Cooling fan circuit on 6.2L

I did a eFan swap on my 2001 Tahoe. It's canibalized from multiple years. I believe a ~2005 GMT800 fan harness and junction block then the 700W severe duty fans from a 2012 6.2L. My notes are below:


All three relays in the junction block failed (high / low select, fan 1, and fan 2). I'm curious if the GMT800 circuitry isn't strong enough for the 6.2L fans. However, I don't have a GMT900 around to look at how they are wired up. Does anybody have pictures or anything of the junction block where the fuses and relays are for the GMT900 radiator fans?

Thanks!

Top left corner, 5 pin control relay on bottom. No amp markings, but rockauto seems to show 35 and 40 amp relays for this style.
This is a 5.3, but I have been running the 700w fans for 2-3 years.

Are you running the fans at 100%?
Generally the secondary fan kicks on at 231 degF with 90% duty cycle.
The standard cooling fan tuns on at around 201degF with 44% duty cycle..

Wow thanks!!!!! VERY helpful. I those are same style relays I have. I will see about getting higher amp ones.

Bit confused are they PWM fans? My controller, the p01, is discrete, either on or off with no duty cycle setting.

I assumed they were, but now that you mention it, my settings say 'discrete' so I am probably wrong about the PWM..
I thought my AC fan had run at different noise levels before, but I could be imagining things..

Usually I hear about the fan connector burning up..

Looks like the gmt900 has a thicker wire in the diagrams..
5.0 cross section vs 3.0 cross section on the GMT800.
Not sure if that has anything to do with it at all..

FWIW, even with my 5.3, it's difficult to maintain much lower temp than stock without one fan running nonstop.
(Didn't try lowering second fan enough to help out at lower temps though..)
This is with a 180 t-stat, which warms upto 186-188degF if cool enough outside..

Great help! I was trying to find the factory service manual circuit diagram for the fans on the 6.2L GMT900 with heavy duty fan package (tow package?). You have a copy of that by chance? I'd like to compare everything.

I had suspected the wiring would be a lower gauge in the bigger fan package as you mentioned...

Maybe the noise you heard was the low versus high speed fan modes? High speed is much louder when it worked on my truck.

Matthew Jeschke said:

My controller, the p01, is discrete, either on or off with no duty cycle setting.

Click to expand...

Unless I'm mistaken, you're supposed to have three settings:
OFF ... over 45MpH, and/or the coolant is cool enough for whatever reason
Lo . . . under 44MpH but over 29MpH, should be only mode possible, even with AC on, even with very hot coolant
Hi . . . under 29MpH, this mode should not be achieved unless the AC or coolant is very hot

Matthew, when Lo fan setting is commanded, do both your fans come on, or just one?
'cause if it's just one, you need to modify the system to have both fans running, slower for Lo mode, quicker for Hi mode.
(Older V8s would only turn on just one fan for Lo mode, but this is inferior to both fans on with both Lo & Hi modes.)

Marky Dissod said:

Unless I'm mistaken, you're supposed to have three settings:
OFF ... over 45MpH, and/or the coolant is cool enough for whatever reason
Lo . . . under 44MpH but over 29MpH, should be only mode possible, even with AC on, even with very hot coolant
Hi . . . under 29MpH, this mode should not be achieved unless the AC or coolant is very hot

Matthew, when Lo fan setting is commanded, do both your fans come on, or just one?
'cause if it's just one, you need to modify the system to have both fans running, slower for Lo mode, quicker for Hi mode.
(Older V8s would only turn on just one fan for Lo mode, but this is inferior to both fans on with both Lo & Hi modes.)

Click to expand...

I'd be interested to delve into the fans' operation more deeply. What's the source for this, and does it go into more detail?

Scrappycrow said:

I'd be interested to delve into the fans' operation more deeply.
What's the source for this, and does it go into more detail?

Click to expand...

Here's an old one:
then scroll down to 'F-car LT1 (Camaro/Firebird) Cooling Systems:'

Scott Mueller said:

In ’93 and early ’94 models with AC, the two cooling fans are independently operated by the PCM at a high fixed speed by using a single relay for each fan.
Late ’94 and newer F-car models operate both fans simultaneously in either a low or a high speed mode by using 3 relays.
In low speed mode, the fans are powered in series. In high speed mode, the relays operate to power both fans in parallel, resulting in a higher speed of operation.

Click to expand...

What I was trying to get at was that if Matt's 'Lo fan' setting only operates a single fan, he ought to have it rewired to update it.

Thanks, @Marky Dissod!

Matthew Jeschke said:

Great help! I was trying to find the factory service manual circuit diagram for the fans on the 6.2L GMT900 with heavy duty fan package (tow package?). You have a copy of that by chance? I'd like to compare everything.

I had suspected the wiring would be a lower gauge in the bigger fan package as you mentioned...

Maybe the noise you heard was the low versus high speed fan modes? High speed is much louder when it worked on my truck.

