That is exactly what I thought I was going to do. Currently have both pressure sensors installed and wired up for that purpose. However, during my testing, I found that regardless of the dodge AC sensor reading, the fan would not speed up. If you look at the wiring even more you find that the switch in the dash tells the PCM to turn on the AC. The PCM sends a PCI data bus communication signal to the ECM, which in turn grounds a pin to energize a relay which powers the AC clutch.

So to make a long story short, it seems as though, without the communication from the PCM to the ECM that the AC should be running, the ECM ignores the AC pressure sensor readings?

I may have screwed something up though...

I just went out and tried to get the fan to speed up manipulating the fan control wiring. Didn't have any luck... It has a 12V fused source, a 12V signal used for the park interlock (which I don't understand), a ground, a fan speed signal which was 2.2V at idle and stayed there when I rev'd the engine. And a sensor supply that was 5V. I tried everything I could think off and nothing seems to matter? No change in fan speed?

It's so stinkin' hot outside

 

Finally got it figured out. The Park Lockout Solenoid control wire is what you use...

With 12V on this wire to the fan clutch, it is free wheeling or slipping the maximum amount. With this wire grounded, it is locked up 100%. The closer to ground or zero volts the less it will slip.

I should be able to verify this tomorrow...

 

Maybe it's just because it's late and I'm tired, but I'm not sure I follow you. What did you end up hooking up where?

I guess it should wait until you verify this tomorrow.

 

Verified it this morning. If you ground the Park Lockout Solenoid control wire (as it is labeling in the pinout section of the service manual under Viscous fan drive), after about 60 seconds the fan will ramp up in speed until it is fully locked up. Man does it pull some air, when I rev'd it up it sounded as though an airliner was taking off...

I'm going to run this to a manual switch in the cab for now and then go get my AC recharged. Later, I'll come up with some way for it to kick on automatically and use the switch in the cab for an override switch if needed.

 

Ok, so now that you've got the fan working and were able to manipulate fan speed and it's verified that it's working correctly, I'd like to see if the ECM will speed up the fan when it senses A/C pressure.

I know you said in your earlier testing that it didn't, but I also thoght you said that you may have not has something right with the wiring. If wiring's correct now, I'm hoping the fan will speed up with the A/C on.

 

I'll do some more checking once the AC is charged and let you know...

 

Dodge Cummins doesn't use a PCM for the a/c system, everything is controlled thru the ECM. You have three components, the a/c pressure transducer (high pressure switch), the a/c clutch solenoid, and the vistronic fan drive. It seems that the ECM receives a signal from the pressure transducer then sends a signal to the vistronic fan drive.

 

Page 8W-10-44 of my service manual (2003) shows that the PCM provides the ground path for the A/C compressor clutch relay which then allows the relay contacts to close and energize the AC compressor clutch.

Also, under the PCM operation section in the manual it states that the A/C request and select are inputs to the PCM and that the A/C clutch relay is an output...

However, this manual applies for gas engines as well, so the description section may not apply to the diesel vehicles.

If only the ECM was used I would expect the ECM to have an input signal of some type to tell it to turn the AC on (based upon the dash switch settings), however I can't find one...

 

Were you able to get the system charged today and do any testing?

 

No, I won't have time to get it charged until atleast friday or saturday...

 

Seems all you need to do is put a "T" and run both the Ford sensor and the Dodge a/c pressure transducer in the line from the compressor to the condensor (high pressure line).

With the ECM knowing that it has high pressure it should lock up the clutch to pull more air.

 

That's the theory, which 5.9Ex will hopefully be testing once he's had a chance to get the AC charged. My engine's not quite in the truck yet, so I don't have the ability to test it at the moment.

 

The pressure transducer is $88.10 from Dodge, that is list. If I get time I will hook it up tonight. I have read the wiring diagrams front to back and that is all it should take.

BTW, the Cummins has an ECM that controls both the engine and the transmission, so what would be the purpose of it having a PCM too. PCM's are only for gasoline vehicles, not diesel.

The ECM has one input and that is pressure. It has two outputs (grounded signals), one to the vistronic fan and one to the a/c compressor clutch relay. The ECM doesn't need to know the a/c is on, just needs to know when to protect the system.

 

However, the ECM needs to know when to turn on the A/C clutch... If it ran all the time then yes it has all the inputs/outputs necessary. However, when you rotate the **** on the dash to turn the A/C on, how does the ECM know to kick on the compressor?

 

ECM doesn't need to know its on. You kick on the compressor when you turn the switch, the ECM just provides the ground to the relay which it can kill to protect the compressor.

The ECM sends a ground to the a/c compresser clutch relay. The relay has 4 wires. Battery (+) hot all times, ignition fused which comes from the control switch in the cab, ground from the ECM and signal to the clutch when the relay is energized. ECM puts out a ground unless it gets an over pressure from the a/c pressure transducer, then kills the ground signal so the relay de-energizes stopping the a/c compressor. So the ECM doesn't need to know the a/c is on, it is there to protect it.