PCM Tuning - can the thresholds be changed?

JayArr

Active Member
Hi

I'm wondering about how much the PCM can be modified when flashing. I've read that codes for the fan can be removed so that a regular fan can be installed but I've got a slightly more complicated scenario.

When I tow, I get P0495 codes, Fan exceeded 1600RPM for 81 seconds. It only happens if the engine RPM has been over 1800RPM for 120 seconds and the PCM commands 0 percent duty cycle from the cooling fan. I get these when I go up a steep grade in 2nd or first towing the trailer, the engine revs high and the fan spins too fast and by the time I've reached the top I've got a CEL.

The P0495 code doesn't invoke limp mode or affect power/efficiency of the vehicle but it does mean that I wouldn't notice if there was a second code because the CEL is already lit.

@Limquat - can the parameters be changed? Could the 1800RPM be changed to 2400RPM or could the 81 seconds be changed to 300 seconds? I don't know how far you can go when you reprogram, is it this granular or is it just an ability to disable the code from turning on the CEL?

JayArr
 

Blckshdw

Likes lights and stuff
Moderator
Obviously I can't give you an in depth answer like others can, but looking in HP Tuners, here's the options I see from the DTC menu from my stock tune. So you could tell it to not turn the CEL on at all, that would allow you to see it if you get a different code. Or just to not report at all.

1629741700986.png
 

Mooseman

Master Blaster
Moderator
Probably would be the same in HP Tuners but maybe has more. I'd have to grab an old pcm and see if I can modify them. I know DTCs can be turned off, that's for sure. There is a table for fan engagement vs. temperature and A/C high side pressure. Not at home right now but I could certainly look at it unless @limequat posts an answer first.
 

MRRSM

Lifetime VIP Supporter
The P0495 Code should probably be thrashed out first from this list of possible causes before deciding to modify the PCM Calibration:


(1) Broken or Damaged Wiring Harness
(2) Malfunctioning Fan Clutch
(3) Cooling Fan Clutch or Bearing Defective
(4) Cooling Fan Relay Defective
(5) ECM Issue Pin/Connector Problem (e.g. corrosion)

Source:

https://www.obd-codes.com/p0495
 
For curiosity sake, do you monitor the actual fan speed and if so at what engine rpm does it go over 1600 rpm??
 

Blckshdw

Likes lights and stuff
Moderator
No idea. Mine's an 05 4.2L EXT. I had PCM of NC mail order tune mine.
 

Mooseman

Master Blaster
Moderator
I know 02 saw a couple of changes and maybe 03 for issues with the fan clutch. In the Tech 2 section, there is a link to ACDelco TDS calibrations available for each year and vehicle. On my phone right now.
 

MRRSM

Lifetime VIP Supporter
The OP's profile does not include the Year of Manufacture, but just in case it is 2002... this is an interesting TSB that seems related here:

Chevrolet
Chevrolet TSB 01-06-04-052
Make: Chevrolet
Models: 2002 TrailBlazer
Number: 01-06-04-052
Date: 8/31/2005
Title: On-Board Diagnostic (OBD) System Improvements

Service Engine soon Light On, DTC P0014, P0116 Or P1683 Set, Delayed Transmission Downshift And/Or Excessive Fan Noise, Id # Yt04032 (Reprogram PCM)
 

Mooseman

Master Blaster
Moderator
At a computer now. Found these for 02-03 and 04. 05 had no update.

02 Calibrations.png03 Calibrations.png04 Calibrations.png

04 is especially interesting as it addresses an overheat condition.

I used VINs I found on the Net. If you want to check with your particular VIN, you can go here. It's free.

 

Mooseman

Master Blaster
Moderator
Just found in another post, @JayArr 's is an 05. I have edited his profile to reflect that.
 
Thanks for all that regarding calibrations. I have seen these before and have checked on my VIN. As it sits I have the same calibration as came in October 2001.

I wondered due to posts I have seen over the years. My base desired fan speed is NEVER less than 496. I have seen others screenshots showing less. And my desired fan speed does not increase at the levels seen in the post by @Blckshdw . It will increase but it takes a higher level of A/C pressure and/or coolant temperature.