Click to expand...

there's no difference in the 500w vs 700w setup on the gmt900's, same wiring, same relays.
the differences are external (fan motor, shroud design, fan blades), however the shroud mounting dimensions and harness are identical.
I would recommend more robust wiring (power & ground) and higher quality relays if not using oem.
also keep the relays away from heat as much as possible
like mark said in my experience the problems have been motors & motor harness connection, there was even a recall on the certain gmt900's for the motor harness melting.
pull the motor harness off and inspect for burnt/melted pins, if you see that then it's either the motor burning out or poor ground. in either case replace the motor harness at minimum.

mikez71 said:

I assumed they were, but now that you mention it, my settings say 'discrete' so I am probably wrong about the PWM..
I thought my AC fan had run at different noise levels before, but I could be imagining things..

Click to expand...

My mechanic buddy told me PWM fans have another low gauge wire going to them to control the duty cycle and he didn't see that on our setup. Maybe safe to assume they are discretely controlled w/o variable fan speed.

mikez71 said:

Usually I hear about the fan connector burning up..

Looks like the gmt900 has a thicker wire in the diagrams..
5.0 cross section vs 3.0 cross section on the GMT800.
Not sure if that has anything to do with it at all..

Click to expand...

I'm looking at the wiring diagram from the FSM for the GMT900 provided by @swathdiver I cannot see the wire diameter. Below is the diagram from the GMT800...


Not sure if I'm reading this correctly...

When looking at the gmt800 wiring diagram the positive line is 8mm^2 ~ 8 AWG... On the low end that's around 40 amps.
The return path / ground is 0.35mm^2 ~ 22 gauge. I'm not as concerned about the return line as it's adding resistance to the load which maybe helping my cause... if my thinking is correct. If I enlarge it then the feed line will have to be enlarged as well.

mikez71 said:

FWIW, even with my 5.3, it's difficult to maintain much lower temp than stock without one fan running nonstop.
(Didn't try lowering second fan enough to help out at lower temps though..)
This is with a 180 t-stat, which warms upto 186-188degF if cool enough outside..

Click to expand...

Good to know... Thanks, helps confirm my theory that there's very little headroom with the factory radiator.

Marky Dissod said:

Unless I'm mistaken, you're supposed to have three settings:
OFF ... over 45MpH, and/or the coolant is cool enough for whatever reason
Lo . . . under 44MpH but over 29MpH, should be only mode possible, even with AC on, even with very hot coolant
Hi . . . under 29MpH, this mode should not be achieved unless the AC or coolant is very hot

Matthew, when Lo fan setting is commanded, do both your fans come on, or just one?
'cause if it's just one, you need to modify the system to have both fans running, slower for Lo mode, quicker for Hi mode.
(Older V8s would only turn on just one fan for Lo mode, but this is inferior to both fans on with both Lo & Hi modes.)

Click to expand...

Here's a screen grab of my fan settings in HP Tuners:


I'm not sure if enable speed and disable speed set to 0 is defeating the Vehicle speed trigger for the low speed fan? If memory serves me correctly, I only ever got the high speed fan (currently broken so I cannot confirm at this moment).

HP Tuners parameter labeling and context help are a bit vague. I assume AC Fans Disable Speed turns the fans off all together over a certain speed, and isn't necessarily linked to the AC being on or off? After all what's the point of energizing fans if you get a larger volume of air from velocity of vehicle.

I should note, I don't have the pressure sensor for the AC so those parameters are unchanged. I lied in the tune a bit to get dual fan control, this released one of the PCM pins which was in control of an AC function to command the high speed fan. Maybe some logic that's not quite right w/o the pressure sensor? I'm wondering if it's forcing the high speed fan on if the AC is on... Additionally, I do a lot of trail driving so well under 45 MPH. Actually under 20 mph, and it's HOT here so I run the AC. This may have further provoked the underlying issue.

Doubeleive said:

there's no difference in the 500w vs 700w setup on the gmt900's, same wiring, same relays.

Click to expand...

It seems so. Both are 40 amp circuits per fan. I did notice the Rock Auto catalog lists some 40 amp relays for the 6.2L GMT900? Possibly for the 700W fan assembly?

I assume the motors are different between the 500w and 700w?

Perhaps I have an issue with the low speed fan... I measure 2 ohms for the high speed fan. However, I cannot seem to get a reliable reading on the low speed fan (partly harder than heck to get the probes on it). I think it's 6ohms? Can anybody confirm this?

Doubeleive said:

the differences are external (fan motor, shroud design, fan blades), however the shroud mounting dimensions and harness are identical.
I would recommend more robust wiring (power & ground) and higher quality relays if not using oem.

Click to expand...

For sure! I'm seeing this now. The factory wiring has virtually NO headroom in the design!

Doubeleive said:

also keep the relays away from heat as much as possible
like mark said in my experience the problems have been motors & motor harness connection, there was even a recall on the certain gmt900's for the motor harness melting.
pull the motor harness off and inspect for burnt/melted pins, if you see that then it's either the motor burning out or poor ground. in either case replace the motor harness at minimum.

