Bad Speedometer/Odometer Readings

[Smaug]

Speedometer registers 5 mph faster at an air temperature of 70 deg and over, about 20 mph between say 40 deg and 60 deg, as much as 40 mph faster at 35 deg and colder BUT the Odometer reads 1/2 of 1 tenth under per mile or 9.5 mi. for 10 mi.

What the heck is going on?

Gear driven transmission output with electric sensor attached to same and wired to speedo.

 

[boggs1227]

??? , wow never heard of that ! but the transmission gear that is hooked to the speedometer cable on your model should not have a electronic sending unit of any sorts .i dont know how air temp's should play a role here....,when driveing down the road does your speedo jump around alot ? if so it's a common accurance with these first gen trucks .either is 3 things :

cable is worn(needing replacement) and or needs to be lubed

speedo gear in the transmission has worn a tooth and needs to be replaced

or the speedometer head it's self is bad or worn / needing to be oiled / replaced


hope this helps . . . . btw: mines got rabbit syndrome (lol) it's fine till you get to 55 but after that it goes into a hopping fit of about 5 mph off of a swing at any given speed past that .

 

[Maniak]

The speed sensor that is on the cable near the transfer case/transmission is for the cruise control.

As mentioned, the speedo is run by cable with a driven gear on the end of the cable and a drive gear in the transfercase/transmission.

I've seen cables stick when colder but the symptom was a jumping speedo.. not reading high.. I'd suspect the speedo head before the drive/driven gears/cables for a speedo reading high especially since the OD doesn't change like the speedo. If it was before the speedo head the OD would be doing the same as the speedo.

You can take just the speedo out of the cluster and have it rebuilt... You can even still drive w/out it... you just won't know how fast or how far you have gone.

~Mark

 

[Smaug]

Reply To: boggs1227

No jumping around. And you'd think that once the interior, especially the dash which gets quite warm, heats up the speedo would get more accurate. But no, the inaccuracy is totally affected by outside temp.:scratch:

Speedo cable is worn and or needs to be lubed. = hope that's all.

Speedo gear in the transmission has worn a tooth or is just worn out
needing to be replaced. = well better than the last alternative.

Or the speedometer head it's self is bad or worn / needing to be oiled (or replaced?) = bummer, I don't want to have to pull the dash and speedo head.:thumbdwn:

 

[boggs1227]

yeah ,pulling the dash apart may sound bad but it's pretty easy .i can do mine in about 20 minutes thats from starting to pulling the speedometer head out of the dash.

 

[Smaug]

Reply To: Maniak

The speed sensor that is on the cable near the transfer case/transmission is for the cruise control. OK, got Cruise, still got to look for the speedo cable then.

I've seen cables stick when colder but the symptom was a jumping speedo.. not reading high.. I'd suspect the speedo head before the drive/driven gears/cables for a speedo reading high especially since the OD doesn't change like the speedo. If it was before the speedo head the OD would be doing the same as the speedo. Not what I want to be the problem...:thumbdwn:

You can take just the speedo out of the cluster and have it rebuilt... You can even still drive w/out it... you just won't know how fast or how far you have gone. Don't I have to pull the dash?

 

[Smaug]

Reply To;boggs1227

Well the next time I'm out your way you can show me how... Ha...Ha...

 

[CDW6212R]

FYI, the speedometers are driven by wind resistance/shear. Those older models have a cable spinning a half wheel(metal round wing), very close to the needle drive. The spinning of the part that close to the inner metal ring(attached to the needle and odometer), it makes the untouched part spin.

Likely something has contaminated those parts, they should be extremely clean. Oils from your fingers will mess up the speedometer calibration. Other things can go wrong also, but that is common if the assembly is ever handled. Take it out and use rubbing alcohol to clean it. Look at it also for damage, bent parts etc. Good luck,

FYI, electronic speedometers use an electric motor to spin the parts, doing the same thing. Don't touch them with bare fingers.

 

[Smaug]

FYI, the speedometers are driven by wind resistance/shear. Those older models have a cable spinning a half wheel (metal round wing), very close to the needle drive. The spinning of the part that close to the inner metal ring (attached to the needle and odometer), it makes the untouched part spin.

Likely something has contaminated those parts, they should be extremely clean. Oils from your fingers will mess up the speedometer calibration. Other things can go wrong also, but that is common if the assembly is ever handled. Take it out and use rubbing alcohol to clean it. Look at it also for damage, bent parts etc. Good luck,

FYI, electronic speedometers use an electric motor to spin the parts, doing the same thing. Don't touch them with bare fingers.

This is looking like the trouble shooting is more work than it's worth. But it is beginning to look like it's the speedo head that's screwed up.. But would your idea account for the Speedo reading over/faster and the Odo reading under/slower?

Anybody know where I can find a GPS fed Speedo/Odom??? Not a mapper like TomTom, they got no odom function.

 

[boggs1227]

...worse come to shove you can always go to a local yard and buy another speedomoeter head for cheap and see if that cures your problem ,but yes this mean pulling the unit out of the dash....really not hard to do ,i would buy a haynes book this will tell you how to do this .

goodluck.

 

[CDW6212R]

Ditto, the answer is usually buying a new speedometer. I have my old 93 parts, 112,622 miles.

 

[Smaug]

Since the Speedo is only about 5mph off at 70 deg and above wouldn't the speedo reading correct itself when the cabin temp reached say 70 deg F.... If it is the Speedo head? Well it doesn't, which seems to say the problem is on the outside of the truck not inside????? Any thoughts?
==========
The Odometer variance IS due to the fact I run tires sized @ 235/75/15 instead of 225/70/15 (old OEM). The 1/2 of 1 tenth reading less than actual distance traveled is correct, constant and acceptable. I don't yet know in which direction (up/faster or down/slower) and how much of a variation the speedo should show. How about you guys???