I am curious about @JayArr engine speed to obtain a fan speed of over 1600. I seldom run at higher engine rpm. With 287 thousand miles on the engine I have no intention of introducing any unwarranted duress but if the engine rpm he runs isn't too high I might check to see what my fan does at a similar rpm.
 

Blckshdw

Likes lights and stuff
Moderator
I wondered due to posts I have seen over the years. My base desired fan speed is NEVER less than 496. I have seen others screenshots showing less. And my desired fan speed does not increase at the levels seen in the post by @Blckshdw . It will increase but it takes a higher level of A/C pressure and/or coolant temperature.

When I had the stock tune downloaded and saw that, I thought it had to be wrong somehow, but I've never had any overheating issues. Even in the FL summers, with the AC on, in parking lot traffic on the interstate, the hottest it's ran is 210. And that was after an hour at highway speeds. If I'm not running the AC, it will stay between 195 and 200. And it only gets to the upper range if the transmission gets warm enough to bump that up.
 

limequat

Hobbyist
Hi

I'm wondering about how much the PCM can be modified when flashing. I've read that codes for the fan can be removed so that a regular fan can be installed but I've got a slightly more complicated scenario.

When I tow, I get P0495 codes, Fan exceeded 1600RPM for 81 seconds. It only happens if the engine RPM has been over 1800RPM for 120 seconds and the PCM commands 0 percent duty cycle from the cooling fan. I get these when I go up a steep grade in 2nd or first towing the trailer, the engine revs high and the fan spins too fast and by the time I've reached the top I've got a CEL.

The P0495 code doesn't invoke limp mode or affect power/efficiency of the vehicle but it does mean that I wouldn't notice if there was a second code because the CEL is already lit.

@Limquat - can the parameters be changed? Could the 1800RPM be changed to 2400RPM or could the 81 seconds be changed to 300 seconds? I don't know how far you can go when you reprogram, is it this granular or is it just an ability to disable the code from turning on the CEL?

JayArr
In terms of the code is basically turning it on/off and enabling/disabling SES light
 

JayArr

Active Member
Thread Starter
Hi All

It's all fairly new - rad, fan, water pump, rad hoses, block flushed etc. I even repaired the wiring harness with a new plug to the cooling fan last year. This isn't a fault it's an anomoly.

I just did a 22 day vacation in the mountains of BC hauling our travel trailer. For those of you who haven't been to the northwest the towns are all in the valley at the bottom with the river and to get to the next town you have to drive over a pass between two mountains and down into the next valley. The passes are usually about 3600 feet (above sea level) so depending where you start the road rises a couple of thousand feet to a summit then you get to drive down the other side in second trying not to burn your brakes as you descend LOL.

Think about driving up the side of a mountain hauling a 5000lb trailer, the engine revs up as it gears down. By the time I've climbed a couple thousand feet I have been down through the gears to first and may have spent the last minute or so of the climb to the summit in first at engine=3500 or 4000RPM. It's easy to end up over 1600RPM fan speed. for 81 seconds

If the fan clutch locks up as I start out of the valley I think it's hard for it to slow down and lose it's momentum with the engine revs so high so despite the PCM calling for 0 duty cycle the fan just keeps spinning above 1600 and then the code sets.

The code only sets on an ascent from where I end up in first gear near the top while towing.

Without the trailer and without the mountain I doubt it would ever happen.

Thanks for the clarification Limequat!

JayArr
 

JayArr

Active Member
Thread Starter
Here's another idea...

We only tow the trailer a few weekends a year and for one extended 2-3 week vacation. The rest of the year the Envoy is our grocery getter.

I have a spare PCM. it came with the engine I bought from the wrecker.

Would it be wise to have it tuned to be a "towing PCM"?

Can the shift points be manipulated so the vehicle is better at hauling?

Lastly: Could I just undo the bolts and swap back and forth between the two?
 
Lastly: Could I just undo the bolts and swap back and forth between the two?


