2006 Sierra Radiator Fans, no Low Speed.

georg · September 11, 2014

2006 GMC Sierra WT, 4.3l, manual tranny, 2wd, no A/C. In hot weather, idling in traffic, coolant temp will climb to 227F before the fans kick in at high speed and rapidly bring the temperature down to 194F. The fans do not come on at all, at any speed, until the temperature hits 227F. I have a ScanGauge and can accurately monitor temperature.

I also have a Tech2 and it can turn on Cooling Fan relay 1, relays 2 &3, and relays 1,2 &3. They all work normally and the fans come on at the appropriate speeds. So the issue is not with the relays or the fans. The PCM is just not commanding the fans on at low speed.

There was a thread on cooling fans

http://www.gm-trucks.com/forums/topic/150659-2006-gmc-sierra-cooling-fans-question/

This is a copy and paste from that thread

Cooling Fan Control - Two Fan System
The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block. During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the low speed fan relay coil, closes the relay contacts, and supplies battery positive voltage from the low fan fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan s/p relay and the right cooling fan. The result is a series circuit with both fans running at low speed. During high speed operation the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. After a 3 second delay, the PCM supplies a ground path for the high speed fan relay and the cooling fan s/p relay through the high speed cooling fan relay control circuit. This energizes the cooling fan s/p relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the high speed fan relay coil is energized closing the relay contacts and provides battery positive voltage from the high fan fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

Important

The right and left cooling fan connectors are interchangeable. When servicing the fans be sure that the connectors are plugged into the correct fan.

The PCM commands the low speed cooling fans ON under the following conditions:
1. Engine coolant temperature exceeds approximately 94.5°C (202°F).
2. A/C refrigerant pressure exceeds 1447 kPa (210 psi).
3. After the vehicle is shut OFF if the engine coolant temperature at key-off is greater than 101°C (214°F) the low speed fans will run for a minimum of 60 seconds After 60 seconds, if the coolant temperature drops below 101°C (214°F) the fans will shut OFF. The fans will automatically shut OFF after 3 min. regardless of coolant temperature.

The PCM commands the high speed fans ON under the following conditions:
1. Engine coolant temperature exceeds approximately 104.25°C (220°F).
2. A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).
3. When certain DTCs set.

At idle and very low vehicle speeds the cooling fans are only allowed to increase in speed if required. This insures idle stability by preventing the fans from cycling between high and low speed.

For low speed operation there is mention of AC refrigerant pressure. Since my truck does not have AC, it is never going to see high refrigerant pressure. Is this what is preventing the PCM on my truck from asking for low speed operation?

I have the GM service manual for my truck and it has the exact same info that I copied and pasted above. It says the fans come on at low speed under certain conditions but it does not specify whether low speed comes on when ANY of the three conditions mentioned are satisfied or whether temperature has to exceed 94C AND refrigerant pressure is high.

The truck has been like this since new and the fans have never come on at low speed.

AS an aside just what does the second last sentence mean? "cooling fans are only allowed to increase in speed if required"

Doug_Scott · September 11, 2014

If I am reading this correctly, from day one in 2006, your truck has never ran the fans in low speed mode. Now, after 8 years of it behaving this way, it is now important enough to look into it? It looks like the very base model truck you have does not contain the more elaborate or complex cooling fan control circuits. When vehicles first started getting electric fans they were simple on at temp, then off at temp. They were usually on at 210, off at 190 or 195.

Second last sentence means the pcm will consider quality of idle when considering increasing fan speed. It is simply trying to avoid complaints from customers regarding rough idle when power consumption from the fans force the alternator to go to full output in order to prevent running battery down.

georg · September 13, 2014

After 8 years of use I noticed no low speed operation for the first time when I was boosting the other truck this summer. I do not tow with this truck and overheating has never been an issue, so I never paid any attention to fan operation. While not having low speed isn't a big issue, I thought it would be nice to have less variation in temperature for head gasket durability, etc.

I have no experience in programming, but it seems to me that all that is required is programming the PCM to request low speed when the temperature reaches 94.5C. I don't understand why my PCM is not doing it, it is supposed to do it according to the service manual.

I am going to try jumping the PCM pins with the appropriate resistors to indicate high A/C pressure and if that does not work I am going to look into PCM programming.

Doug_Scott · September 13, 2014

I would imagine GM decided that it was simply not required to have a low speed fan. They could not just enable it, there is at least one sensor missing, and there may be others that operate in a different range.

