On the way home from work my speedo went wacko and was bouncing between 45 mph and 60 mph. At 2300 rpms, it should have been at 65mph. To make matters worse the cruise control tried to keep up with the bouncing speedo.
Anybody have this before?? Tips on fixing it?

 

What truck? If it is the 89, I was under the impression that they still had mechanical speedometers. Possible broken drive cable?

The cruise control gets an input from the speedometer; so if it is bouncing around, guess what the cruise does.

If you have an electrical speedometer like on the 93-96 trucks, it is probably either the VSS mounted on the rearend or the PSOM itslef.

Robert

 

On the 89 the first thing I would do is pull the speedo and then lube the cable.

 

It's on my '88 ZF-5.
Do I pull the speedo cable from the dash - as oil doesn't go uphill to well.
Spray oil or heavier pour oil??
Sounds like an all day job getting the dash out, although I do have to pull my wiper relay box out and resolder that too.

 

You just pull the cable itself out of the jacket from inside the truck, that is why you remove the speedo. Then slowly slide it back in using wheel bearing grease on your fingers to put a coating on the cable as you slide it back in.

You do not pull the entire dash, just the instrument cluster.

 

I got the same problem on my 1992 f150. When I reach about 60mph it bounces around. I think it has to do with the sensor mounted on the top of the rear end differential of my truck but I'm not sure yet. Just started happening about a week ago.

 

Pull and clean the sensor, a new sensor is about $10-15 at the dealership, if the sensor doesn't do it then it's the PSOM, one word of caution, if you peg the electric speedo it will mess up the PSOM and cause the bouncing issue, go ahead and ask how I know.

 

Lubed it all up and still No-Go.
It works if I put a drill on the end of the cable that goes into the transfer case. The plastic gear looks to be worn in the middle of the gear, kinda like this )(. I'll try replacing the gear and see what happens.

 

Yea I think that's how i screwed up mine. I need to switch out the speedometer for one that goes to 120mph.

 

I replaced the sensor in the rear end on my 92' and it's a hell of a lot worse then it was before I replaced it!

The speedometer starts twitching soon as I start driving and will not stop. I think I have a wiring or a connector issue. Going to play around with it to see what happens. Take some brake cleaner and clean the connector best I can to see if that helps. Then I will go from there.

 

My truck did the same thing, and most gear drive speedometer fords do it. A quick fix for it is to ziptie the cable that the gear is on to the crossmember and get the gear on an angle so it grabs. Kinda have to ziptie it tightly. The other fix is to buy a new gear of course. I have mine ziptied up and haven't had an issue in 2 years and 10k miles.

 

Not to thread hijack, but a bouncing PSOM equipped speedo can be caused by the ABS brain too. My speedo took a dump one day and I spend a week chasing it. Troubleshooting until I was sure the PSOM was bad, so I replaced it. Then I tried a new VSS just in case.

The schematics show the only other thing on the VSS signal circuit is the ABS brain. Unplugged it... = problem solved! Tried another one from the junkyard, still no good.
My solution... who needs ABS anyhow? I grew up driving without it, I certainly don't need it now!

 

After replacing the gear and lubing the cable I am still having a problem.
It bounces from 48-55 at 2300rpm. It should be steady at 65at 2300rpm. I could care less if the speedo works or not - I just want my cruise control to work!!

 

How hard is it to disassemble the speedo itself and lube it? I ain't ever had a mechanical one apart.

 

What is the plug and electric wires just above the speedo cable, on the transfer case?? Could this be causing a problem??
On an 80 miles trip it starts out at 55mph doing 2300 rpm. By the end of the trip it is at 45-48mph at 2300 rpm. 2300 rpm should be 65mph.