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Ask Kenny Z. Your Own Question
Kenny Z.
Kenny Z., Ford Senior Master Tech
Category: Ford
Satisfied Customers: 5447
Experience:  Auto Trans, Drivability and Electrical Specialist
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97 F250: 4x4..intermittent flashing OD trans light..PCM..dealership

Customer Question

97 F250 HD 4x4 7.5L with intermittent flashing OD trans light, transmission slams between gears, and code 452 (Veh Speed sensor inop) in PCM. Replaced VSS, no change. Replaced PCM 3 times (under dealership diagnosis), no change. Speedo/Odom works all the time. Have breakout box for analysis. Need a real diagnosis besides a dull stare and "Replace PCM" again... thanks in advance for any clues...
Submitted: 9 years ago.
Category: Ford
Expert:  Kenny Z. replied 9 years ago.

I am doing some checking,back soon with some ideas............

Expert:  Kenny Z. replied 9 years ago.

If the ABS lite is staying off and the speedometer never quits then we can assume all the wiring is good from the vss to abs module and cluster.If the vss was bad you would have alot more going on then just trans issues.The vss+ and- signal to the pcm and speed control servo is supplied from the instrument cluster on pins 2 and 7 from the cluster,wire colors pin 2 pink/orange, pin 7 gray/black.The PSOM(programmable speedometer odometer module) actually generates a new vss signal for the pcm and speed control.If you have speed control and it works (while the problem is occurring) then we can assume the psom is ok as well.If thats the case I would then proceed to go after the 2 wires I mentioned earlier.The first wire I would go after is the gray/black wire (circuit 679) between splice S246 and connector C185 at the PCM for the positive side.For the negative side I would check the pink/orange wire (circuit 676) from splice S216 through connector C202 and connector C185(pcm connector).I would also be sure to check for good continuity to chassis ground on this wire.If I didn't find an issue with the wires I would then overlay them with new wires pin to pin.

Main harness, near T/O to speed control amplifier

Main harness, near T/O to G200 (see diagram for location)




Customer: replied 9 years ago.
Reply to Kenny Z's Post: Hi Kenny...

   Thank you for your idea, sounds far more informed than any I have gotten from the dealers. I am going to drive until the problem comes up, then engage the Speed Control to see if it works. I haven't ever used it before. If it fails as well, sounds like you might have a different thought on the cause (PSOM?). If it works, I will proceed as you suggested. I will reply soon and tell you what I found.

By the way, if you help me find the cause of this (I've spent months and hundreds of $$), I'm going to be real happy to give you a good bonus for your time, too. Thanks for looking into this, it's good to talk to someone who knows what's going on...
Expert:  Kenny Z. replied 9 years ago.

Its no problem, I really like going after these kind of problems.Keep me informed and hopefully we can get it fixed.

Take care

Kenny Z. and 2 other Ford Specialists are ready to help you
Customer: replied 9 years ago.
Reply to Kenny Z's Post: Kenny...

Took test drive and found when problem starts and OD light flashes, the cruise control immediately fails. Speedo/Odom still works normally. Later, OD light will extinguish, cruise control works, and all operates normally again.

Should I follow your original advice, or does that change things...
Expert:  Kenny Z. replied 9 years ago.
There is a couple of ways to go about it.Either we trace it down and fix it by eliminating wires by overlaying them,which involves pulling the psom making up overlays with pins and installing the overlay.This is a lot of work for a ford tech - are you planning on doing this or do you have someone to do this for you? I am assuming the wires from the splices to the pcm are ok since the cruise fails.You would have to go after the wires from the psom to the splices now.My gut feeling is the psom is bad but in these type situations I like to know the wiring is good.So its up to you how you want to proceed.The positive thing is you are a lot closer to fixing this thing!
Customer: replied 9 years ago.
Hi Kenny...

This question is from a month ago, so I'll recap. My 97 F-250HD has a bad intermittent tranny shift problem. (See above) You suggested the problem might be a bad PSOM not sending the speed signal to the PCM, either cause of a bad wire or bad PSOM. After some further tests, this is what I found.

Performed DS1-5 Pinpoint Test for Code 452, PSOM diagnosis.

DS1) PSOM resistance between test pin 3 and 6 at breakout box checks ok at 22,210 ohms (between 21,000-55,000 ok). Goto DS4.

DS4) RAB sensor resistance checks ok. Goto DS5.

DS5) Check PSOM Output voltage. Measured AC volts between Test pin 3 and 6 at 50 mph. Voltage is 4.37 AC, spec calls for maximum of 4.0V. If voltage is over 4.0, the routine says "REPLACE PCM".
This explains why dealers keep saying to replace PCM, although the PCM has nothing to do with that reading.

My question to you is this, if the PSOM output voltage is out of spec, shouldn't you replace the PSOM, not the PCM? From the diagrams you sent me, the PSOM sends the speed signal voltage to the PCM and the Speed control. If the PSOM was bad, this would explain why the PCM controlling the transmission and Speed control fail at the same time. The speedometer always works, but do you think it's possible the PSOM is sending good signal to the instrument cluster, but bad signal to the PCM? I just can't see how changing out the PCM would help an out of spec PSOM output voltage. My thought is to replace the PSOM because of the overvoltage output. I don't know if a 10% overvoltage is enough to cause problems, but maybe. I would think if it were a bad splice, then the voltage would read 0.

I'm sending another $20 for your time to check my logic here. If I'm missing something, please let me know. By the way, thank you for the schematics, they really helped me understand what the problem could be.

Expert:  Kenny Z. replied 9 years ago.

Good to hear back from you,

I still think a continuity check is in order from the psom to the splice and the ground side of the circuit both performed while bending and moving the wires that are accessible.If no faults found then I would be more comfortable replacing the psom.There is nothing wrong with the pcm.The voltage reading is only valid if the problem is occuring,if its not acting up at 4.37 volts then we know this voltage is acceptable even if its a little out of range...........Thanks for the $20