That "feature" may only be enabled on trucks with A/C to prevent slow or rough idle when engine is hot and a/c is on. At idle, on hot days, the a/c compressor will not cycle as often as it does when rpm gets over about 1500 rpm, and the load from that, plus the load from the fans, may make the alternator attempt to output max amperage, which makes it harder to turn the alternator, that brings the idle down to a point that it starts to get rough. With an automatic transmission, the pcm cannot just raise the idle speed up, due to the driver not expecting it when sitting in bumper to bumper traffic.

I just don't get why after 8 years of having no issues with it working this way, that now all of a sudden it is important.

georg · September 16, 2014

You may get lucky and find that "hot wiring" the pcm to impersonate the high side a/c pressure hitting 210psi, will run the fan at low speed. Problem then becomes, you have hard wired it in, the fan will never stop since it meets the requirement of high side pressure at 210 all the time.

Thanks, Doug, I appreciate the info.

Because of "AND", both 1) and 2) should exist for low speed operation. Once the temp has dropped, 1) is no longer satisfied and that should cause the PCM to withdraw the request for low speed operation. At least, I hope.

I don't even mind an engine operating temperature of over 100C. But I don't think this almost 20C variation ( fan kicks in at 108C on my truck according to the ScanGauge) in coolant temperature once the fan kicks in is good for the engine.

Doug_Scott · September 14, 2014

I would imagine that if it has not hurt the engine in the last 8 years, it is likely quite safe to operate in that range. I would make damn sure that I have checked a wiring diagram for my options, and not use a diagram that contains options I to not have.

Programming is very easy when done at 5th generation level. Problem is PCM/ECM is not written in 5th generation language. I know it sounds easy to do, but, I would be very mindful of basically hot wiring the PCM. If you are going to do this, pay careful attention to voltages. Most sensors will not run on 12v more than once..

Going back to your first post, I think the manual missed one word in the first section and two words from second section.

The PCM commands the low speed cooling fans ON under the following conditions:
1. Engine coolant temperature exceeds approximately 94.5°C (202°F). AND
2. A/C refrigerant pressure exceeds 1447 kPa (210 psi).
3. After the vehicle is shut OFF if the engine coolant temperature at key-off is greater than 101°C (214°F) the low speed fans will run for a minimum of 60 seconds After 60 seconds, if the coolant temperature drops below 101°C (214°F) the fans will shut OFF. The fans will automatically shut OFF after 3 min. regardless of coolant temperature.

The PCM commands the high speed fans ON under the following conditions:
1. Engine coolant temperature exceeds approximately 104.25°C (220°F).AND/OR
2. A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).AND/OR
3. When certain DTCs set.

You may get lucky and find that "hot wiring" the pcm to impersonate the high side a/c pressure hitting 210psi, will run the fan at low speed. Problem then becomes, you have hard wired it in, the fan will never stop since it meets the requirement of high side pressure at 210 all the time.

I think you will find that GM was more focused on getting the a/c pressure down than they were at getting engine temp down to thermostat opening temps.

georg · September 12, 2014

Before I installed the ScanGauge I only had the dash gauge to go by and you really don't notice the difference between 220F and 200F. Also, in traffic, with the hood down, you don't hear the fans much above the surrounding traffic noise. Recently, on a really hot day, I had to boost a box truck for about an hour (I eventually figured out I had to turn my headlights on to get decent charging voltage to charge the box truck's batteries) and , with the hood up and not much to do I was monitoring the behaviour of the fans.

What I plan to do next is to jump the pins at the PCM for the(non-existent on my truck) A/C pressure switch to indicate high pressure to the PCM and see if that will convince the PCM to turn on the fans at low speed. If that does not work I am going to look into programming the PCM to enable low speed operation of the fans.

georg · September 12, 2014

My truck does have the elaborate and complex cooling fan control circuits (the three relays in series/parallel configuration). The only thing missing is the request from the PCM for low speed operation.

Doug_Scott · September 16, 2014

Thanks, Doug, I appreciate the info.

Because of "AND", both 1) and 2) should exist for low speed operation. Once the temp has dropped, 1) is no longer satisfied and that should cause the PCM to withdraw the request for low speed operation. At least, I hope.

I don't even mind an engine operating temperature of over 100C. But I don't think this almost 20C variation ( fan kicks in at 108C on my truck according to the ScanGauge) in coolant temperature once the fan kicks in is good for the engine.

You are correct, it does not require the AND gate. Logical OR gate can cover all the combinations correctly.

The fans will only come on while in slow moving traffic or at idle. Once the truck is moving faster than 10mph the fan will likely not ever come on assuming you not drafting the vehicle in front of you or using the Alaska thermostat.