Speedo reads slow and bounces

[-stew-]

My speedometer bounces at low speed, it reads slow, and I've never seen it indicate over 45MPH. While I had the instrument cluster apart for new bulbs and a cleaning, I spun the speedo with my drill and it would bury itself and acted like it had no issues being spun by the drill. Anybody had this issue? Any tips where to start looking for a solution?


Truck is a 1990 V3500 with a TH400 and a NP205.


Thanks.

 

[Deuling]

The speedo in my 85 also bounces like crazy

 

[nvrenuf]

I have to ask... are you sure it's a '90? Both '90 & '91 had electric speedo's, they were not cable driven.

 

[Fordum]

Well, if its cable driven, and reads low but goes up at all speeds just not as far, then its usually a ratio problem and can be cured by a speedo gear change.

Bouncing is usually a dry cable. Most of the time, you can take the cable loose at one end or the other, pull the core out, lube it and slide it back in.
You will have to twist it slightly just as it goes the last bit to alighn it with the socket.
I have seen the speedo come apart inside and let the magnet touch the driven part, but it usually destroys it's self in a few miles.

I have also seen the core break in the cable, but they usually quit reading when that happens.
I suppose its possible the ends could rub and cause a reading, just never seen it.

 

[-stew-]

I doubt its a ratio problem. The truck has taller than stock tires, but it acted the same way with stock sized tires.

 

[jeff in co]

I have to ask... are you sure it's a '90? Both '90 & '91 had electric speedo's, they were not cable driven.

I'm also curious on the year thing.

When you removed the housing for the light bulbs, was there a cable connected to the speeds that you had to unhook?

If its a cable, I would lube it or replace it with a new one. Those are the easiest. It could possibly be a bad gear within your tcase but I think that's pretty remote....

 

[-stew-]

It is a cable drive. I don't know for certain, but I don't think the 88-91 one tones used the electric speedo due to the 205 transfer case.

 

[Z71paramedic]

The speedo in my 72 was doing this. I switched to all electric autometers thinking it was a gauge/cable problem. It still did it so I figured it was the gear. When I tore the output on the 205 apart I found that I had a metal gear that wasn't stripped. Turns out that there are no teeth on the shaft/speedo gear to hold it in place. It is held in place with pressure from the rear bearing and a thrust button-type spacer on the shaft. Mine would get warm and start to slip. I added a bead of RTV to the back of the speedo gear and it solved my problem. Not sure if your setup is the same but it's a thought.

 

[nvrenuf]

It is a cable drive. I don't know for certain, but I don't think the 88-91 one tones used the electric speedo due to the 205 transfer case.

Fwiw, the 205 was available with a vss switch for those years. My old 91 v3500 had an electric speedo and vss 205.

Not trying to argue with you, just sayin. The cable is most likely the problem and I'd bet you'll have to buy one for an 89.

 

[-stew-]

Also not to argue, but 91's were available with the 4l80e, 90's were not, prolly explains your vss. Lots of hanky panky with these last square bodied trucks.