Code 85 4l80e

[doop300]

Ok so this is last resort, I swapped a 6.2 diesel and 4l80e out of a 91 suburban into my 90 blazer, then put a banks turbo on it. My check engine light comes on intermittently. When I apply even the slightest amount of throttle it goes away but with no throttle its back.
I got a code 85 out of it, which is PROM. I think it has to do with the trans but im not sure. I tested the alternator and its good and swapped in two other sensors from another 4l80e that I have and nothing changed.
im not sure what to do next, any help would be great.

 

[sweetk30]

looks like its " undefined gear ratio "

did you redo / modify the drac or swap it in from doner vehicle ?

the drac is the little white box behind dash under radio held in to black clip bracket. this unit ( drac ) controls the axle gear ratio info to the speedo and computer.

need to modify it for the ratio in the axles you have . i do these up cheep modifyed and all you got to do is know the ratio and tire dia all the way around NOT tire hight. then flip the switches and boom set.

 

[doop300]

I thinks its the original one from the blazer but I swapped in 3/4 running gear with 4.10 and never had a problem but that was a 350 700r4 setup, ill take a look and get back to you thanks

 

[doop300]

Ok so I swapped out the white box with the other one I had and no change, went for a drive and had no check engine light, let off the pedal and the lihht comes on, give it throttle and light goes out. Im not sure what to do next

 

[doop300]

Ok so I went on the highway for the first time today and the trans keeps down shifting out of overdrive. Is this just a bad sensor or what? Any ideas?

 

[doop300]

Forgot to say that I only got the one code (85). Otherwise the light just turns on and off and flickers along with the tranny symptoms.
thanks in advance for any help.

 

[dyeager535]

Got my '91 manual here.

The manual is pretty lame, it's not giving me much in the way of definitions, just a bunch of acronyms.

The flow chart starts out assuming speedometer is operational and no other codes are set.
Clear codes.
Verify PSM operation see PSM check (No reference what PSM is, or where the check is located, and Pressure Switch Manifold isn't listed in the index either. Looks like GM rushed the manual)
Raise drive wheels
Range selector in "D" (3rd gear)
Is input speed close to engine speed +/- 100RPM?

If input is not within +/- 100, the input speed sensor is bad. If it reads correctly, take engine up to 1800RPM, then "is input speed close to output speed?". If no, refer to output speed sensor. If yes, problem is intermittent.

To me it sounds like you need to verify the PSM is working first (manual says PSM indicates to the PCM/TCM what gear range has been selected), however you do that. Since GM says that's an assumption before testing anything else, you might chase your tail if you don't test it first.

All gears BUT overdrive the PCM/TCM calculates gear ratio from the input/output speed readings, then compares to what the speed reading SHOULD be, based on the selected gear. So that might give some goofy results if you put it in OD and then go drive I suspect.

I'll try and take the manual to work tomorrow and scan these two pages.

 

[dyeager535]

PM me your email if you want the two 1991 R/V service manual pages for Code 85, I've got them scanned.