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I have a 94 4l80 with an older style reverse apply pin and housing that I overhauled and cannot get the thing to go in reverse. I watched Nick's transmission videos on youtube to assist in the work. In his video he shows how to measure the reverse low band clearance by measuring from the surface of the case to the higher side of the reverse piston. If I remember correctly he was shooting for .150 from surface of case to the piston which in turn made me shorten the apply pin by .100 After Installing the transmission and not having reverse I decided to drop the pan yesterday and lengthen the pin .120 by welding material on. For two short moments yesterday it had reverse and now I cannot get the thing to go back into it. I have read alot on making sure this is being done correctly but the information I am finding is vague. Is the travel of this pin really only .120 from resting position to apply? I guess I need to spot on to get this to work correctly?
Can I swap in a newer style reverse pin/piston and housing to start fresh? The total length of the pin before adding material was 4.260 and now the pin length is 4.380. The transmission is commanding full pressure of 180psi at idle for some reason. This is in a older 1993 6.5 diesel K2500. Please enlighten me with some words that can lead me to finishing this, I have been at it hard going on a week trying to get this thing right.
Finally got the reverse working somewhat. I had to offset the shift lever a tad and give it some pedal and it would finally engage while revving down? It's like it is barely engaged maybe I need to add .050 to the pin length? Can I use a newer style reverse housing and piston on this older 4l80?
I have never tried to us the late servo and cover from a late to early . So cannot advise. But yes the servo travel for the low/rev is somewhat limited. I generally set up during the build to be as close as possible without the band dragging the drum. Kinda hard to do trans in though. But I would say yes add a little to the pin . See what happens. I rarely see any issues here except broken piston. But I started using the bilet ones for the late models. Haven't done an early one in years.
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I welded and grinded the pin to 4.425" so I could get closer to Jakes .070-.080 reverse/low piston travel spec and the reverse sort of engaged better. Hard to tell since the truck still took alot of pedal to get it to move still. The only thing I can think of is to add another ~.050 to the pin since while installed all the weight is pressing the inner hard parts towards the valve body vs when its upside down getting assembled?
So with the truck on and running the force motor is putting out no voltage when checked with a test light? Isn't it supposed to be pulsing to reduce the pressure? Where would the tcm be on a 93 K2500 diesel?
Last edited by Guy with a Chevy; Mar 19, 2025 at 06:02 PM.
jakes spec is probably tighter for transbrake applications where this travel is more critical. this is fairly easy to check with an old cover and a dial indicator. sounds like you need to get a scan tool next.
there is a difference in valve bodies & force motors 91-93 vs 94+ they claim they interchange but I've always swapped to the newer PCM on the gas stuff. check your other post i suspect you've made a wiring error or something simple like a blown fuse and not getting power to the transmission. if you're at 180 psi (full pressure) it's probably in fail safe mode (no power).
From my understanding since this is a 1993 diesel k3500 it has no PCM or ECM just a TCM so no scanning available. I'm going to make a measuring housing with a spare I have and cut it to expose the piston and weld a nut on the top so a bolt can press on the pin and measure the travel off the bolt with a dial indicator. The scary thing is is when I went to fix the truck it already had a spliced connector on it so that's why I need to find the TCM so I can back probe to the wires and make sure they're going where they need to or even if the TCM is working since I'm not getting any kind of voltage for the EPC and full line pressure at idle.
From my understanding since this is a diesel k3500 it has no PCM or ECM just a TCM so no scanning available. I'm going to make a measuring housing with a spare I have and cut it to expose the piston and weld a nut on the top so a bolt can press on the pin and measure the travel off the bolt with a dial indicator. The scary thing is is when I went to fix the truck it already had a spliced connector on it so that's why I need to find the TCM so I can back probe to the wires and make sure they're going where they need to or even if the TCM is working since I'm not getting any kind of voltage for the EPC and full line pressure at idle.
i just edited my post above, but yes the early diesels had a standalone transmission controller. they will communicate through the OBD1 port. with that being said if you're at full pressure @ idle the trans probably in fail safe because no power. check basics like power to the trans (i test with a light to make sure the wire will pass amperage), power at correct pin, good grounds, etc. i would also go over the wiring you spliced. this sounds like the trans wasn't working so everyone decided it must need a rebuild when in reality it was probably an electrical problem.
The other 3 solenoids have power as they should but that comes from ignition? I have a friend that is going to search all data for the tcm location and possibly some diagnosing material. I haven't seen an odb1 connector under this dash maybe I should look harder. I'm getting conflicting info on the location and don't want to take both bottom sides of the dash loose hunting for it. With the way the wiring was exposed I wouldn't be surprised if the thing grounded out and fried itself because the trans fuse is still intact
The other 3 solenoids have power as they should but that comes from ignition? I have a friend that is going to search all data for the tcm location and possibly some diagnosing material. I haven't seen an odb1 connector under this dash maybe I should look harder. I'm getting conflicting info on the location and don't want to take both bottom sides of the dash loose hunting for it. With the way the wiring was exposed I wouldn't be surprised if the thing grounded out and fried itself because the trans fuse is still intact
On new style connector pin "E" you should have 12 Vdc. check with a test light or headlight bulb and make sure it is BRIGHT. The force motor should have 12Vdc at pin "C". Also check resistance to ground (on the battery to the 4L80E case) and make sure you have low resistance (under 1 ohm, ideally 0.5 ohm or less). I would verify wire integrity back to the controller, possibly have a bad one. it should be behind the glove box on top of the heater box, at least that's where the gasser stuff is on the early GMT400s. definitely get a factory manual and/or diagram.
