PSOM or PCM

8644 Views 27 Replies 3 Participants Last post by SigEpBlue, Nov 18, 2009

Ltsboards

Discussion Starter · Nov 14, 2009

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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SigEpBlue

· #2 · Nov 14, 2009

Follow this procedure EXACTLY, from the Ford TSP/PCED:

DS1 DIAGNOSTIC TROUBLE CODES (DTCS) 29 AND 452: CHECK PSOM RESISTANCE

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?

Yes
GO to «DS4».

No
GO to «DS2».

DS2 CHECK CONTINUITY OF THE HARNESS CIRCUITS

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?

Yes
GO to «DS3».

No
SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».

DS3 CHECK HARNESS CIRCUITS FOR SHORTS TO POWER AND GROUND

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?

Yes
GO to «DS4».

No
SERVICE short circuit. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».

DS4 CHECK RABS SENSOR RESISTANCE

l Key off.

l RABS disconnected.

l Measure the resistance of the RABS.

l Is resistance between 1300 and 1550 ohms?

Yes
GO to «DS5».

No
REPLACE the RABS. REMOVE breakout box. RECONNECT all components. RERUN «Quick Test».

DS5 CHECK PSOM OUTPUT VOLTAGE

WARNING:
THIS TEST STEP REQUIRES ANOTHER PERSON TO ACCOMPANY THE DRIVER TO MAKE MEASUREMENTS AND RECORD DATA. ALL APPLICABLE SAFETY PROCEDURES AND TRAFFIC LAWS MUST BE FOLLOWED.

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?

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

No
REMOVE breakout box. RECONNECT components. REFER to the Instrument Cluster, Speed Control or Brake Group in the Service Manual for further diagnosis.

Ltsboards

Discussion Starter · #3 · Nov 14, 2009

ok quick question about that and it may be stupid. Whats the breakout box?

SigEpBlue

· #4 · Nov 14, 2009

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 (fordfuelinjection.com).

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

joshhath

· #5 · Nov 14, 2009

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.

Ltsboards

Discussion Starter · #6 · Nov 14, 2009

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?

SigEpBlue

· #7 · Nov 15, 2009

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.

Ltsboards

Discussion Starter · #8 · Nov 15, 2009

Ok ill get in there and back probe the wires. Ill let you know what i find out.

Ltsboards

Discussion Starter · #9 · Nov 15, 2009

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.

SigEpBlue

· #10 · Nov 15, 2009

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.

Ltsboards

Discussion Starter · #11 · Nov 15, 2009

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?

SigEpBlue

· #12 · Nov 15, 2009

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:
http://www.supermotors.net/registry/2742/69150-2

Ltsboards

Discussion Starter · #13 · Nov 16, 2009

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.

SigEpBlue

· #14 · Nov 16, 2009

Continue on to step DS3. :thumbup You're getting warmer.

Ltsboards

Discussion Starter · #15 · Nov 16, 2009

It says if one is over 5 to repair that circit. One is over 5. How do i repair it.

SigEpBlue

· #16 · Nov 16, 2009

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.

Ltsboards

Discussion Starter · #17 · Nov 16, 2009

Thats what I thought:doh0715: ok let me get to work.

Ltsboards

Discussion Starter · #18 · Nov 16, 2009

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?

SigEpBlue

· #19 · Nov 16, 2009

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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Ltsboards

Discussion Starter · #20 · Nov 16, 2009

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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