Click to expand...

Went out and inspected the connectors to the fan motors. They are good.

@ScottyBoy helped me source the harness from a GMT800. I did notice on my buddy's 2005 Silverado that the factory put a little box by the battery which I don't have. I assume it's a terminal block to provide an extra battery tap to the fan assembly...?

I tried to wire the battery tap off the stud in the underhold junction block but it blew the stud fuses... So I ended up wiring positive feed directly to the battery.

Matthew Jeschke said:

My mechanic buddy told me PWM fans have another low gauge wire going to them to control the duty cycle and he didn't see that on our setup. Maybe safe to assume they are discretely controlled w/o variable fan speed.


I'm looking at the wiring diagram from the FSM for the GMT900 provided by @swathdiver I cannot see the wire diameter. Below is the diagram from the GMT800...

View attachment 486316
Not sure if I'm reading this correctly...

When looking at the gmt800 wiring diagram the positive line is 8mm^2 ~ 8 AWG... On the low end that's around 40 amps.
The return path / ground is 0.35mm^2 ~ 22 gauge. I'm not as concerned about the return line as it's adding resistance to the load which maybe helping my cause... if my thinking is correct. If I enlarge it then the feed line will have to be enlarged as well.


Good to know... Thanks, helps confirm my theory that there's very little headroom with the factory radiator.


Here's a screen grab of my fan settings in HP Tuners:

View attachment 486317
I'm not sure if enable speed and disable speed set to 0 is defeating the Vehicle speed trigger for the low speed fan? If memory serves me correctly, I only ever got the high speed fan (currently broken so I cannot confirm at this moment).

HP Tuners parameter labeling and context help are a bit vague. I assume AC Fans Disable Speed turns the fans off all together over a certain speed, and isn't necessarily linked to the AC being on or off? After all what's the point of energizing fans if you get a larger volume of air from velocity of vehicle.

I should note, I don't have the pressure sensor for the AC so those parameters are unchanged. I lied in the tune a bit to get dual fan control, this released one of the PCM pins which was in control of an AC function to command the high speed fan. Maybe some logic that's not quite right w/o the pressure sensor? I'm wondering if it's forcing the high speed fan on if the AC is on... Additionally, I do a lot of trail driving so well under 45 MPH. Actually under 20 mph, and it's HOT here so I run the AC. This may have further provoked the underlying issue.


It seems so. Both are 40 amp circuits per fan. I did notice the Rock Auto catalog lists some 40 amp relays for the 6.2L GMT900? Possibly for the 700W fan assembly?

I assume the motors are different between the 500w and 700w?

Perhaps I have an issue with the low speed fan... I measure 2 ohms for the high speed fan. However, I cannot seem to get a reliable reading on the low speed fan (partly harder than heck to get the probes on it). I think it's 6ohms? Can anybody confirm this?


For sure! I'm seeing this now. The factory wiring has virtually NO headroom in the design!


Went out and inspected the connectors to the fan motors. They are good.

@ScottyBoy helped me source the harness from a GMT800. I did notice on my buddy's 2005 Silverado that the factory put a little box by the battery which I don't have. I assume it's a terminal block to provide an extra battery tap to the fan assembly...?

I tried to wire the battery tap off the stud in the underhold junction block but it blew the stud fuses... So I ended up wiring positive feed directly to the battery.

Click to expand...

yes the 500w motors and 700w motors are different, more windings in the 700w, they also mount different and use different blades depending on the shroud and model. I have the 700w hummer fans in mine because too many sellers online say there fans are k5l which is simply not possible

Here's the wire cross sections I was looking at circled in blue..

Ah thanks, I see they are using a lower gauge wire in the connector harness... 3mm2 versus 5mm2 in the GMT900.

Thanks!

the gmt900 fans are brushed standard on off. they do use the relays to give it hi/lo setting. but they aren't pwm. the next Gen up started pwm and brushless fans.


if your really wanted to get crazy and hptuners allows you to select pwm output, you can use the control box from a c6 corvette to run the brushed fans with pwm. boxes are expensive and you'd need 2 of them. but also there's a more common Ford pwm box that works the same way and can be had for like 20$ each on ebay.

mine being a hybrid, gmt900 I get 3 speeds, lo/med/hi. but no option in hpt to switch to pwm. if I did, I'd love to put the 2015 and up rad/brushless fan combo in there. would be sweet. all the cooling when needed and nothing wasted.

Take the load off ur factory system and use this. Was just talking about this in another forum. Amazon.com and its cheap and comes with the ledger for where the wires go in the PCM harness. And or partsgeek.com

I've seen those. I'm trying to keep an as factory configuration as possible. However, maybe worth canibalizing one of those for the wire... Otherwise my next question was... where can somebody get 8 or 10 gauge bulk GLX J1128 compliant wire or otherwise for under hood conditions?

ebay has gxl wire..
search popped up delcity.net

Del City and Waytek are my sources for stuff like wiring and associated stuff like terminals, connectors, tools, etc.