On new style connector pin "E" you should have 12 Vdc. check with a test light or headlight bulb and make sure it is BRIGHT. The force motor should have 12Vdc at pin "C". Also check resistance to ground (on the battery to the 4L80E case) and make sure you have low resistance (under 1 ohm, ideally 0.5 ohm or less). I would verify wire integrity back to the controller, possibly have a bad one. it should be behind the glove box on top of the heater box, at least that's where the gasser stuff is on the early GMT400s. definitely get a factory manual and/or diagram.
Yes test light was bright on E and on A,B and S were a little less bright as expected. C & D gave nothing. I will check the case ground resistance tomorrow thankyou for that suggestion. I think you may be right about the glove box and shouldn't be too hard to remove.
You can scan the TCM need a gm tech 1 or 2 or a snapon brick/ other aftermarket scanner here.
Reverse is 100% mechanical there is no TCM involvement at all. If it doesn't have reverse it's not a computer problem. There is a fuse marked ecm/ign in the fuseblock to the left of the steering column which provides ign power to the C16 pink/ b;ack TCM and shift solenoids. Battery power orange A12 for the tcm comes off a fusible link on the junction box on the firewall under the hood on the passenger side under the black cover above the ac drier. C1,C2 ground inputs for pcm
TCM is behind the glovebox on the right side once you pull out the black glovebox liner.
DLC connector is mounted to bottom side of dash right by the hood release its a black rectangle connector mounted on a metal tab and screwed to the lower edge of the dash with a 7mm screw its about 6" to the left of the panel that drops down under the steering column.
Last edited by 01WS6/tamu; Mar 20, 2025 at 12:00 AM.
When I worked at the chevy dealer many years ago, We had a recall to replace the internal and external plug over various issues on this, The kit is still available. The Old connector had leak and connection issues.
The OBD1 connector many times on those was basically just hanging around someplace near the steering shaft under dash . Sometimes mounted and well sometimes not. Beware also some 94 and 95 had both OBD1 and OBD2 connectors but the OBD2 did not do anything, Saw this a couple times and was like weird.
So I finally found the ALDL stuffed under the dash. The codes it displayed in flashes were
DTC - 22 Throttle position sensor error (signal low)
DTC - 58 Trans fluid temp sensor circuit error (low voltage)
DTC - 59 Trans fluid temp sensor circuit error (high voltage)
DTC - 75 Digital EGR solenoid #1 circuit error
DTC - 80 Transmission component slipping (I thought it flashed 81 not 80?)
Besides something not being right with the wiring I also think I need to remove the trans and start fresh since I cant get the reverse/low band clearance set up right with out creating drag in drive or get it to engage properly in reverse. Does anyone have the pin out associated with that pcm behind the glove box?
So I finally found the ALDL stuffed under the dash. The codes it displayed in flashes were
DTC - 22 Throttle position sensor error (signal low)
DTC - 58 Trans fluid temp sensor circuit error (low voltage)
DTC - 59 Trans fluid temp sensor circuit error (high voltage)
DTC - 75 Digital EGR solenoid #1 circuit error
DTC - 80 Transmission component slipping (I thought it flashed 81 not 80?)
Besides something not being right with the wiring I also think I need to remove the trans and start fresh since I cant get the reverse/low band clearance set up right with out creating drag in drive or get it to engage properly in reverse. Does anyone have the pen out associated with that pcm behind the glove box?
Is this the TCM?
Last edited by Guy with a Chevy; Mar 20, 2025 at 05:30 PM.
Yes thats TCM
I'd replace the external connector trans connector and the wiring harness in the pan to start with just so you have a baseline of a good solid connection. Solder and heat shrink everything. several of those codes are electrical circuit related and that plugs a mess by the picture. This will probably take care of several codes at one fail swoop
TPS is mounted on the injection pump pretty common back when to see those bad. Aftermarkets are junk hunt around ebay and find a GM military surplus new one. I've got a 98 6.5 and have gotten a few parts for it from various military overstock successfully. You have to use a DVOM to adjust it properly if you don't have a scanner to watch the values change when you tighten it up.
Ignore the EGR code useless BS and will not cause any adverse tranny operation.
Last edited by 01WS6/tamu; Mar 20, 2025 at 08:11 PM.
I did replace the internal harness and Pigtail so all good there and since the old pigtail was spliced in I wanted to double check the TCM side of the wiring. After unplugging the TCM yesterday it would not show any of those codes over again and I started to get voltage on the force motor right under 5 volts which is normal right? I took it for a drive and now it's shifting perfectly except for reverse. Reverse requires me to throw it quickly from drive or first gear manual selection down to reverse quickly and it engages firmly?
Last edited by Guy with a Chevy; Mar 21, 2025 at 10:22 AM.
nice! if you still have trans temp and EGR code they may be related. on the gas stuff they share a ground along with the MAP sensor. unfortunately i don't have a diagram for this era diesel to confirm.
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4l80e Nightmare 
