Hello,

I have a problem with my 2001 Ram 1500 3.9L 2WD truck. When I turn on the A/C, I get a trouble code for the TPS voltage high and sets the check engine light. I also get bucking and sagging at ~1600-1800 RPM. The only way to operate the vehicle normally is to remove the A/C clutch relay.
Another earlier post from SST_Ram says a faulty PCM will cause 12 vdc to be sent to the TPS to cause this to happen.
Another post says the cost of replacement is ~$480.00 and the dealer still has to flash the PCM.
Does anyone have schematics or troubleshooting notes for this problem and for the PCM itself. I can replace the fault chip(s) if I knew what they were and how to get to them.
Thanks in advance for the help,
Steve

 

I am willing to bet the electronic schematics for the actually PC board/s for the PCM are going to be either EXTREMLEY hard to get, or impossible, more like the latter of the two. Unless B&G or Hypertech, companies that program the PCM's aftermarket would have some info on it.

Look into the junkyard. That maybe an option for you for a PCM, and cheaper.

 

Get one from a junk yard, and then have the dealer flash it. You will get a bunch of crap in the dealer flash, but this is probably your cheapest route. The used pcm doesn't have to match your truck exactly, but the closer the better. IIRC Motorolla manufactures the pcm's. You could try to contact them to get the schematics, but they will probably consider them proprietary. I doubt that they will give them to you.

Also, if you haven't done it yet, you could download the 2001 service manual from the Merry Christmas sticky at the top of this forum. It has a list of the pin-outs for the pcm. This would enable you to open the pcm up and see wich chip(s) that they are controlled by. The chips you should be able to get. Worth a try??????

 

Since my last posting in this topic, I called the Dodge dealership in my area. I asked if the PCM was still within the mfg. warranty. At first they said it wasn't. I politely said I thought it was as per my warranty manual that came with the truck. He put me on hold for a minute. When he came back he confirmed if the PCM is indeed the problem it will be covered by the mfg. 8 year/80,000 mile coverage.
I have an appt. for Friday to take it in. I'll be sure to let you all know how we made out. Thanks for all your replies.
Steve

 

Since my last posting I did some more clarifying of the troubleshooting steps and wording of the description of problem and notes. I'd like to do this for the next guy that has a similar problem, trying to help him as you all have helped me in the past. I tried to keep all the tabled results in columns, otherwise none of the results will make sense. See below entry.

DESCRIPTION OF PROBLEM(S)
Since late last year, shortly after the vehicle purchase, I noticed a “sagging” of the engine at ~1600 to 1800 rpm. It would run fine above and below that rpm. I notified the service manager at the time. The problem became more pronounced about two weeks ago when the truck started to “sag” and “buck” at that rpm, followed by a slight surging in rpm just above ~1800 rpm. At that time I noticed the check engine light came on with a trouble code of “P0123 – TPS sensor high voltage.” I checked the TPS static and dynamic operation as “good.” I also got a MAP sensor code of "P0121 – TPS voltage does not agree with MAP". The MAP sensor voltages were also within specs. The “no power” pin-to-sensor continuity checks were fine between the PCM connections and the sensors. Since no other explanations were plausible, I replaced the TPS and MAP sensors and reset the codes. Everything was fine for a day or so, then I happened to observe that within a few seconds of turning on the A/C, the TPS code came back and the engine started acting up at the same spot mentioned above. When I checked the static and dynamic voltages as stated above, the A/C was in the “off” position. I pulled the A/C relay, reset the code, started the truck and turned on the A/C. The code didn’t come in with the A/C relay out. I next replaced the A/C relay and disconnected the A/C clutch connector located above the valve cover. I again started the truck and energized the A/C. The code and erratic engine performance returned. I also swapped the wiper relay with the A/C clutch relay with no change in symptoms.It appears that anytime the engine is running, with the A/C relay inserted, and the A/C circuit is energized, even if the A/C clutch connector is disconnected, the check engine light comes on, the code "P0123" is set, and the engine runs erratically at ~1600-1800 rpm. I found a discussion thread on the internet “Dodge Forum” website from "SST_Ram." Someone had the exact same problem as I have. Their dealer replaced their PCM module. They proved that when the A/C was energized, the PCM module put 12 vdc down the TPS sensor line, causing the code and "sagging" and "bucking" at RPM problem.


CONDITION A/C RELAY TPS VOLTS TPS SIGNAL A/CSWITCH CHK ENGINE LIGHT?

KEY ON ONLY OUT 5.22 .58 OFF NO
KEY ON ONLY IN 5.22 .58 OFF NO
START ENGINE OUT 5.22 .58 OFF NO
START ENGINE IN 5.22 .58 OFF NO
START ENGINE OUT 5.22 .58 ON NO
START ENGINE IN 5.22 >/=.85 ON YES
DISCONNECT CLUTCH OUT 5.22 .58 OFF NO
DISCONNECT CLUTCH IN 5.22 .58 OFF NO
DISCONNECT CLUTCH IN 5.22 >/=.85 ON YES

NOTES
1. TPS VOLTAGES ALWAYS MEASURED AT IDLE
2. VOLTMETER READS SAME VOLTAGES EITHER ATTACHED TO CHASSIS GROUND OR TPS SENSOR GROUND
3. DIGITAL VOLTMETERS RESPOND VERY SLOWLY TO FAST VOLTAGE FLUCTUATIONS. TPS SIGNAL VOLTAGES, AS A/C IS ENERGIZED, MAY ACTUALLY BE HIGHER THAN VOLTAGES INDICATED

Enjoy,
Steve

 

Sorry the tables didn't come out aligned. If you want a clean copy, reply with your email address and I'll try to send you a .doc file with the data.