Problem child f650 help
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1st Gear
Joined: Oct 2025
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Problem child f650 help
Hey there guys,
I have a very troubled 2007 f650 that’s been making me bang my head against the wall
Long story short in may of this year I decided to change out the pto gasket on my Cummins/allison f650
and the transmission went into limp mode
This is a tow truck I own, I’ve rebuilt a lot of Allison’s and diagnosed a ton of trouble some f650 transmissions issues with this combination in the past.
Figuring I got some gunk in the transmission valve body possibly I did a new filter, fluid flush and still in limp mode
Keeping a spare engine and transmission for this truck after that I figured one of my solenoids was clogged I changed the valve body
this is from a fresh reman Allison changed out with wiring harness and still in limp mode
left scratching my head with no logical explanation the pto was shimmed correctly, I threw a tcm at it. No change
I did not have the capability to purchase a scanner for this with Allison doc I tried a harbor freight zeroc or whatever it’s called and got a generic unhelpful code for the transmission.
So just for the hell of it I put the reman transmission in
no change
From then forward I’ve changed every wiring harness
cleaned every ground and still in limp mode
after I changed the wiring harness to the new one I had a crank no start but would start if I jumped the solenoid
installed the old wiring harness(engine, transmission, firebox to the hvac controls) no internal harnesses have been changed. No crank no start.
I have 5 volts at the relay from the neutral safety relay in the Allison relay box when cranking is applied
can high/can low bus lines are all within specification and the tcm is communicating with the ecm can’t remember the specific diagnoses I did but it checked out
so my question is does anybody have any idea on what to do, will not shift into reverse or forward,
check trans light on.
im sure its gonna be the dumbest thing ever
tcm has power and ground.
could someone also please please share with me the starting circuit and engine/ transmission control wiring diagrams.
I have a very troubled 2007 f650 that’s been making me bang my head against the wall
Long story short in may of this year I decided to change out the pto gasket on my Cummins/allison f650
and the transmission went into limp mode
This is a tow truck I own, I’ve rebuilt a lot of Allison’s and diagnosed a ton of trouble some f650 transmissions issues with this combination in the past.
Figuring I got some gunk in the transmission valve body possibly I did a new filter, fluid flush and still in limp mode
Keeping a spare engine and transmission for this truck after that I figured one of my solenoids was clogged I changed the valve body
this is from a fresh reman Allison changed out with wiring harness and still in limp mode
left scratching my head with no logical explanation the pto was shimmed correctly, I threw a tcm at it. No change
I did not have the capability to purchase a scanner for this with Allison doc I tried a harbor freight zeroc or whatever it’s called and got a generic unhelpful code for the transmission.
So just for the hell of it I put the reman transmission in
no change
From then forward I’ve changed every wiring harness
cleaned every ground and still in limp mode
after I changed the wiring harness to the new one I had a crank no start but would start if I jumped the solenoid
installed the old wiring harness(engine, transmission, firebox to the hvac controls) no internal harnesses have been changed. No crank no start.
I have 5 volts at the relay from the neutral safety relay in the Allison relay box when cranking is applied
can high/can low bus lines are all within specification and the tcm is communicating with the ecm can’t remember the specific diagnoses I did but it checked out
so my question is does anybody have any idea on what to do, will not shift into reverse or forward,
check trans light on.
im sure its gonna be the dumbest thing ever
tcm has power and ground.
could someone also please please share with me the starting circuit and engine/ transmission control wiring diagrams.
Fleet Mechanic
Joined: Jul 2004
Posts: 1,516
Likes: 198
Wow. The only thing I can suggest is the transmission getting 12v.? Some of these trucks had a wire straight to the batteries with an in-line 10A fuse that powered the transmission.
Maybe this guy' video can help:
Maybe this guy' video can help:
Last edited by Louisville Joe; Oct 21, 2025 at 11:31 PM.
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westcoaster15
1994.5 - 1997 7.3L Power Stroke Diesel
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Sep 14, 2010 10:35 AM
