My truck throws P0501 code as soon as it travels even a quarter mile at road speed, there are no other symptoms.

2002 Dodge Ram 2500 Cummins with 47RE auto.

This truck had the transmission rebuilt before I got it and one of the front wheel bearings replaced. I have replaced the other front wheel bearing and wheel speed sensor, the transmission speed sensor and the rear axle speed sensor(which causes complete brake failure BTW). I have also replaced the ECM. The ABS appears to work OK, as hard breaking on gravel causes pedal pulse. Does anyone know what the P0501 code really means? Is it a difference in one of the 3 wheel speed sensors or a difference in the transmission tail shaft sensor and wheel sensors? Also does the ABS module actually store codes or just send VSS to the ECM and it sets code. BTW the speedo is working well. I had a Smarty tune installed, but changed back to stock for test with no change. When I had the Smarty installed, I also changed the tire size to 30" in the ABS which made the speedo dead on. I talked to Bob Wagner @ Smarty and he doesn't believe it is an ABS or Smarty issue. Any help is greatly appreciated, as this drives me nuts.

 

 

 

did you read through the article yet?

 

 

the ABS computer can store codes that are only readable using very specific tools, but this code is not in the ABS computer. 

Thanks for the reply, yes I have read every article I could find including that one. It doesn't say what actually causes this code, just alludes to speed differences, but not what speed differences. I don't know if it is the ABS saying the three wheel speeds don't match or it is a difference in the ABS VSS and something else. I would think if it was in the ABS that it would set one of the brake MILs.

 

it's not the AB Scomputer giving the code the p0501 is a pcm code.

last time I check a VSS sensor was like $15.  I would replace it for good measure.  I think the code is saying that hte ABS computer is not reporting a speed from the VSS when speeds for the rear sesnor is greater than 20 mph

As I said in my OP, I have replaced both the speed sensor in the tranny and rear differential with no change.

 

I was looking for the 3 letter acronym to see if you posted that you changed them 

I did some more testing today. If I unplug the ABS I get an ABS warning light, but no P0501 and of course no speedo. If I unplug just the two front wheel sensors I get an ABS light, working speedo and just the P0501 code. I then hooked up my scope and went for a drive. The two front sensor outputs at the ABS module have almost identical frequency (roughly 330 HZ) and amplitude of about 8 VPP at 30 MPH. there is a very slight difference in frequency due to the right tire having more wear. The rear sensor is about 15 VPP and about twice the frequency. I would not think the slight difference in tire size would cause this, or it would set just going around a turn in the road. Anybody know if these signals are normal?

Front wheel scope shot

L front and rear scope trace, sorry for the scope leads:

 

Edited September 6, 20168 yr by RMaynard

Well I am out of ideas. Checked the signal from transmission speed sensor at C2, pins 25 & 28 of PCM, it is a fairly decent sine wave that is about 6 VPP. Checked the ABS speed signal at C2 pin 27 of PCM, it is a nice TTL square wave (5 VPP) and is about 100 HZ at 40 MPH. The PCM has been reflashed and the sticker says it is P56040218AC which appears to be the correct PCM for this truck. Is it possible the PCM programming is bad? I looked on Tech Authority and there is no listing for this module or my truck by VIN, which I thought odd.

I'm not sure if this video will be of any help.

Dodge 1999 RAM Cummins 5.9L NV4500 5spd. 2WD
ABS sensor signal from the rear differential  @ 6 Mph.
(YELLOW) pin1+ and pin8- @ abs connector.
Out to PCM (BLUE) pin 12 @ abs connector.
 

In my never-ending quest of reverse engineering the CCD BUS i spent a lot of time with the ABS unit.
Anyway your issues seems to be the programming of the PCM.

It seems that the frequency of the differential rear speed sensor should be divided by 10 and that will give you the nice square wave frequency.

 

EDIT:

Ok so I did some math it appears to be 25Hz per mile.
formula:
25Hz x 40mph = 1000Hz

To get what the PCM requires
We're going to divide 1000Hz \ 10 = 100Hz

 

Edited September 7, 20168 yr by Chris O.

I appreciate you taking the time to do this. It looks like the two trucks are pretty much the same. That CCD bus is a mystery. It appears the ABS is the only module that sees wheel speed sensors directly. It processes them and sends the square wave to the PCM, the thing I don't know is does the ABS send speed information over the CCD bus to the instrument cluster for the speedo or does the PCM process the square wave signal and send speed info to the CCD bus. I am going to look for a used PCM and give that a try, but I could not find if the PCM needs vehicle specific programming or not. Looking on Tech Authority my part number is not listed at all which makes me wonder.