I once installed a used PCM (same year) and did the security relearn (30+ minute). It was only for the experience and so I could record the serial data bus while doing that. Afterwards I reinstalled the original. I had to do the whole 30+ minute relearn again even though this was the original PCM.
 

Mooseman

Master Blaster
Moderator
It could be tuned with a "tow/haul" mode tune and be swapped as needed. Could have custom shift points and different fan speed controls. It can be "hot swapped" by not disconnecting the battery as I had done it many times before even if it's against all rules for handling the PCM.

I'd have it tuned to have the fan run faster earlier or at a lower temp as a pre-emptive when you start going uphill. It should also have better shift points like the tow/haul mode in other trucks.

@limequat would be the expert on this.
 
Last edited:

MRRSM

Lifetime VIP Supporter
--+--- A Radiator and Heater Core Flush and Spray Out of the Radiatior and Cooler Finned Cores after removing the Front Grill.
 

JayArr

Active Member
Thread Starter
Well, a 30 minute relearn isn't too arduous and I do own a Tech II clone. I would probably only do the swap for the long summer trip and use the regular PCM for weekend trips.

I guess I need to do some research into what the current shift points are and where the tow/haul ones should be set.

Can I set the VIN# on both units to match my current chassis?

What happens to mileage? does each pcm have an "odometer" such that mileage on the tow haul unit stays on that unit?
 
Well, a 30 minute relearn isn't too arduous and I do own a Tech II clone.

If I understood @Mooseman post it sounds like for him it didn't require the relearn?? I only did mine the two times, once to a used PCM and I can understand that need for the relearn. But when I replaced the original PCM it again required the relearn. That is the part that puzzles me.
 

Mooseman

Master Blaster
Moderator
If it's tuned, VATS can be turned off and a relearn shouldn't be required. And it can be set to the proper VIN so your speedo will work correctly. The gearing is set by the VIN during programming, which you can do with your Tech 2 and Tis2000. It will likely also require a CASE relearn which you can also do.

What I'd do is get a used PCM, reprogram it with your VIN, send it to @limequat for tuning as a tow/haul unit, then install it and do a CASE relearn. If it needs a relearn, so be it but neither should need it again.

Typically, trucks with a tow/haul mode will shift at higher RPM and won't shift into OD unless it's under really light throttle cruising.
 
A couple things here with this code setting bother me.

If this document from a GM website is correct the code cannot set if the engine rpm is greater than 3200 rpm, and the fan has to be over 1600 rpm for like 100 seconds while it is not commanded on. (800/1000 cts @ 125ms per count).

Screenshot_20210825-180038.png

Does the engine not get hot enough to command some fan while towing up the mountain?

Today I recorded an attempt to get my fan to go that fast. It wouldn't. With the engine at full operating temp I selected 2nd manually and ran up to 4000 or so and held it for about 30 seconds or so. Granted this is not a long time but my fan speed made no indications it was going to go any faster.

Screenshot_20210825-175510.png

Has anyone else experimented with reading their fan speeds over a range of operations? 1600 just seems high to me for a properly functioning fan clutch at 0% duty cycle.
 

Mooseman

Master Blaster
Moderator
A slight deviation from subject matter,,, anyone know at what rpm is the fuel cutoff supposed to happen on a 4.2 while doing this Crankshaft Position Variation Learn??
I seem to remember mine would cut it off at around 5500-6000 rpm. I know it's higher than the rpm limiter in park.
 

Mooseman

Master Blaster
Moderator
It could be 5000 but it's definitely not pegged, I'd say 3/4 of the swing, at about 2 o'clock. I do know it's nerve wracking when you're not used to it. If you don't need it, no worries.
 

JayArr

Active Member
Thread Starter
TJ, here are the results I got last Sept when I was comparing the Behr to the Hayden.

At 3000RPM the Behr was spinning at close to 1600RPM with command = 0% duty. Climbing a hill at 3000-4000RPM would easily go over the 1600 and if it's a big hill it would last more than 81 seconds.

I didn't reve past 3000 in those tests last year because I never conceived of this anomaly.


