I have a 1991 f-250 7.3 with an E40D trans. When I let off the gas pedal and coast, the tach drops to zero rpms and the overdrive dash light starts to blink. When I accereate back on the gas, the tach returns to the right rpms and the light stops blinking. When this happens the transmission shifts and acts normally. The tach also works fine. Anyone have a cure to fix the culprit. Thanks for any help.

 

check your TPS on the side of the pump. I would be looking for loose connectios.

1991 F350 CC Dually in process of conversion from 7.3 IDI Diesel to stroked 460 Gasser (521 CID).
E4OD Pulling 3300lbs.

 

You need to take your rig to a tranny shop and have the E4OD scanned for trouble codes - this shouldn't cost much. Some shops will do it for free. A flashing overdrive lamp is your tranny's way of telling you that it's sending a trouble code. Page 119 of your Owners Manual mentions this phenomenon. Any code(s) are stored in the transmission control module until uploaded to a code reader. Don't get nervous just yet - It could be something simple and cheap like a bad signal from the TPS (throttle position sensor) or the VSS (vehicle speed sensor), or a noisy tach signal. I happen to have the same rig as yours ('91 7.3 w/ E4OD).

 

 

I've had the same problem before. My batteries were toast.


When you rev the motor up the alternater is outputing sufficient current to support the electronics and at idle the output drops off.

Get your batteries load tested.

 

mine was doing that only when the tach quit the trans went into limp mode(shifted really hard)replaced the tach sender mounted on pump drive cover and all is good.
hey travis how come your dumpin the deisel?do the guys over at TDS know about this?

 

Mine is doing this right now, just started this morning. I know little of transmission.

Started shifting hard, and overdrive light flashed off and on in idle, once up to speed its like the transmission constantly fluxes trying to settle in on a gear but there's really no true shifting going on. Sound familiar? I hope its not a big problem, I have had more trouble with this truck than any other behicle and am about to throw in the towel. Its a 94 350 turbo deisel. Any ideas?

 

Before you take it to a tranny shop, both of ya, check your VSS or Vehicle Speed Sensor. It is located in the rear diff, top towards the front. Has an electrical connection going to it.
That little sensor in there (use a 1/2 inch wrench to undo the bolt, then a flat tip screwdriver to pry it out, GENTLY) lets the tranny know when to shift and stuff. When you put it back in, make sure to seat it all the way in. The bolt just keeps it there, does NOT help to seat it in.

My 94 started to do the same thing, I bought it after it sat a year or so.
I pulled that sensor and it fell apart in my hands. The new one worked for a day, until it started doing that same thign again. (OD light blinks, spedometer is jumping, etc).

I pulled the sensor again, and cleaned the metal off it, and it worked again for 2 days. Cleaned it off again, and it is pretty good for a while now. But I have to check it every 2 weeks or so, less and less each time.

One thing to note, the tranny seems like it is hard shifting when that happens. Alot of places will tell you to rebuild the tranny. The sensor costs 23 CDN. Try that first.

GunNut, if the price is right, I might be interested in yours, IF it is a CC - and if you get too tired of that old beast.

 

Thanks for the input. I picked up a new tach sensor today, mine has exposed metal on the wires at the sensor end and looked pretty ratty. I'm going to plug this one in tonight and see if that's any help, then I'll check the VSS (will probably do so just for good measure, at least locate it and "get to know my truck".

Being a newbie to all this deisel and Fords in general, what is "CC"? I may indeed be selling soo, and price will be right (because if you search my posts you'll see I have issues with coolant getting into the engine, haven't fixed it yet, its intermittent and only happens sometimes). The truck has a lot of minor problems that are simply age related, like leaky gaskets on caps and drippy power steering and minor easy to fix things that are more annoying than anything else, has a new starter, new pos batter cable, new tires, new brakes in front, hell, I've put in a lot of little "new" things. Cooling system passed a pressure test just fine but I'm still slowly losing coolant.

Where are you at? I'm way the hell down here in Texas. Not selling yet, but if I do I'll let you make me an offer.