I have been fighting this problem for 8 years and decided to tackle it once and for all this week.
I have a 4 cylinder Cummins (ISB170) in my 08 Dakota. I could never make the Dakota ECU happy enough to run the AC so for the last 7 years I just had a pressure switch in line with a temp probe in the evaporator and a toggle switch on the dash. It worked pretty well but now I want to get the proper system running.
I spent the last few days making the ECU happy and it looks like I have no errors in the system. I can monitor the system and watch the dash switch signal go to the ECU and its request for AC but it wont turn on the compressor relay. I can also engage the compressor with the software so I know the AC relay and compressor are good and would receive a signal from the ECU. There is just one part of the puzzle missing and I'm at the end of my rope. I have a low/high side reading of 85 PSI when the compressor is not running and then when I manually engage the compressor I get 18 PSI on the low side and 180 PSI on the high side.
From what info I can find the 85 PSI should be in the correct range and not be inhibiting the engagement?
Any clues as to the last piece of this puzzle would be greatly appreciated!!
AC diagnostic screen.PNG (172.54 KiB) Viewed 5136 times
18 psig is too low, your evaporator will make ice in it's fins at that pressure. ECM is probably disabling compressor to keep you from wrecking it since you are probably low on charge.
Have it recovered and charged to the underhood sticker amount.
Shouldn't it first turn the pump on in order for it to "see" the low pressure?
I thought the 85 PSI would be within spec to turn the pump on. I realize its low once its running and have a local shop who built all the lines and did the first charge 7 years ago so I will have them do the recharge but I need to know the system will run once they do that.
FYI, "I have a low/high side reading of 85 PSI when the compressor is not running"
this 85 psi in this scenario is termed static charge and is about what anyone would expect.
I do not know for sure, but the 85 should not be the signal to turn on the compressor, because that number exists only when the system is off.
I would expect the thermostat setting, however that is determined by the ECU, would be the trigger.
Agree with Detroit
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" I need to know the system will run once they do that."
You have no way to know that, but it is the next step in the process. I feel as though you have, as of yet, an unidentified control issue
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I appreciate all the replies and info.
What determines the Tstat setting?
It does not show up in the diagnostics. As far as the ECU looks all systems should be go.
You are right, it should engage the compressor once, see the pressure go too low and disengage. I don't know how Dodge's algorithm for compressor protection works, but I'm sure there is one, maybe someone else knows.
The ECM doesn't have a thermostat setting. Do you have an ATC control head or a manual control head? The control head is in charge of whether or not the compressor is requested, the ECM is in charge of doing it. There is some compressor clutch engagement torque compensation that happens and the ECM controls that and clutch engagement so they are synchronized.
DetroitAC wrote: ↑Sat Jul 10, 2021 6:58 pm
Do you have an ATC control head or a manual control head? The control head is in charge of whether or not the compressor is requested, the ECM is in charge of doing it. There is some compressor clutch engagement torque compensation that happens and the ECM controls that and clutch engagement so they are synchronized.
Not sure which I have, it does not have zones, pretty basic. I watch the ECM and when I push the A/C button on the dash the ECM shows an A/C request and turns the A/C dash light on. You can see that on the screen shot that I posted.It also shows that all the conditions have been met for A/C as far as the ECM is concerned.I will dig deaper into the other modules, maybe the BCM has an issue with something but I don't see any codes from it.
Yes the ECM has a tach signal from the stock crank/cam sensors and runs the dash, I always try to replicate as many sensors from the old engine as possible. I used HP Tuners to turn off a lot of the engine sensors and used resistors to fool the rest so I have no DTC's from the ECM. I have done quite a few engine swaps in the last 30 years but mostly to older vehicles that had a straight forward A/C system with no ECM involvement.
