I have a 91 7.3 diesel with an E4OD trans. Transmission is new, less than 15k on it.
The problem is, my tach drops out, and the transmission goes to neutral. I replaced the Engine Speed Sensor, and TPS at the time I installed the new trans. Where do I begin looking? Should I just bring it to a trans mechanic who is knowledgeable on this trans?
I love the truck, but the trans is so sensative to sensors, it's driving me nuts.

 

When the tach drops out, does the speedometer drop out as well ?????

More information please.

Bob

 

No, the 91 has a mechanical speedo with a speed sensor on the cable. This problem began on a 2400 mile trip during the last 600miles. At first it felt like a skip or water in my fuel. It would be so quick, it was hard to see the tach drop. Now the tach drops to zero even at an idle. The engine is still running fine, but the trans goes into nuetral, or has no gear. It will not pick up a gear until the tach starts functioning.

 

For what it's worth, I had an 81' (manual trans) that kind of had the same symptoms.
Turned out to be one of the connectors on the horseshoe connector on the coil was arcing (not a good connection) and put on a pretty good "light show" when I popped the hood at night.

A simple new wire connector fixed it.

Hope some of this helps.

Bob

 

I will check connectors, and wires associated with the Engine Speed Sensor. But this is diesel,(no coil) and the tranny is automatic overdrive. I feel I am becoming an unwilling expert on the Ford E4OD trans.
I am beginnig to wonder if a 6 speed standard conversion will be easier, and cheaper than resolving these E4OD issues.

 

Sorry for the simple questions, but, is the tach good (not shorting out) ???

And is the fuse that is in the tach circuit (I think it's fuse #8) seated in the fuse holder nice and snug. A sloppy fitting fuse can cause the symptoms you're having.

Do you have any "tap in connectors" in the tach circuit that may be loose ???

You're losing signal/voltage/ground somewhere.

Bob

 

I will check those items, fuse, connectors. As far as I know nothing has been tapped into the tach circuit, but I will check. I will check wiring to rear of instruments. But I'm not sure how to check if the tach itself is bad, with an intermitent problem. However I will begin with what I can check. I will try a new ESS also.

 

But you did say "Now the tach drops to zero even at an idle."

Does that mean that the tach works correctly for a short period of time then it doesn't work at all ???

Hopefully you've found the solution at this point.


Bob

 

The tach normally works. It is gradually getting worse. At first it would feel like water in the fuel (a skip), the tach would flinch, and the truck would smooth out, and the tach would return to normal rpm. Now even at idle the tach will drop to zero for a second or two, then return to normal idle rpm, but the engine doesn't seem to actually drop rpm. IF, I am accellerating, and the tach drops out, the trans will slip or go to neutral, and the engine will race. When the tach returns to reading rpm, the trans will slam into gear. This makes it feel like a trans problem, but I believe it is a control problem. The computer is reading no engine speed, so it tells the trans, to release whatever gear it is in.
I guess what I am trying to avoid is throwing parts at this problem, because these sensors get expensive, and every time I have a shop put it on their computer they charge me $65 plus time at $85/hr. IF I could find a diagnostic computer for a 91 ford, I would buy it. I just am looking for a starting point with this issue.

 

Great !!! Thank you for the clairfication.

Now, hear me out........others have scoffed at this suggestion but here it is and why I'm pointing you in this direction.

Based upon your description, it (the tach circuit) is giving the symptoms of something failing/shorting.
Obviously, the tach signal has to "get into" the computer so that it can "tell" the transmission how to function.

The PCM, ECM, computer, or whatever term you'd like to refer to it as, can develop an internal problem due to age.
The internal problem is with the capacitors used on the circuit board within the computer.
Long story short, the capacitors used will fail DUE TO AGE.

Here's a link for you to click:
A9x ECM's (and same years ECM's) Failures Due to Age

Because you've described SOMETHING that is FAILING and hasn't failed totally yet, you may want to pull out your computer, open it up and see if there's any signs of burnt IC chips, resistors, diodes, or LEAKING CAPACITORS such as the above link describes.
Electrolitic capacitors, such as the ones used in our computers, that leak, are in fact starting to fail. ie. intermittant failing(s)..........see where I'm going with this ???

It may not be what in fact is causing you problems, but as you say, "every time I have a shop put it on their computer they charge me $65 plus time at $85/hr".
Save yourself some dough and give it (the computer) a check.
If everything looks O.K., then at least you've narrowed down the possibilities by one item.

Give it a check and let us know what you find.

Still here till the end,

Bob

 

I just looked at my Haynes manual and yes indeed, the tach signal goes straight into the "Transmission Electronic Control Assembly, a.k.a. TECA, a.k.a. the "Computer" at pin #36.

Strangely enough the tach is also serviced by the "A/C Clutch Resistor Diode". Hummmm.....somewhere else to look.


If I were a betting man............I'd place my bet on the TECA (Computer) having a or some leaking capacitors.

Still hanging with you,

Bob