RPM Drop and Engine Turned off when shifting down

[W351]

Hi,

I have an issue with my 1998 K3500 7.4, RPM Drop and Engine Turned off when shifting down, let me tell you a brief about my project:
1998 K3500 with NV4500 5.7 swapped to 7.4 taken from a suburban, 2 days before I did 411 swap and it is running perfectly however i still have the subjected problem and the speedometer is not working.

Any advice??

You must be registered for see images attach

 

[Rock Hard Concrete]

I don't have any advice but just wanted to tell you that is an absolutely gorgeous truck.

 

[W351]

@Rock Hard Concrete Thanks bro

 

[Schurkey]

Connect a scan tool, find out what the computer wants you to know.

Verify fuel trims, IAC and EGR operation, along with checking EVERY sensor and the other computer outputs.

Check fuel pressure.

Did the speedo work before the 411 and engine swap?

 

[W351]

Yes the speedo was working after the engine swap but it stop after the 411 swap, any thing related to the engine harness since it is originally for auto trans 4l80e ?

 

[Schurkey]

Guessing that this is more a matter of the 411 computer than the harness. Whether auto or manual, either way there's going to be a speed sensor, so as long as the harness is plugged into the sensor, I don't think that the harness is at fault.

But I'm not at all knowledgeable about computer swaps.

Perhaps a moderator will move this thread to the LSX + Computer Swaps sub-forum

 

[1998_K1500_Sub]

I assume you reprogrammed the 0411 with appropriate tables / firmware for the 7.4 engine & manual trans combo. Do tell.

I've noticed on one manual trans truck (1995 S10 4.3L with OBD1.5 ECU... that was the transition period when they starting putting the ECU in the engine bay instead in the cab) is that the ECU commands the engine idle RPM higher when the truck is in motion. Notably, pushing in the clutch and shifting to neutral, so as to coast, causes the ECU to maintain the engine RPM to some higher idle RPM (perhaps 1200, from memory) until the vehicle speed approaches zero (or maybe even after reaching zero), after which time the ECU will gracefully lower the engine RPM to normal.

Dunno if this above is related to your issue, but (a) you stated you've got no speed indication and (b) you notice this problem when downshifting, which I assume means you're off the throttle. I can't help but wonder if the ECU's is mis-informed about the operating conditions and is not fueling (injector PW) / sparking (timing) / airing (IAC) the engine appropriately, hence the issue.

It's just a thought.

And yes, that truck is beautiful

 

[GrimsterGMC]

I assume you reprogrammed the 0411 with appropriate tables / firmware for the 7.4 engine & manual trans combo.

I've noticed on one manual trans truck (1995 S10 4.3L with OBD1.5 ECU... that was the transition period when they starting putting the ECU in the engine bay instead in the cab) is that the ECU commands the engine idle RPM higher when the truck is in motion. Notably, pushing in the clutch and shifting to neutral, so as to coast, causes the ECU to command the engine RPM to some higher idle (perhaps 1200, from memory) until the vehicle speed approaches zero (or maybe even after reaching zero), after which time the ECU will gracefully lower the engine RPM to normal.

Dunno if this above is related to your issue, but (a) you stated you've got no speed indication and (b) you notice this problem when downshifting, which I assume means you're off the throttle. I can't help but wonder if the ECU's is mis-informed about the operating conditions and is not fueling (injector PW) / sparking (timing) / airing (IAC) the engine appropriately, hence the issue.

It's just a thought.

And yes, that truck is beautiful

It almost sounds like a DFCO situation where the fuel cuts off when off the throttle.