Idle, RPM=620

Command = 0
Haydon - Desired=304, Actual=752
Behr - Desired=304, Actual=736

Command = 100%
Haydon - Desired=752, Actual=752
Behr - Desired=816, Actual=816


2000RPM

Command = 0
Haydon - Desired=304, Actual=1400
Behr - Desired=304, Actual=1260

Command = 100%
Haydon - Desired=2560, Actual=1520
Behr - Desired=2608, Actual=2608

3000RPM

Command = 0
Haydon - Desired=304, Actual=1520
Behr - Desired=304, Actual=1568

Command = 100%
Haydon - Desired=3200, Actual=1950
Behr - Desired=3584, Actual=3456
 

JayArr

Active Member
Thread Starter
I checked my manual for the Envoy, I have the paper manual for 2005.

If does not have the same specs for this code, your screenshot is for light duty trucks and I guess they are different.

Specifically, my manual does not have the condition that the RPM be between 1200 and 3200.
 

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JayArr

Active Member
Thread Starter
Does the engine not get hot enough to command some fan while towing up the mountain?

The temp never went above 89 degrees C (192F), even in first gear at the top of a summit. After all the problems I had last year I watched this like a hawk for about the first ten days of the trip.

According to the chart BlackShadow posted above the commanded duty cyclce remains 0% until the coolant hits 210F so even climbing a mountain the fan is just free-wheeling if the cooling system works and the cooling system seems to be doing it's job quite well, it doesn't overheat.

I think maybe I just have a fan that runs a little to fast at commanded duty cycle=0% and engine RPM over 3000.
 
does not have the same specs for this code, your screenshot is for light duty trucks and I guess they are different.

The more variations I see in "service information" the less trust I have that there even exists accurate information! Everything gets farmed out these days and accuracy suffers.

Here is where I got the document from which the screenshot came...



I have never purchased a new fan clutch, have pulled maybe 7 or 8 from Upull yard, for testing purposes. The recorded data I posted yesterday is from a Behr unit. I think I will see what that same unit does when commanded on.
 

JayArr

Active Member
Thread Starter
With all your testing have you pulled the mode 12 freeze frame data to see conditions at the time of the code set?

No, I didn't do that, I only had my phone and a ODBII module with me, the Tech II was left behind this trip. I had to clear the code up to a few times a day and I wasn't thinking of the freeze frame data.

If I've cleared the code with the phone, does that clear the freeze frame data as well? If it's still in there I can retrieve it tonight but if not it'll have to wait until next time because the trailer is parked and the code only sets with it on.

If I have time tonight I'll go out with the TechII and record the RPM of the fan at 4000RPM with command=0%DC. I'm betting that it will be above 1600RPM.
 
So I just came back from an errand during which I recorded a series of events where I used one phone to command the fan to 50% for 20 seconds while a second phone recorded engine rpm, fan desired rpm, and fan actual rpm. Here is a snippet from that recording. Interesting how much more responsive the fan disengagement is at higher engine rpm. I knew it is pretty much required that the engine rpm needs to be at least as high as 2000 rpm but I was not aware of how much quicker the clutch disengages at closer to 3000 rpm. This is a Upull Behr unit.image.jpg
 
If I've cleared the code with the phone, does that clear the freeze frame data as well?

I think yes, freeze frame data would be cleared with a code clear.

If you can capture the freeze frame it may offer a glimpse but then again might not.

I just happen to have a capture of mode 12 freeze frame data for a P0495 code. The "04 95" can be seen near the end of the second and last lines.

I do not know if the mode 02 freeze frame captures the same data. Mode 12 is what the Tech 2 displays.You can see it captures like 30 some odd parameters. The PID is displayed in the 2nd and third byte after the highlighted columns. The data follows after that and would need to be decoded but that is easy enough for a lot of the data. Oddly enough the actual fan speed is not captured! Among data captured are engine speed, throttle position, accelerator position, engine load, MAP, engine temp, vehicle speed, #1 injector pulse, various transmission parameters, etc..

Screenshot_20210826-164701_aGrep.jpg
 

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