2nd Gen Non-Engine/Transmission Why does boost over 20lbs make the speedo erratic?

[Deezul1]

Coming home today towing my TT my speedo would start jumping up 10-15 mph and jump around and then settle back down again only to do it again later. The RPMs stay rock solid with no rapid movement at all. I then noticed that when the speedo would go weird was when the boost hit 18+lbs and it didnt matter what speed or gear. It was fine when I was towing the trailer Friday morning. It was raining today most of the trip but why the reaction to the boost? This is a 12 valve truck with no computer controls on the engine. The boost is mechanical with plastic tubing from the intake manifold to the gauge.

TIA

John

 

[vssman]

Just a guess, but the speedo is "driven" by the PCM. Maybe when you put a load on the engine that produces more than 20 lbs boost some sensor/wire gets an intermittent connection. I'd first check all 3 connectors at the PCM, then the crank sensor and charging system. Just a thought that the engine may be rocking a little under load.

 

[Deezul1]

Also have a problem with the cruise control now. It will surge when more than level ground is encountered. It will accelerate a little and let off then do it again and again with varying amounts of throttle applied, kinda like a new driver that hasnt figured out how to hold the throttle steady so its on, off, on off but never real hard and only when boost is above 8-10lbs. Dont know if boost is the issue or not.

 

[scotmartin]

Sounds like you are having a problem with the rear ABS sensor. That sensor also drives your spedo and cruise control. Maybe, just maybe you are having a problem when more torque or vibration are encountered. Hence the problem with high boost and rough roads.



Scot

 

[vssman]

John,



It still sounds like an electrical problem. Your 97 has a VSS sensor located on the drivers side transmission. I'm not sure how the abs is driven on that model year. 98 & later did away with the VSS and used the abs sensor for vehicle speed. Again, check all of the connections at the vss, pcm and maybe crank sensors.



Paul

 

[Deezul1]

Thanks, I will keep looking. The ABS is rear wheel only and the sensor is in the Rearend housing seperate from the speed sensor on the trans.

 

[Michael Hughes]

I know on 99s with 5 speeds only have the sensor in the rear end.

 

[Deezul1]

When the fully computer controlled engines (24 valves) came out they changed a bunch of stuff.

 

[ddillman]

I was feeling a slight surge with cruise on to. Then I added some mods. and it got worse. It turned out to be my fuel pump going out . Have you checked fuel pressure?

 

[Deezul1]

Problem solved, I removed the VSS connector at the trans and cleaned it good with brake cleaner and let it dry and moved the wires around a bit and now its back to normal.