans properly

What exact engine are you using? A lot of the later model vehicles use PWM fan control by default, not just on/off.
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Old 03-19-2018, 11:01 PM   #6
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Re: PCM is not grounding fans properly

Quote:
Originally Posted by jrpontiac View Post
I posted this in the electrical forum as well.

I am working of the relay setup for my engine fans on my LS conversion. I got a dual fan setup with each fan triggered by a relay that is activated by the engine PCM. Each fan has it's own relay. The PCM activates the fan relay by triggering the ground to the relay. My problem is that when the pcm triggers the fan (about 190degrees) that fan only runs 1/2 speed, maybe less. The relay is also rapidly clicking, both relays do this. When I bypass the PCM ground with a wire clipped to a solid ground the fans run at 100% and the relays don't click.

So I assuming I have a grounding issue, my problem is I don't know how to improve the ground since it terminates somewhere in the PCM.

The fans have their own ground, which I believe is adequate because when I activate the relay by grounding it to the neg battery post the fans work great. When I let the relay be activated by the pcm ground switch the fans don't work so great.


Any suggestions?
What engine did you swap? Also, what year?

Quote:
Originally Posted by racecougar View Post
Had the exact same problem with the high speed fan output on my EEC. Low speed works perfectly. I ended up triggering the high speed relay via a switch that overrides the low speed relay (and I have yet to need to use it; the fan cools the engine adequately using just the low speed). I would be interested to hear if you find a solution, as I'd love to have the EEC properly control both speeds.
Same question. What engine (and year)?
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Old 03-19-2018, 11:32 PM   #7
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Re: PCM is not grounding fans properly

2005 5.3l with a 1999 pcm. When I had the pcm flashed I had them activate the fans. I added a couple pins to the harness
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Old 03-19-2018, 11:47 PM   #8
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Re: PCM is not grounding fans properly

a lot of those early ecm's only controlled a single fan and wouldn't run 2 fans....I had close to the same issue...I had to send my ecm back so they could set it up for dual fans...works fine now...
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Old 03-20-2018, 12:10 AM   #9
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Re: PCM is not grounding fans properly

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Originally Posted by jrpontiac View Post
2005 5.3l with a 1999 pcm. When I had the pcm flashed I had them activate the fans. I added a couple pins to the harness
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Originally Posted by mongocanfly View Post
a lot of those early ecm's only controlled a single fan and wouldn't run 2 fans....I had close to the same issue...I had to send my ecm back so they could set it up for dual fans...works fine now...
Yep. It sounds like the PCM not set up correctly to control electric fans (or, possibly, the added wires aren't on the correct pins). Since the fans work properly when the fan signals are grounded, everything from the relays to the fans appears good.
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Old 03-20-2018, 10:51 AM   #10
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Re: PCM is not grounding fans properly

I've heard of the relays clicking real fast when the fan on and off temps are not set right in the tune, like the PCM tries to turn the fan on, but as soon as it does, it thinks it needs to be turned off. Might be worth checking with your tuner to see exactly what the temps are set at.
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Old 03-20-2018, 10:55 AM   #11
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Re: PCM is not grounding fans properly

Could the PCM be using PWM to control the fan speed? That would wind up just cycling the relay and making it buzz/click.
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Old 03-20-2018, 06:21 PM   #12
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Re: PCM is not grounding fans properly

Quote:
Originally Posted by dayj1 View Post
Same question. What engine (and year)?
2002 Suburban DBW 5.3L here. I can confirm that the fan settings are correct in the tune file...I wrote the file. Altering the commanded "ON" temp for the high speed corresponds with the temp at which it attempts to switch to the high speed relay. If it was PWM, how would it receive feedback?

Quote:
Originally Posted by mongocanfly View Post
a lot of those early ecm's only controlled a single fan and wouldn't run 2 fans....I had close to the same issue...I had to send my ecm back so they could set it up for dual fans...works fine now...
What was done to "set it up"?
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Old 03-20-2018, 06:36 PM   #13
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Re: PCM is not grounding fans properly

I misunderstood how the fans work. There are 2 fan wires, I thought they triggered at different temp, one triggered at a lower temp than the other. I guess I don't know how it is suppose to work.

I had my pcm flashed by lt1 swap. and he activated Blue pin #42 and red pin #33. I thought one pin was triggered at a lower temp than the other, but when both ran they were at 100%. I have 1 relay and fan setup on the pin #33 wire and the other relay and fan to pin #42. I'm running a dual fan setup.

It sounds as if my fan is suppose to be running at low speed, so maybe that is correct. But I don't know why my relay is clicking constantly.

What temps are the 2 pins suppose to be triggered. At what temps will the fans be running 100%. Is this something the programmer would set?
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Old 03-20-2018, 07:13 PM   #14
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Re: PCM is not grounding fans properly

Yes, the tuner sets the fan(s) on and off in software.

The early trucks (99-05 ish) Didn't use electric fans. Just a big mech fan. So if the PCM is from one of those apps then tuner has to get things right to activate those outputs in the PCM. I have read more than once of guys missing a thing or 2. Specifically the fan output Red C2 33 has to be repurposed in the software. On the trucks is operates a door in the HVAC system, And can cause issues like you state.

When you say low speed fan do you mean just one fan running ?
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Old 03-20-2018, 07:34 PM   #15
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Re: PCM is not grounding fans properly

At this point I'm not sure how the fans are suppose to be working. Is there a low speed fan that becomes a high speed fan once the engine hits a certain temp? Is the low speed fan always a low speed fan? And a high speed fan that only runs high? Im just confused on what either fan is suppose to be doing. Going into this I thought the fans ran either 100% or not at all, and were triggered at diff temp. It sounds like they are suppose to be doing something different.
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Old 03-20-2018, 07:39 PM   #16
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Re: PCM is not grounding fans properly

Do you have a 2 relay or 3 relay fan setup ?
The 2 relay type operates each fan independently. Each fan is either 100% on or off. One fan or both fans can be on if the PCM commands it. Uses 2 outputs from PCM.

