PCM repair

[bigd]

Where is the best place to send my PCM for repair.My PCM actually work good except signal for AC is dead.I tested wire coming out of PCM,it is suppose to have 12v.I thing I burned out that part when chasing dead wires for something else.  Thanks.

[Haggar]

D,

 

no, the PCM when controlling the AC clutch relay, only connects the negative side of the relay coil to ground.  It will NOT give you a 12v +  signal.  It will only become continuous with ground.

 

See the wiring diagram I attached.

 

Let me know if you need to know how to test that.

 

GL

 

Hag

2001 FSM Ram 8W-42-5.pdf

[bigd]

Thank you Haggar.Yes I will need help with testing this problem.

[Haggar]

D,

 

Ok,  I am going to assume you have a Volt ohm meter (VOM).   (digital or analog is fine) 

 

Set the meter to measure ohms.  Connect one lead to the NEGATIVE side of the battery.  (you could connect it anywhere on the chassis just make sure it is connected to ground, and VERY GOOD.)

 

Remove fuse J and fuse 11. (I put this step in before I realized the FSM tells you exactly which contacts are which) This will keep you from accidentally blowing those or your meter.  (fuse 11 also has the central timer relay on it, so automatic locks and such may not function properly while it is out)

 

With one end of the VOM still connected to ground, use the other to probe the slot that corresponds with terminal 85 on the clutch relay socket .  (if you are unfamiliar with the din terminal designations, there should be a wiring diagram of the relay on its side.  also sometimes if you look closely near the pins on the relay, the terminal number may be molded or printed there.)

 

I have attached the page from the FSM which shows the actual pin designations to locations in the PDC. 

 

When the ac is commanded ON.  your VOM should show a very low resistance to ground.  (0.1 ohm type number)  It should be similar to the same number you get when touching your two probes together.

 

When the AC is commanded OFF. you should read infinite resistance, or the same number you seen when holding the two probes very far apart.  (mine actually reads OPEN LINE, not a number) 

 

This is how it should react when operating properly.  You will at least know that the PCM is not your problem.  (well trouble shooting is more complicated....  but I can step you through this if need be.) 

 

HTH
 

Hag

 

 

2001 FSM Ram 8W-10-2.pdf

Edited July 19, 2017 by Haggar

[bigd]

Thanks again Haggar.I will be home tomorrow and will try this.

[Haggar]

D,

 

No worries.  The above will tell you if you are getting a command out from the PCM.   If you do see it switching, you will know your problem is just in the compressor clutch relay, or compressor clutch circuit, or the clutch itself. 

 

We kindof started this trouble shooting backwards.....

 

For our AC to work many things must happen.

 

1.  A request to the PCM for air conditioning must be made.   This is an electrical part of the knob on the dash.  You only get AC request in certain positions. (see pages 8w 42-3 and 8w 42-4)

 

2.  The PCM must see the proper signals from both the high and low pressure switches in the lines.  (see the FSM pages 24-20 to 24-22 I attached) 

 

3. An output signal will be generated (the one you are testing in the post from above) to change the states of the relay.

 

Pins 22 and 23 on the PCM are the above inputs.   (see FSM page 8w-42-3  and 8w-42-4)

 

Pin 23 (AC select input)  will see a ground signal  from G201 Through the fan switch on the light green/white wire. (please excuse my ignorance in this area.  I have not had the actual heater controls out and actually tested them... I am totally making a mental model from the wiring diagram...)  You can check this actual request by using the ground ohms test like the previous post.  (with AC in OFF, there should be no continuity.  As you move through max ac etc, you should see the meter become continuous with ground, until you hit certain modes that don't call A/C then you will get A/C signal again with defrost modes.

 

Pin 22 (A/C switch sense)  Again from the G201 connector you should see ground anywhere along this path.  Both the high pressure and low pressure switches are in series.  The high pressure switch is normally closed. (it opens at approximately 450psi)  The low pressure switch is NORMALLY OPEN. It only closes if there is enough refrigerant in the system to have more than 37-43 psi on the switch.  

 

 

I hope this helps.   It is very odd for AC troubles to be a PCM issue.  Of all the A/C problems I have seen, it was never the PCM, but always somehow the sensors or wiring or command to the PCM to be the issue.

 

Hag

2001 FSM Ram 8W-42-3.pdf

2001 FSM Ram 8W-42-4.pdf

2001 FSM Ram 24-20 to 24-22.pdf

[bigd]

Hi Haggar

I have it tested and while truck running on ON and OFF position I have 0 reading.When only ignition ON in OFF position it read 30 ohms and when switch is On it read 0.

