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6.5L Turbo Diesel 1992-00 6.5TD - Discussion forum for the 6.5L engine.

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Old 03-28-2006, 01:01 PM
6.2 G-30 van 6.2 G-30 van is offline
Join Date: Mar 2006
Posts: 5
Default 4L80E trans no overdrive or lockup

Hello all , I hope this is the correct forum to post my question under. When I lose overdrive and torque converter lockup on my 1993 4L80-E transmission, I spray engine degreaser then blast with carburator cleaner around the outside of the fuel injection pump (driver side) .

This last time the trick didn't work, so I took off the throttle position sensor from the passenger side of the pump and took it apart to clean it. This helped smooth the shifts and there were no more delayed upshifts, but still no over-drive or lock up. I even disconnected the batteries to clear the computer.

This same effect happened long ago when I discovered one of the thin wires broke off the computer harness connector (I remove the computer at the end of each day to prevent its theft.) After I fixed the wire the trans shifted normal.

Is there an electronic solenoid that needs replacing on the trans? Thank you in advance for any suggestions.

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Old 03-28-2006, 02:39 PM
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john8662 john8662 is offline
Join Date: Sep 2001
Location: Lubbock TX, USA
Posts: 4,191

Check for codes, you might get a code for the tranny that could point to where the problem is.

At the very minimum, I think that replacing the TPS would be a wise move!

Solenoids have been problematic on early 4L80E transmissions, so it's possible that one of them (the one that controlls OD) could be malfunctioning.

Hopefully others that are tranny guru's will chime in here...
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Old 03-29-2006, 09:19 AM
LanduytG LanduytG is offline
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Join Date: Mar 2000
Location: Greenfield, In
Posts: 1,407

First thin I would do is take a look at the brake switch. Then move to the connector on the trans itself. Also if the cruise control does not work its a good indication of these two areas.

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Old 03-31-2006, 09:34 PM
damork damork is offline
Join Date: Apr 2000
Location: IL
Posts: 810

A brake switch is not too expensive, if I recall it is around $40. I'm about to change mine out although I bought it a while ago for the same reason. GM techs say that removal is simplified if you don't try to save the retainer circlip and just plan on buying a new retainer when you install the new switch. It really is located in a cozy spot.
350k+ miles of 6.2/6.5 experience
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Old 04-01-2006, 12:14 AM
signgrafix signgrafix is offline
Join Date: Jan 2006
Location: Louisville Kentucky
Posts: 85

if you do not fiqure out the problem by Monday. I will ask a friend of mine who owns a Transmission Shop for you.
1994 chevy turbo diesel 3500 4WD extended cab.
4 inch Pinnacle exhaust from the turbo back
Kennedy OPS harness (yet to be installed)
FSD cooler (yet to be installed, planning on placing in fender to pick up air from intake)
Bet top extendable mirrors.
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Old 04-25-2006, 12:44 AM
bluuzman bluuzman is offline
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Join Date: Apr 2006
Location: Finland (Arctic Circle)
Posts: 31

Hi, I recently lost the TC lockup and cruise (4L80E). At first, they did work from time to time but TC had quite rough action (like on/off). Sometimes everything worked just fine until they were lost completely.

Got code 28 pointing to pressure switches detecting the shift position. The problem was probably in the multi-pin connector at the transmission but as I did not really fix anything, just disconnected and connected the plug a few times, code disappeared and the lost functionality came back, I can not be sure what was really the reason.

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