The 3 relay setup runs both fans in series for low speed. Then it runs both in parallel for high speed. They always run together.
Uses same 2 outputs from the PCM.

Basically the PCM doesn't care which system you use but you have to get the relay wiring right for each type.
If your PCM was done by LT1 swap Id be surprised if it isn't done right. That guy knows his stuff
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Old 03-20-2018, 07:44 PM   #17
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Re: PCM is not grounding fans properly

That really depends on what fan arrangement you're running, how you have it wired, and how you're controlling it in the PCM.

In my case, I'm running a two-speed fan. I use pin 42 from the blue connector to trigger the low speed fan relay. I attempted to use pin 33 from the red connector to trigger the high speed fan relay, but that signal kicks on and off rapidly when the coolant temp reaches/exceeds the "ON" temp in the tune file. The low speed works perfectly, turning on and off at the temps I've set in the tune file. I've since ditched the pin 33 output to the high speed relay and instead control that relay via a switch, though I've yet to actually need to use it.
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Old 03-20-2018, 08:49 PM   #18
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Re: PCM is not grounding fans properly

cougar....I don't know what he did to fix the "setup".....I'm not a computer guy..and hate wiring

stormin....my 02 engine had elec fan....

on mine there is no high/low speed on the fans...its either on or off
the psi harness is built to run dual fans thru seperate relays...my 02 ecm was only designed for a single fan....like i said...my problem was very much like yours...I sent the ecm back to the tuner and he did whatever he does and fixed it....on mine fan 1 comes on at 175....fan 2 comes on at 185....
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Old 03-20-2018, 09:25 PM   #19
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Re: PCM is not grounding fans properly

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That really depends on what fan arrangement you're running, how you have it wired, and how you're controlling it in the PCM.

In my case, I'm running a two-speed fan. I use pin 42 from the blue connector to trigger the low speed fan relay. I attempted to use pin 33 from the red connector to trigger the high speed fan relay, but that signal kicks on and off rapidly when the coolant temp reaches/exceeds the "ON" temp in the tune file. The low speed works perfectly, turning on and off at the temps I've set in the tune file. I've since ditched the pin 33 output to the high speed relay and instead control that relay via a switch, though I've yet to actually need to use it.
That's what happens when the tuner doesn't "repurpose" the pin in software to control a rad fan, From controlling HVAC door.
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Old 03-20-2018, 09:39 PM   #20
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Re: PCM is not grounding fans properly

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That's what happens when the tuner doesn't "repurpose" the pin in software to control a rad fan, From controlling HVAC door.
I'll dig around in the tune file more when I get time. If anyone else does their own tuning here, I'd be very curious to know what parameter I've apparently missed. The fact that I can control the temp at which it starts cycling would say that the EEC knows to ground that pin based on the input from the ECT, it just cycles instead of holding steady.
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Old 03-20-2018, 09:45 PM   #21
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Re: PCM is not grounding fans properly

After a very quick Google search, it seems this is a common issue.

Solution if you're using HPTuners to write your files: Use version 2.24, go to Systems, then A/C, change the recirculation door to "Fan #2".

Only problem for me: I'm using SCT Advantage to write my files. Ugh.
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Old 03-20-2018, 10:05 PM   #22
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Re: PCM is not grounding fans properly

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At this point I'm not sure how the fans are suppose to be working. Is there a low speed fan that becomes a high speed fan once the engine hits a certain temp? Is the low speed fan always a low speed fan? And a high speed fan that only runs high? Im just confused on what either fan is suppose to be doing. Going into this I thought the fans ran either 100% or not at all, and were triggered at diff temp. It sounds like they are suppose to be doing something different.
Just to clear this up for you, the way you have it wired, one relay for each fan, they should come on at different temps, but run at full speed. The actual temperature that the fans come on is settable by the tuner, so I cant say for sure what temp it should be. So basically, when the low speed temp is reached, one of your fans should come on, at full speed, then when it reaches the second higher temp, the other fan should come on at full speed.

I would email Brendan and have him look at the file to make sure they actually got enabled, he's only human, and things do happen from time to time. Hope this helps.
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Old 03-20-2018, 10:11 PM   #23
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Re: PCM is not grounding fans properly

Just in case anyone stumbles across this thread in the future, does their own tuning, and happens to use SCT Advantage...

I believe this would be the parameter that you would need to change from a 1 to a 0 in order to get pin 33 to output a steady ground instead of cycling at the "ON" temp for fan #2.

That said, I don't foresee myself getting around to switching the wiring back for the high speed fan relay anytime soon since I haven't had reason to use the high speed yet.
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Old 03-22-2018, 11:14 AM   #24
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Re: PCM is not grounding fans properly

Got info from Brendan, great guy.
Fans are either on 100% or off.

First fan on at 201 off at 197
Second fan on 204 of at 201

My obd reader shows the first fan coming on 190.

I'm going to check my grounds and wiring once again and see if I can get the clicking to go away.
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Old 07-19-2018, 05:07 PM   #25
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Re: PCM is not grounding fans properly

Update in case anyone runs into this in the future. It was mentioned above as well. The wire controlling the low temp relay was the only one clicking. I set it up so that both fans are activated by the hi temp PCM wire. All works good, fans come on about 205, no clicking. I'm not able to backtrack the wires to see if #42 or #33 is the low temp wire. Whatever pin was used for the low temp didn't like the situation.

A delay in my update because the entire drivetrain was swapped from the GMC to a 77 c10
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