Quote

does the ABS send speed information over the CCD bus to the instrument cluster

NO

Quote

the PCM process the square wave signal and send speed info to the CCD bus

YES

 

Let's start with some info about CCD bus first.
CCD bus system was introduced in 1988 (end-of-life about 2003).

 

The Chrysler Collision Detection (also referred to as CCD or C2D ) data bus system is 
a multiplex system used for vehicle communications on many Chrysler Corporation vehicles.

Within the context of the CCD system, the term “collision“ refers to the system’s ability 
to avoid collisions of the electronic data that enters the data bus 
from various electronic control modules at approximately the same time.

 

The Chrysler system allows an electronic control module to broadcast message data out onto the bus 
where all other electronic control modules can “hear” the messages that are being sent.

When a module hears a message on the data bus that it requires, it relays that message to its microprocessor. 
Each module ignores the messages on the data bus that are being sent to other electronic control modules.

 

On the CCD bus:
 The Controller Antilock Brakes (CAB)
  CAB is listening to VIN number messages from Powertrain Control Module (PCM). (Only few of the Vehicle identification number characters)
  CAB is listening to diagnostic scan tool messages. (Reading codes, Activate the ABS solenoids and pump , Speedometer calibration) 

ABS(CAB) On the CCD bus Is only sending one message once every one second, Status message. (Control of the lights in the Cluster gauge ABS, Brake, 4WD Lock, Brake switch sense etc.)

If the ABS(CAB) Status message is missing on the CCD bus the Cluster gauge will illuminate the amber ABS light.

 

Edited September 7, 20168 yr by Chris O.
Few more additions.

So is the PCM just blasting CCD messages out to the other devices?  Is it a bi directional communication?

 It is sort of bi directional communication.
 Cannot communicate between each control modules at the exactly same time.

 For example if you connect scan tool, you can send specific message to the ABS(CAB) asking for DTCs codes.
 But the reply will be on the next message transmitted from the ABS(CAB).

and the PCM is the "host" of the ccd network?

No not really, you can remove the PCM.
The body control module, abs, cluster gauge, ecm, still can talk to each other.

 

There's just one problem:
Bus Bias And Termination.

The voltage network used by the CCD data bus to transmit messages requires both bias and termination.
At least one electronic control module on the data bus must provide a voltage source for the CCD data bus network known as bus bias, and there must be at least one bus termination point for the data bus circuit to be complete. 

Now I'm not sure if the PCM is supplying the voltage source or the cluster gauge?

Quote

So is the PCM just blasting CCD messages out to the other devices? 

The messages on the CCD-bus are flowing in a series a bytes. One message is typically 3-6 bytes long.
They are repeating the same thing over and over barely changing the bytes. 
Essentially the communication is done via sending sequence of numbers between 0 and 255.
Every message starts with an identification byte (ID byte). The closer it is to the zero the more important information it holds. In case of message collision (two modules are trying to say something at the same time) the message with the lower ID value wins the bus.

I checked the CCD bus a couple of years ago and think the bias is from the cluster and there is a terminating resistor in the PCM.

Does anyone know how bitchy the system would get if I subbed another PCM from an identical truck to see if that solved the p0501 code. Neither have SKIM modules. I assume the PCM stores VIN and mileage info.

Solved. The ECM was bad, apparently it was not processing the 5V speed signal on pin 43, this caused it to throw the P0501 code as it was receiving the speed on the CCD bus. In fact when I swapped the ECM I checked for codes and there were none set, this confirms the code is set in the ECM, as I did not clear them before the swap. The P1693 in the PCM just went away after the swap also. The strange thing is, it did not seem to have any effect on the vehicle performance, makes me wonder why they even bother to have two separate VSS signals going to the ECM. I hopes this helps someone else.

What did you do for a new ECM?

dripley, I just got a used one and programmed it with my Smarty. From what I understand they are all the same from 98 1/2 to 2002, auto or stick, the only difference is programming.

This is the third ECM. The first one got thermally intermittent and the gauges would all quit working and the engine idle changed. The second was the P0501 code. There are three things that kill electronics, heat, vibration and voltage spikes. So what did Dodge/Cummins do? Bolted the ECM to the engine block and provided no filtering for the spikes from: VP44, lift pump and alternator, stupidest thing I have ever seen.

My next project is to relocate the ECM to the inside of the cab with an extension cable, I have spare plugs now from all the failures. As these things have been enduring this abuse for 14+ years, I suspect they are going to be dying like flies from here on out, and they are getting scarce and expensive.

BTW when you install a smarty tune it is normal for the ECM to throw a P0601 (Fuel calibration) error. The replacement ECM didn't throw any codes with the stock tune from my Smarty. Hope this saves someone some grief.

Edited September 22, 20168 yr by RMaynard