Erratic Speedometer?

[xccter]

My son has a '91 4.0 Explorer 5 speed (no cruise control).

The speedo needle pegged all the way past max and got stuck on the bottom of the cluster.

I took a small wire and put it through the lens where the trip odometer button goes through and as soon as I dislodged the needle it returned to 0.

It worked fine for several months but has now become erratic while driving to the point of being unreliable.

Is this speedo actuated by an electronic sensor of some sort, rather than just a mechanical cable to the trans?

Thanx.

 

[beauboy]

Yes it is. It is also excited by a magnetic spool. I've never heard of an electronic failure causing the erratic movement. It's usually a bad speedo. Mine did this about 4 years ago. I replaced the speedo cable at first. This didn't fix it. I checked on a new speedo and found that it would cost me around $580 just for the replacement speedo. I then went to a "U Pull It" junkyard and purchased one for only $42. I also paid the extra $5 for a one year warranty. Replacing the speedo did the job.

Others will jump in with other options. These are the only two I had to do.

 

[little x]

Nope, it's a cable that runs to the tranny. Completely mechanical.

 

[boggs1227]

if it's mechanical then it a small colored gear that the cable hooks into .the speed sensor gear can become worn and out of spec so to speak ....cheap part. if it's the speedo head then thats another story.i have heard people fixing them but thats a little past my expertise because they work of the ideas of spin rates with loaded springs ....could always replace it that may be a better idea for most people .

but i want to say that a 98 witha v-8 is a electronic type sensor ..not sure about that as seenig i don't own a 98 v8 x .


mine is acurate up to 65-70 mph then it really starts to hop pretty good ,but when i'm going down the road and am on flat even road it will read steady but as soon as i hit a dip i can see the speedo jump till the truck settles back down ...must be the gear or cable ends getting worn .

 

[beauboy]

Whoops. Perhaps the elec I saw in there was only for the back lighting. Thanks for the correction, little X. However, if the cable replacement doesn't fix it, then it's the speedo. Neither is a costly replacement (if you use a junkyard for the unit).

 

[Maniak]

There is also a drive gear in the trans/transfer case. That gear does wear down over time.. Somewhere around 250k miles I finally replaced ours.. It helped the bouncing speedo. Its not "hard" to get to.. at least on the transfercase. You pull the pinion out (one but nut, after you have the drive shaft off), reach in there, pull the old one out, and put on a new one..

edit: to check the drive gear, pull the speedo cable/driven gear out of the transfercase/transmision and look in the hole (or put your finger in there). In our case the drive gear was very worn and you could see it.. It had worn down to be concave instead of straight across.

~Mark