While no engine on and with ignition off It read around 60 ohms in booth  positions.

Little background,I did frame of restoration on this truck.Before disassembly it was all working.When I put it together IOD fuse was blown because I shorted mirror wires.I didnt recognize it.I was chasing wires ,testing it and while testing ac wires I did hear something in PCM hapened,lik short(like when you touch positive and negative wires). After

I found short on mirror and replaced IOD fuse everything work except AC.I did test compressor by applying 12 v and it is working. If I touch Wire coming out of PCM with light probe compressor start working.Thanks for your time and help,      Dmitar.This name explain my English.

Edited July 20, 2017 by bigd
spelling

[Dieselfuture]

It almost sounds as you may have a lose ground somewhere 

[bigd]

Hi Dieselfuture

I did check all grounds,cleaned and tested,everything look good.Almost everything is new on truck and everything is in working order.

[Dieselfuture]

You might have something cooked in side PCM after all. It could be something as simple as a resistor.  Don't know if it's hard to open and look for obvious burnt parts, may worth a try. I just fixed a 95 suburban, ac wasn't getting as cold as it should and pressure was good in system. Ended up being a vacuum solenoid that shuts a valve to heater core, but it wasn't a solenoid it wasn't getting power. Traced down to control on the dash, it wasn't putting out juce. Ended up being a resistor in control board. I think it was 3.8 ohms I didn't have one so there are 3 at 1.2 total of 3.6 but it works now. Could be something similar with you.

[Haggar]

D,

 

Good.  From your testing you do know that the compressor will work, if it gets the proper signal from the PCM.

 

This could be PCM, or most likely either the command to the PCM from the a/c switch is missing, or the "allow" command from the high and low pressure switches is missing.

 

Can you disconnect your PCM wires and feel confident testing circuits there?(i hope you can, this will get you the most information quickly) 

 

I would very first thing (look at FSM pages 8w 80-69 attached): With the A/C mode switch in one of the AC positions, disconnect the C3 connector.

    Perform 2 tests. 

     1.  Check continuity to ground for pin 22 (on the wiring connector, not the PCM).  This will ensure the ground is connected, and that both high and low pressure switches are in the proper position for the AC to function. (you can do this test with truck  not running.  The position of the ignition switch does not change the wiring.)

 

     2.  Check continuity to ground for pin 23.    This should have continuity to ground in the proper modes.   The continuity should disappear if the mode is changed to a non AC mode.  (This should also work without starting the truck.  you are testing the wires to the PCM and the position of the ignition switch should not make a difference)

 

 

I know what you are saying about the PCM.  It is possible to have killed it.  But, of all the components in the PCM you could have killed, this is the least likely.  I find it VERY hard to believe that a short would have killed only that path, without disturbing other similar paths.  I hate for you to waste your time and money on a very unlikely path without testing the areas where the most problems occur.  

 

Hope that helps!

 

Hag

    

2001 FSM Ram 8W-80-69.pdf

Edited July 21, 2017 by Haggar

[bigd]

Hi .

I did test for continuity pin 22it has on all positions,Pin 23 dont have on any position.What this mean?Thanks.

 

[Haggar]

D,

 

Sorry,  being so late to this.  Had trouble logging into the website.

 

Great job!  

 

Very interesting....  According to the drawings on 8W 42 3 and 8W 42 4.  The ground source is G201.  it comes through the switch (toward the PCM)  and at junction S120  they split apart.      What year is your truck?  Are we using the correct wiring diagram for yo

 

Both pins 22 and 23 should have switched with the switch.   This tells me that there is a wiring problem or there is a wiring problem and a switch problem.  ( What year is your truck?  Are we using the correct wiring diagram for your system?)

 

Assuming the wiring diagram is correct, If you feel certain that both the high and low pressure switches work properly, ( you could retest pin 22, and disconnect the high and low pressure switch connectors, making sure that the ground reference goes away)    you could put a toggle switch in, (connecting ground to pin 23.) this would allow the air conditioning compressor to work.  

 

I hope this helps!

 

Hag

 

 

 

[bigd]

My truck is 2001.I will try yo do this when I come back home next week Thanks again.

[Haggar]

D,

 

Great.  The wiring diagram we are using should be valid then. 

 

Good luck!

 

Hag