Van B350 15 person van has computer problem???

I have a B350 MaxiVan (Van Pool) that has started not shifting into overdrive and the speedometer is dead. When the speedometer isn't moving the van will not shift into over drive. Every so often for no apparent reason the speedometer starts working and then it will also shift into over drive. The check engine light also comes on when all this happens. I just went through all the electrical connectors to the computer and everything else I could get my hands on and nothing has changed. Everything else works great. Anyone have any thing I can check before I take it to a repair shop and start spending big bucks?

Almost forgot. Will it hurt to run it without it being able to shift into overdrive? I'm most concerned on the freeway when you have to go 75 MPH to stay with traffic. I don't want to blow up the engine.

It won't hurt it at all. Vans never used to have overdrive.

Thanks....It just sounds so loud with the fan and all running at a higher rate. Just wish I knew what caused the problem. I hate to replace the computer $500 and find that it didn't fix it.
When the check engine light comes on doesn't it record an error code that can be retrieved? Do you have to take it to a shop to get that code or is there a trick to read it with lights or something in the dash? I know my Jeep has something like that.

Hi, my 1990 B-350 ( with Overdrive!) had the same problem. Post here if the problem is still happend. May i can help you.
Reiner

I used to have the speedometer problem until my mechanic the speedometer pickup and display unit in the dashboard. The lack of speedometer is why the overdrive does not work.. the ECM needs the speed to shift ot overdrive. If there is nothing wrong with the speedometer pickup and display module look at the ECM.

However a bad ECM will usually have more than one problem and they will be unrelated as your two are.

What is an ECM? I did take it to a garage and they said the codes showed a speed sensor was bad. I replaced it for $50. It just screws onto the tail shaft of the transmission. But now it has started doing the same thing again but intermittenly....almost like there is a grounding problem or something...then again it could just be that the part has gone bad...that does happen.

From witch year is your van?

ECM is "Electronic-Control-Module" behind the engine under the hood.
The code only tells: there is no signal from speedometer! The reason may be not only the speedometer! May be wires or bad plugs ore shortened wires in this line.

Yes its correct.....without speedometer signal...no overdrive!

Its easy to test at the time when the problem is happend!
Now you know where your speedsensor is placed.
unplug the two wires (or one plug) on it.....start ignition...check the voltage!
one must have 5 Volts, the other zero! You have to place one wire of the multimeter to chassis or ground, the other to one of the two wires.
If thats ok, start the motor and shortcut and open the two wires very fastly with a short wire together.

If you are quickly enough, (5-10 times a second)normaly you will see a result on your speedometer and the engin get a other noise because rpm changed a litte bit.

If you have checked this, answer here.
Reiner
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I replaced the speed sensor and it seems now to work. I bought a cheap OEM version from Kreagon and it worked for awhile.

I have another problem that started right after my wife filled up a few weeks ago. This is a 1992 B350 with the 360 FI engine.
When it warms up and you are traveling with very minimum acceleration it will "cough" just almost like a backfire through the carb. In fact that pretty much describes it. If you floor it the van takes off like a scared rabbit. I thought it would start failing when I took up a steep hill.....just goes great but if you are going 25mph and ever so slightly start to speed up it will do it's cough. I checked everything but the fuel filter because I have been told it is in the gas tank.
Would a clogged filter do this? I thought it might be water in the tank but I've added some additive that is suppose to clear that up and it still does it's cough.

Any ideas?