Speedo way too fast

[Jersey 93 Ty]

I had an interesting ride to work this morning in my '85 M1009. About 2 blocks into the trip, I notice that the speedometer jumped from around 30mph to roughly 75mph nearly instantly, all without me noticing the fighterjet levels of acceleration that should have accompanied such a jump. As with most problems, my first quick instinct was to punch the gas pedal a bit, which probably brought me up to the 35 to 40mph range in real life, and sent my speedo zooming past 85, briefly pointing straight down, and then coming back down to around 82 or so. Slowing down for the next turn, the speedo did drop to near zero, but immediately came back up to somewhere north of 50mph before the TH-400 shifted into 2nd. This was not the typical "bouncing speedo" problem that so many have described and occurs regularly on my '84... this seems to be a speed multiplying problem. As I got onto a slightly higher speed road, with my actual speed probably in the 50mph range, my speedo was rock steady at 80mph. I also started to notice some sort of mechanical noise coming from the gauge cluster that corresponded roughly to the speed I was actually going. When I stopped, the speedo dropped to zero and the noise didn't occur, but as I increased speed my speedo outran me significantly and the mechanical noise increased. Oddly enough, my odometer still seems to be working perfectly, racking up a correct 12 miles during the trip.

I am really not sure where to even start with this particular problem, but I suppose I'll be able to fix my broken PRND21 indicator while I'm in there also. Any suggestions?

By the way, feeling the 2-3 shift when the dashboard claims you are going 75mph is an interesting experience that I don't particularly suggest anyone try if you can avoid it.

 

[Stonepicker1]

I'd start with the speedo cable first. Disconnect from back of speedo and pull the inner cable out. Clean , lube and reinstall . Drive and see if thats works.

 

[Jersey 93 Ty]

Well, I fixed it through the FM method (F-ing Magic, that is). On my ride home from the office, the mechanical noise got progressively quieter and now the speedo seems to work just fine again. I'll keep an eye on it in the future, and I will try to clean and regrease that cable the next time I pull out the cluster.

 

[mkcoen]

There's a speed sensor that the bottom end of the speedo cable goes through into the TC. Mine was busted when GL was kind enough to trash the TC with a forklift. I pulled it off and just stuck the speedo back in without it. My speedo is now 5mph slow (as tested with a gps) but if you know that then you know what to keep it on. It varies, faster or slower but generally between 3-5mph. If it happens again it's easy enough to take off with a pair of pliers.

 

[Goose2448]

I have a 85 Chevy C20 non MV. My speedo works fine but has a noise when bouning. The faster I go the quicker it is. I cant tell if it gets quieter at higher speeds because the truck is way too loud.

Now I have expenicened a 2-3 shift at over 80 MPH. its kinda cool that you can do 84 MPH in 2nd gear. Now the trans does not shift at that speed unless you keep it in 2nd. Could anyone tell me why it shifts at only 24 and 36 MPH? You can shift it your self but when in drive it shifts at those speeds.

Sorry for the Hijack, but it is relavent.

 

[4x4 Forever]

Had the same problem with a 84 civvy k10 when I was, lets just say a long time ago. A bad speedo was the culprit, I changed it and all was ok.

I would check the speed sensor that mkcoen was speaking about first and lube the cable before deciding to change a speedo. Less $$$ to start with.

It might have gone away for now, but it will be back...