The Diesel Stop banner

1 - 18 of 18 Posts

·
Registered
Joined
·
139 Posts
Discussion Starter #1 (Edited)
I have a problem with my speedo. This started a couple months ago, but haven't had time to get around to fixing it. It always reads higher than the actual speed, but not consistant. It can be anywhere from 3 to 15 mph off, which is causing shifting issues. I replaced the vss in the differential being it was cheap and the most obvious place to start. Still not right. When I get on the throttle, it will run up to 35-40 quick while I'm still around 25 or so. Then my speed will catch up to speedo- but never right on. I took it out on the parkway and I immediately buried the needle in the odo, even though I was at 65. I verified my speed with GPS. Searched around and can't seem to find any threads with similar issues. Anything else electronic that I should check, or the speedo drive? I don't get any abs lights or flashing od light.The needle doesn't jump around or do anything out of the ordinary. Any help would be appreciated.
 

·
Registered
Joined
·
139 Posts
Discussion Starter #2
If the cable was bad, wouldn't it jump around or read lower in speed? I wouldn't give a damn, but it feels like it goes into 3rd and 4th way too soon. I end up driving alot with the od off. Anybody?
 

·
Registered
Joined
·
3,553 Posts
Isn't the VSS on the rear for ABS? Check the one on the trans. I think there is a speedo sender there.
 

·
Registered
Joined
·
139 Posts
Discussion Starter #4
I don't know if there is one on the trans also. I've been reading all over and that wasn't mentioned. I'm kind of confused how a sensor in the diff would work with a cable driven speedo in the first place- or even on the trans.
 

·
Registered
Joined
·
6,680 Posts
when there is a mechanical speedo pickup it is only for the gauge.

RABS and E40D read the rear axle.

Later-model trucks use the rear axle speedo pickup for speed, too.
 

·
Registered
Joined
·
6,680 Posts
it really shouldn't have any effect.

I don't have my book at hand right now, but IIRC the speed signal actually gets to the trans computer from the back of the cluster (PSOM)
 

·
Registered
Joined
·
16 Posts
Had the same problem w/ my 91 4x4. Replaced the VSS and both the drive and driven gears for cable. Solved both issues. MHG
 

·
Registered
Joined
·
139 Posts
Discussion Starter #9
got a chance to get under the truck today to take a look at the trans where the speedo cable exits. Sure enough another sensor. I don't know how I couldn't think to just look there first. I've looked at the back of the trans a hundred times and never did it ever sink in that the cable goes thru a sensor. I'll look to change that out asap. I'll let you know if that was it.
 

·
Registered
Joined
·
139 Posts
Discussion Starter #10 (Edited)
Finally got a chance to put in the new sensor where the speedo cable exits. Speed still not right. It climbs real quick, then as you settle in it gets close. But still too high.Anything else before pulling the cluster? I could spin the cable easy, so that shouldn't be an issue. What a PITA!
 

·
Registered
Joined
·
139 Posts
Discussion Starter #11
So I changed both sensors, rear and trans. Gear on cable is good. Cable is good. Truck still reads high mph causing trans to shift way too early. It's making me stomp the accelerator to kick it down, and drive with out the OD engaged around town. Going uphill is a problem, especially when I have to keep up my speed.
Is there someplace else I can look or is it pretty much the speedo in the dash that needs to be looked at. If the cluster is the only place left to look, does anyone have a diagram on what comes apart and needs replacing? Any help would be greatly appreciated.
 

·
Registered
Joined
·
6,680 Posts
In the 1992 there is a module on the back of the cluster. Not sure about your truck.
 

·
Registered
Joined
·
16 Posts
As far as I know the cluster is mechanical driven on the 91s. VSS sends electrical singnal to tranny. You said cable driven gear is good, there should be a drive gear inside housing where cable exits tranny. On my 4x4 this was on back of transfer case. The gears are nylon so they can strip causing the problems you are having. If you have to replace the gears be sure to count the teeth so the speedo and VSS read correctly. I had same problems on my 91 with a stripped drive gear, replacing it solved problems. Good luck.
 

·
Registered
Joined
·
139 Posts
Discussion Starter #14
As far as I know the cluster is mechanical driven on the 91s. VSS sends electrical singnal to tranny. You said cable driven gear is good, there should be a drive gear inside housing where cable exits tranny. On my 4x4 this was on back of transfer case. The gears are nylon so they can strip causing the problems you are having. If you have to replace the gears be sure to count the teeth so the speedo and VSS read correctly. I had same problems on my 91 with a stripped drive gear, replacing it solved problems. Good luck.
Right. The gear on the end of the sensor is good. Nothing stripped. I replaced the sensor and reinstalled the gear, so I had a good look at it.
 

·
Super Moderator
Joined
·
11,717 Posts
What did the gear inside the trans look like?
 

·
Registered
Joined
·
139 Posts
Discussion Starter #16
I didn't get a look at it- I'm thinking the first to go would be the easily replaced part. I hope. I'm going to look again when I get a chance, prob not today. The gear on the output shaft nylon also? I see you have exp with the transmission. Would anything else cause the speedo\early shift problem? I replaced the tach pickup last summer because the display had become erratic, then stop altogether. Then right after that, I noticed the speedo reading too high. I checked it against my gps. Within the first 15 min of driving it registers at least 15mph over. If I'm on the hwy sometimes more. After I've been driving awhile, the speed will come down somewhat. But not right. It always jumps up quick when I pull away from a light or stop sign. The closest I got was 4 mph to actual speed. I have some hills to cross and it won't shift down unless I kick it. Getting to where I don't want to drive it till I figure it out. Any help would be great.
 

·
Registered
Joined
·
139 Posts
Discussion Starter #17 (Edited)
You got me thinking so I pulled off the vss to look inside the trans to see how the gear on the output shaft looked. Looked inside and everything is good. I took a pic of the speedo drive gear connected to the sensor in case somebody needed a visual.
 

Attachments

·
Registered
Joined
·
16 Posts
Mine is a 4x4 so VSS and cable were on back of transfer case. There is another nylon gear inside TC that drives the gear in your pic. On mine that was the gear that was missing some teeth. I am not familiar the w/ 4x2 so just guessing that it is a similiar setup. Hopefully somebody that knows for sure will post
 
1 - 18 of 18 Posts
Top