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Discussion Starter #1
A week ago I noticed when I got in my truck and started it the OD light showed off, seemed strange but pushed the button and it the light went out. All was fine for a few days untill Wednesday night. The wife and I were headed out and all of a sudden the light started coming on I would push the button and it would turn off. This continued for the whole evening. Next day I pulled the column apart and checked the wires to the switch found a small worn spot, I repaired that checked the rest and put it back together. Still did the same thing! The next day I took it to a trans shop thinking maybe it was flashing ireatic codes, my scanner won't read the trans codes. The tech came out scanned the truck ran some self test and it passed. So he took a test drive with me the light started turning on and he said on the scanner it showed it was activating and deactivating. I figured it had to be a short maybe the swhtch itself? I went and got a new assembly installed it and still the same issue. I the crawled under the truck and checked connections at Mlps, trans connection and pulled cleaned and checked wires to VSS. I'm stumped. All the info. I can find always points to a flashing light which I don't have I have a light turning off and on on its own and no trans or engine codes.
 

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There are two ways that this could happen. The most likely, by far, is that there is another damaged spot in the wiring between the switch and the PCM. Nothing near the trans or VSS could possibly cause this to happen.
The other possibility, though it is a longshot, is that the PCM is bad. That's not very likely, but it is possible.
 

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Discussion Starter #4
Okay. I checked the other things as a precaution that the trans was thrown ng a code that wasn't getting logged. Longshot but as I said I want my truck back I was grasping at anything. I think I found a pin out of the PCM so I will test wiring on tcs circuit and the light circuit I already know I have 12v at the center wire. Thanks for your help!
 

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You can download the Ford service disk free from here: Ford Service Manuals - Google Drive
Burn it to a DVD, copy that to a new partition on your computer HDD with a new drive letter, install from that location, and you'll always be able to access it instantly.
 

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Alright so I tested the wires could never prove a short to ground. I decided to cut pin 29 (TCS) at the PCM connector and jumped it straight to the shifter just to eliminate wiring and make sure the PCM wasn't causing the issue. Sure enough as you've been saying Mark it must be the wiring as everything worked perfect on an hour long test drive. My question now is... Between the trans control switch and the PCM are there any components in line other than the bulk head connector? I plan to pull the dash and find the wiring issue but have to head back out for work for a few days so I was planning a temp fix for now. Want to make sure I'm not going to be screwing anything up by bypassing a component? Thanks Mark for the help so far.
 

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Discussion Starter #8 (Edited)
Well appears I spoke too soon. After another test drive then filling my tanks the issue appeared again. So on the LG/W wire I have 12v with the od light off when the light is illuminated the voltage goes to 0. On the R/Y wire constant 12v then on the T/LB I have fluctuation of .5 to 2v with the TCS depressed the T/LB wire has a momentary 12v. I don't think my meter is fast enough to catch the trigger when it changes state on its own.
 

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My question now is... Between the trans control switch and the PCM are there any components in line other than the bulk head connector?
There is not.
 

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Discussion Starter #10 (Edited)
Alright so I'm down to the PCM it self I believe. I cut the wire going to pin 29 at the PCM so there is no signal on TCS side of the circuit no way anything could be telling the PCM to turn off the OD. I drove a few miles and the light came on and it shifted from 4th back to 3rd then the light went out and it shifted back to 4th. Is there any way a failing Torque converter or BOO switch could cause the OD to turn off and on. Or am I really looking at a failing PCM?
157173
 

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Discussion Starter #11
There is not.
Okay I have checked all grounds and power to PCM. I was able to complete the KOEO test and the switch request test. I jumped both pins 29 and 79 directly to the switch as well as tested power on center wire. I have no codes. Are there any other components are or inputs that would force the overdrive to shut off or turn on?
 

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Okay I have checked all grounds and power to PCM.
OK, but none of them would cause this problem.

I was able to complete the KOEO test and the switch request test. I jumped both pins 29 and 79 directly to the switch as well as tested power on center wire.
Have you removed the pins to the OD switch and light from the PCM and tried running new wires? Somewhere in those wires is a short to ground. There is also a very, very small chance that a bad PCM is causing it.

I have no codes.
This type of problem will never set a code. The PCM can't tell if you are pushing the button or if it is getting a false reading from a short.

Are there any other components are or inputs that would force the overdrive to shut off or turn on?
No, there is not.
 

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Discussion Starter #13
I have ran new wires for both the light and TCS, straight from the PCM connector to the new switch and handle assembly. Even ran with both wires (light and TCS) de-pinned from the conector and not connected to the switch and it still turned off and on while watching it in the scanner and driving. I'm sending the PCM off today for testing we will see what I hear back. Thanks again for all your help.
 

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Discussion Starter #14 (Edited)
OK, but none of them would cause this problem.


Have you removed the pins to the OD switch and light from the PCM and tried running new wires? Somewhere in those wires is a short to ground. There is also a very, very small chance that a bad PCM is causing it.


This type of problem will never set a code. The PCM can't tell if you are pushing the button or if it is getting a false reading from a short.


No, there is not.
Just wanted to give anyone following or in the same situation my soloution. PCM went out for testing and repair to Diesel Technology of Chattanooga Jonathan was awesome to work with and he went above and beyond to get this thing working again. After he spent the day on the test bench we emailed back and forth a few times he sent back a repaired unit along with as much diagnostic info and ideas he could come up with. Well today I installed the unit back in the truck have been driving it all day and so far seems to have done the trick. I want to thank Mark for all the info and help on this one. Its nice to have the truck back on the road.
 
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