Electrical problem?

T

Trussman

JAFO
May 22, 2008
9
0
1
Wisconsin
2001 allison has been staying in 5th gear......only way out is to shut down & restart......set a p1721 code.
After scanning all the normal "diesel forums", I find a couple of gents at another "place" had similar issues.....the problem being shorted wires &/or bad connections.
So taking the advice I gleaned from one of those threads, I pulled the red connector at the TCM, and check each pin for a short to ground. Pin 13 is grounded......apparently connected to the pressure switch because pin 13 is no longer connected to ground when I disconnect that plug.

Do I have a bad pressure switch?........or is this how it should be???
 
Mike L.

Mike L.

Got Sheep?
Staff member
Vendor/Sponsor
Aug 12, 2006
15,681
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Fullerton CA
Trussman said:
2001 allison has been staying in 5th gear......only way out is to shut down & restart......set a p1721 code.
After scanning all the normal "diesel forums", I find a couple of gents at another "place" had similar issues.....the problem being shorted wires &/or bad connections.
So taking the advice I gleaned from one of those threads, I pulled the red connector at the TCM, and check each pin for a short to ground. Pin 13 is grounded......apparently connected to the pressure switch because pin 13 is no longer connected to ground when I disconnect that plug.

Do I have a bad pressure switch?........or is this how it should be???
Click to expand...

Code P1721 is now code P0776. This is not electrical. This code means a clutch was over powered. Classic limp.
 
T

Trussman

JAFO
May 22, 2008
9
0
1
Wisconsin
Never limped as far as I know, although my 17 year old son used the truck a few times.......claims nothing went wrong.......but he seems to suffer from poor memory at times, LOL.........

Trans is an SC Level III w/ about 45,000 miles on it. Started hanging in 5th under very easy driving conditions. Spin-on may have had 25,000 miles on it. Fluid looks clean....smells a little from clutch material, but not burnt. Pan (deep) has lining residue in it but no pieces. I would question how well the internal filter was flowing as it could/would suck up the "lining dust". I will pull the valve body and inspect & clean.

The original question remains unanswered......is it normal for a pin on the pressure switch to be shorted to ground?
 
Last edited:
Mike L.

Mike L.

Got Sheep?
Staff member
Vendor/Sponsor
Aug 12, 2006
15,681
232
63
Fullerton CA
Trussman said:
2001 allison has been staying in 5th gear......only way out is to shut down & restart......set a p1721 code.
After scanning all the normal "diesel forums", I find a couple of gents at another "place" had similar issues.....the problem being shorted wires &/or bad connections.
So taking the advice I gleaned from one of those threads, I pulled the red connector at the TCM, and check each pin for a short to ground. Pin 13 is grounded......apparently connected to the pressure switch because pin 13 is no longer connected to ground when I disconnect that plug.

Do I have a bad pressure switch?........or is this how it should be???
Click to expand...

Pin 13 goes to the turbine speed sensor. The pressure switch manifold has 3 normaly open switches and one closed ( reverse pressure ).
 
Last edited:
IOWA LLY

IOWA LLY

Yes, its really me
Feb 23, 2007
2,275
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0
If you have debris in the pan along with that code it is a pretty good sign your trans is toast and will need to be pulled and the damaged clutch packs replaced.

Your spin-on should never have been on there for 25K miles. They should be changed every 6K or so IMO.
 
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