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8644 Views 27 Replies 3 Participants Last post by  SigEpBlue
Ok here is the problem. I am getting code 452 VSS. I have replaced the vss with a new moto one and it did not clear the code. The speedo is working fine as well as all the other gauges. I unpluged the vss to see if the speedo would go out and it did Also did the simple PSOM test and got the full sweep. Based on this i assume the signal is making it to the PSOM. But I am still getting code 452. Now I am assuming that the problem lies between the PSOM and the PCM or the PCM and the transmission. I see the connector 679 GY/Bk is the output of this signal to the PCM. I dont know how to check if the signal is leaving the PSOM and entering the PCM or getting to the transmission so i dont know which is bad.

Is there an easy way I can ckeck if the signal is getting to the PCM? And if it is not do I then assume i need a new PSOM? Im kind of at a loss.
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Follow this procedure EXACTLY, from the Ford TSP/PCED:


Continuous Memory DTC 29 indicates that during the last 80 warm-up cycles, the PCM detected an error in the PSOM output signal. DTC 452 indicates the PCM detected an error in the PSOM output signal during the last 40 warm-up cycles.

Possible Causes:

-- Damaged Rear Anti-Lock Brake System (RABS).

-- Damaged PSOM.

-- Damaged harness circuits.

-- Damaged PCM.

l Key off.

l Disconnect Powertrain Control Module (PCM). Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.

l Install breakout box, leave PCM disconnected.

l Measure resistance between Test Pin 3 and Test Pin 6 at the breakout box.

l Is resistance between 21,000 and 55,000 ohms?

GO to «DS4».

GO to «DS2».

l Key off.

l Breakout box installed, PCM disconnected.

l Disconnect PSOM.

l Measure resistance between Test Pin 3 at the breakout box and PSOM(+) circuit at the PSOM vehicle harness connector.

l Measure resistance between Test Pin 6 at the breakout box and PSOM(-) circuit at the PSOM vehicle harness connector.

l Is each resistance less than 5.0 ohms?

GO to «DS3».

SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».

l Key off.

l Breakout box installed, PCM disconnected.

l PSOM disconnected.

l Measure resistance between Test Pin 3 and Test Pins 6, 37 and 40 at the breakout box.

l Is each resistance greater than 10,000 ohms?

GO to «DS4».

SERVICE short circuit. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».

l Key off.

l RABS disconnected.

l Measure the resistance of the RABS.

l Is resistance between 1300 and 1550 ohms?

GO to «DS5».

REPLACE the RABS. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».


l Key off.

l Breakout box installed.

l Reconnect PCM to breakout box.

l Reconnect PSOM.

l DVOM on 20 volt AC scale.

l Warm engine to operating temperature.

l Measure AC voltage between Test Pin 3 and Test Pin 6 while gradually increasing vehicle speed to 50 mph.

l Is the maximum voltage received greater than 4.0 volts?

REPLACE PCM. REMOVE breakout box. RERUN «Quick Test».

REMOVE breakout box. RECONNECT components. REFER to the Instrument Cluster, Speed Control or Brake Group in the Service Manual for further diagnosis.
ok quick question about that and it may be stupid. Whats the breakout box?
It was a diagnostic tool that allowed a technician to easily probe the PCM's pins; basically a box with two large connectors, one to the PCM, the other to the vehicle harness. The pins called for on the BOB are the same as the pin numbers on the PCM, thankfully.

Without it, you'll have to do it the old-fashioned way, and backprobe the pins at the PCM (not advised since there's a silicone gasket in there), or pierce the wires with a conductive pin (safety pins work great) and measure the signals from that. I'd advise you to find a complete schematic of your PCM, hopefully with the wire color codes included. They're usually in the back of the Haynes manual, or on Ryan's site (

An example: pins 3 and 6 are the VSS inputs to the PCM. #3 is Gray/Black, #6 is Pink/Orange.
Breakout box

check ebay for Breakout boxes. I just picked one up brand new for $200 (retail $800). I also saw a used on listed as buy-it-now for $135 when I was logging in to pay for the one I won. Dangit.
Is buying a break out box worth it in this situation or is my money better spent taking it to a shop and let them figure it out for me?
It's best to just try solving it yourself first. If you can measure the voltage of a battery with a meter, you can troubleshoot this problem. I wouldn't recommend the box for anyone but a Ford mechanic, really.
Ok ill get in there and back probe the wires. Ill let you know what i find out.
OK i got the PCM harness and checked pins 3 and 6 and got 14800 ohms. Low. So now I need to disconnect the PSOM and keep testing. Um where do I diconnect it to get to the + and - inputs.
The PSOM is mounted to the back of the speedometer. So, it requires removing the instrument cluster. Fear not! It's not that hard! Just take your time. :thumbup

Remember that the two main connectors on the instrument cluster require you to press/pinch INWARD before they can be pulled out. The one for the PSOM is a little different.
OK ill get to that as soon as i get a few more free moments. Will the + and - be clearly marked. What should i look for on the back. I think i found that the + is pin 4 4wabs in and - is pin 5 4wabs return. or does the + and - mean Battery in and Ground. Is that right? And im assuming that if i find a short in the wires that it means i need to trace the wire and find where it is broken and repair it?
According to my schematics, at the PSOM, the (+) line is Orange/Light Blue, and the (-) line is Light Green/Yellow.

For reference, you may want to look at this page, but take special note of which years the diagrams refer to:
OK got the PSOM Pins and Pin 3 to + tested over 5 ohms and pin 6 to - was under 5 ohms. Now that means there is a short some where. Where is it? what should i look for next.
Continue on to step DS3. :thumbup You're getting warmer.
It says if one is over 5 to repair that circit. One is over 5. How do i repair it.
Trace the wire back through the harness. Look for physical damage of any kind, and at the end connectors for pull-out and/or corrosion.
Thats what I thought:doh0715: ok let me get to work.
Ok I Omed out the 2 wire going from the 4wabs and the bothe tested the same. Could the short be further back in the line between the sensor and the 4wabs ecu?
It's very important to understand the VS signal path here. Its origin is the sensor on the rear differential. From there it goes directly to the 4WABS control module, and is then passed, unaltered, on to the PSOM. The PSOM then interprets that signal, and creates outputs to the speedometer needle motor AND directly to the PCM.

By your tests, I can only assume there is an open circuit (note that this is different than a short circuit, as "short" denotes an inadvertent connection to ground) between the PCM and the PSOM.

Is your 4WABS functioning normally? IIRC, your speedometer is working properly, so that indicates that the signal is good up to that point, e.g. VS --> 4WABS --> PSOM is working, but PSOM --> PCM is questionable and possibly your open in the signal path.

If you're in doubt, start at the diagnostic procedure's beginning, and follow it through, observing the test outcome jumps, e.g. YES go to step DS3, NO go to step DS4, etc. These diagnostic procedures were designed by Ford to be thorough, accurate, and above all FAST ways to diagnose problems in the dealership service bays. That's why I always recommend consulting them first before guessing or throwing parts at problems. They are the de facto standard in keeping these trucks going strong.
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OK i think I understand, Here is where im lost. On the second step of the test I measured ohes between the + 4 pin O/LB on the PSOM Harness and the 3 pin GRY/BLK on the PSOM. That test measured above 5 ohms and thus the open circuit is here some where. I guess im not understanding the path of this wire.

I see that the O/LB runs back to the 4WABS ECU from the PSOM, and I think the GRY/BLK from the PCM is going to the PSOM? is that where my open is? In the Gry/BLK from PSOM to PCM?
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