A question with a problem I'm having.

[1997chevydriver]

Yeah it's possible the po changed the specs in the ECM for the tires, that's what we are trying to figure out

Sent from somewhere in MN

 

[Dans94Chevy]

Ok well yesterday I had someone run along with me and I had a co-pilot with the GPS keeping track of how off my speedo is and when the speedo says 25, I'm actually doing 20 and when the speedo says 100 I'm actually doing 75. but here is where it gets interesting. I know these trucks have a fuel cutoff at 90 mph. So when the speedo hit 90 I expected it to hit fuel cutoff but it didn't. It hit fuel cutoff when the GPS hit exactly 90 mph. the speedo was buried past a 100 and had been there for a while. So my question is what controls the fuel cutoff?? I need to hook into the PCM and see what its actually saying. Because if its accurate then it must be in the instrument cluster. And I really don't wanna mess with that or the needles.. I plan to check the rear gear ratio today or tomorrow. Anyone have anything else I can test or try or options?? Thanks in advance

 

[sewlow]

The speed-limiter is computer controlled. Somehow it's tied into the speedo itself. I've changed the needles in my '97, and the speedo is not quite right on. It reads faster than what I'm actually going. Checked by GPS. It's not out as far as your's, though.
Don't know if the 4X4's are different than the 2whl.dr.'s, but both my trucks limiters kick in @ 98mph, according to the speedo, not the GPS. Because my '97's speedo reads fast, the limiter kicks in @ 89mph by the GPS, but @ 98 by the speedo. Haven't had time to mess with it lately, but I've been wanting to check to see that if I re-positioned the speedo to read slow, would it actually affect the limiter's cut-out, according to the GPS.
For my black truck I have a programmer for it & with that I've re-progammed the limiter to 135mph (like it'll even go that fast!) & changed the computer to adjust for 29.5" tall tires, & for the gear change from 3.42's to 3.73's.
Those programmers aren't cheap! Around $400.00, and they are limited in what they can do.
Don't know for sure if those gazillion dollar scanners could do the same for you. It's not like I'm messing with that $400.00 programmer all the time. I set it up once for the changes, & have never messed with it again. If the expensive scanners can do that, all you need to know for the re-progamming, is the height of the tires, & the gear ratio.
It could be that the PO had this done for the 35's, & just never had it changed for the smaller tires.
I'd be checking to see which gear ratio is in there first.
That can be done without taking anything apart.

 

[Dans94Chevy]

I checked the gear ratio today. It's the factory 3.73's but not to say your wrong but there is no way the governor (fuel cutoff) is by the speedo. I say this because I pegged the speedo past 100mph and kept going and once the GPS hit 90 mph is down.... I think the tire size has been changed in the PCM. Like I said earlier in the post I plan to do the 0411 PCM swap so I'm hoping it will fix the problem seeing I can adjust tire height

 

[sewlow]

Oh, I've been told I'm wrong before! Sometimes quite loudly! Angrily, too! No biggie! Part of the learning experience! Lol!
Most of these trucks have developed their own little quirks over the years. And sometimes former PO's that owned 'em should of just stuck to driving them, instead of trying to figure out how to use the tools that they got for Christmas on them!
I was just saying what I've experienced with my trucks.

 

[Dans94Chevy]

LOL as have a I. thanks for the input. I have a horrible exhaust leak where the PO tried using exhaust tape to fix it and it didn't work. I trying to figure out which I wanna replace first.

 

[JonZ71]

dan drop the pan. Mine was doing the same thing. the filter was loose causing air to get into the trans lines causing it to do that

 

[Dans94Chevy]

Jon I have already dropped the pan and replace the pressure control valve and replace the filter and fluid. No change. But how would air in it cause the speedo to be off??? I can understand the converter lockup problem. But I'm pretty stuck on the the converter problem is caused by the speedo problem. I could be wrong though

 

[JonZ71]

it's a 94 correct? You can go to the junkyard and find a truck with the same thing you have and take the prom out of it.

 

[JonZ71]

Or wait. i have the same trans as your truck. come to my house and we will change out the gears for the speedo