fisher plow/4L80

[maverick_935]

I have a 1998 gmc 2500 with a 4l80. Last week it lost reverse in the tranny. The trans guy said this was common in these trannies because the voltage drops from 12 volts to about 10 volts when you lift the plow. He says you are not getting enough 12 volts to the rear transmisssion solenoid as you are doing this and it is hutring the tranny.

Does anyone know about this; is it true. If this is true how do I maintain 12 volts to the transmission to protect its computer controled systems.

Any thoughts???

[JohnC]

Low voltage can cause problems selecting between the forward gears, but, as far as I know, reverse is entirely mechanical.

[maverick_935]

can anyone else ad to this. Is reverse in a 4l80 electric and does it have issues with fisher plows?

[JohnC]

OK, I'll add to it. There are 3 solenoids. None are called "rear". There's A and B which control passages in the valve body and allow the trans to select 1,2,3 and OD. the third is the TCC solenoid that controls fluid pressure to the Torque Converter Clutch. the A and B solenoids have no direct control over bands and the TCC isn't involved in reverse, or plowing, for that matter, so I'm skeptical this guy is steering you in the right direction.

The PCM monitors the voltage and current on the solenoids. If it detects a problem it will set a code, default to second gear, and/or disable the TCC (depending on which solenoid coded). If you have no codes and none of these symptoms then you don't have a voltage related issue with the trans.

YMMV, TMSAISTI

[crashz]

The 4l80e and th400 are known to have weak reverse band clutches and willbreak the band with heavy use or shifting while moving in the oppsite direction. Has nothing to do with the voltage.

[Mark Rinker]

Agreed - the 4L80E is a great plow truck tranny, but careful operation to limit wheel spin and 'jamming' the transmission from F to R while still rolling is imperitive to a long and happy service lift.

I had a new operator on my '94 K3500 a few years back, toasted the tranny in one night's plowing. Certain that it was directly related to his abuse of the gear shift lever.