Speedometer jumping ahead, not bouncing?

[Darrman]

I have noticed that from time to time my speedometer will all of a sudden jump and hold a higher than right speed.
Yesterday I was on my way to work, getting on I35, had to rapidly accelerate to about 70-75, not punching it but getting on it alittle more than normal. So I settle to a 70mph cruise, look back at the speedometer a minute later and it says around 85 mph, but impossible because I am only at 2400 rpms and my truck will be at 2900=3000 to pull 85, because of my 32's. Plus the speed is holding but the needle is trembling a bit. When I slow down it drops but keeps doing it when I accelerate again.
Of course today it works perfect again. It comes and goes whenever it pleases, sometimes months pass before it will do it again.
Any ideas what this could be, it does not act like a sticking cable. I have pulled the speedo gear out of the transfer case and its in perfect condition.
Thanks for any help!

 

[FixingDaily]

Edit, 11-7-10

My issue with the speedometer jumping suddenly reappeared. All other items still working great, but the speedo, after 2 months of behaving, just started doing it again. Must be the speedo itself as stated in "Finds" post below.

I still am happy that changing the PCM out fixed so many things though. Just must have been coinsidental that the speedo started to work correctly at the same time.

Good luck in fixing it.

******************************************************************************

Oringinal Post


You are describing the same thing that evolved into a several year search for the cause on my 94 Explorer. I tried changing the speed sensor but that did not help. Then the cruise control went out and after several years, all sorts of things stopped working. After getting lots of good advice, and trying a ton of stuff, it turned out to be the PCM/ECM module was goofy (search fixingdaily for my post on what was going on at the end for me). I got a salvage unit from the local junk yard ($65.00), plugged it in and all was well. The 94 Explorer is now like new, starts and idles correctly, speedometer reads correctly, cruise control works again, good acceleration, etc. The biggest problem in diagnosing this issue was the PCM/ECM won't give you a code telling you that the PCM/ECM itself is really the problem.

I see on e-bay that most of the replacment PCM/ECM units are about $100, assuming you can't get one at a junk yard. Be sure to get the right numbers off your existing unit as they are different depending on auto/manual, 2wd, 4wd, etc. Given the cost of all the other sensors I replaced, only to find the replacements seemed to be doing the same thing as the part I replaced, I think I would have gladly tried the PCM/ECM first.

Good luck in getting it fixed.

 

[ragajungle]

Fixing:
My 89 B2 does it - but on the RPMs. The gauge jumps from normal accelerating (2K) t (4K).... PCM also?

 

[FixingDaily]

Ragajungle,

I never had this problem so I wouldn't want to give you bum advice by saying that the PCM is the cause. Do you have any other issues associated with the erratic tach readings? If there are some other issues you are looking at, then maybe trying to replace the PCM might be a good place to start.

Other basic things you may want to check:

1. Battery and Charging System are in good order.
2. Check to see that all connectors assemblies under the dash that may be associated with the tach are well connected.
3. You may want to disconnect and reconnect the PCM multi-pin connector. This may help on any intermittent connections that may have developed though age and tarnishing.

Other than that, not sure where to go.

Good Luck on getting it fixed.

P.S. Latest item for daily repair, Intermittent ABS Light. I think I fixed it by pulling and re-inserting that ABS relay in the power distro box. Now it does its self test then the light goes out after starting.

 

[FIND]

sounds like your speedo is going bad.