Hello, New here. I have a customer vehicle that is a 1994 Chevy K3500 Gas with a 7.4 engine 4L80E transmission.
The transmission is brand new custom built for this customer.
Driving on flat ground it goes through all gears, going uphill it goes 1-2 no problem, when trying to switch to 2-3 the speedometer says it's dropping speed however it is not. It will not shift out of 2nd and basically puts the truck in limp mode.
Both speed sensors have been changed.
Originally thought it was and ECM/PROM issue however the problem is still presenting albeit better than it was originally. With the old PROM it would not switch out of second at all.
We are at a loss of what else it could be. Please send any information/helpful hints anything my way.

 

Are you still trying to figure this out?

What’s the truck being used for that required a custom built transmission?

Are there any other mods to the truck, such as larger tires, or a rear axle with a gear ratio different than factory?

The speedometer dropping off indicates an issue with the VSS/VSS buffer module and related circuits. Any mods that affect road speed will affect how the VSS signal is interpreted by the buffer module and PCM.

The PCM relies primarily on VSS, RPM, MAP and TPS input to determine shift schedules. The VSS can prevent up shifts in certain circumstances.

First verify the speedometer is reading accurately by checking it against a GPS signal or another vehicle with someone’s help. It should be reading within 5 MPH of the reference signal.

If it’s accurate, and if you have a scanner capable of reading OBD 1 live data (for GM, you’ll need a Tech1 or equivalent), monitor the speed signal while the problem is present. If the signal on the data feed matches the speedometer, a problem with the VSS & related components is indicated. Further diagnosis will be needed.