E40D TCM Compatibility

Thewespaul

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Gentlemen. Working on a 94 idit with e40d today, came in with a handful of issues, one being a wonky tach and trans shifting poorly. Checked my input and output at the tach sensor and I have 2.4v sitting on both sides with key on and off. Followed the power and I’m thinking the computer is shorted internally. I have another computer from a ‘90 f350 with an idi, can I drop it in to see if it remedies anything or is there a compatibility issue? Did some searching and got some mixed answers, wanted to be sure before trying anything.

Thanks guys ;Sweet
 

Thewespaul

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Forgot to add, I have replaced the tach sensor with a known good unit with no change in behavior, the trans is not in limp mode and has no flashing od light, but does take off in second if you don’t manually go down to first.
 

retiredinhomer

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have you seen your overdrive off light on at any time? I ask this because I had a bad cable in the column and the od off light was very intermittent.
 

retiredinhomer

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check the connector on the drivers side of the transmission. Look for green connections and looseness on the plug. I used some tywraps to keep the connection tight.
 

Thewespaul

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Thanks, mlps connector on the trans is cleaned and tight, as well as the connection on the firewall. No od light and I can turn it on with the button, so I know the bulb isn’t burnt out
 

david85

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Really not sure on this one. 1992 eliminated the speedo cable in favor of a common VSS on the diff to drive the RABS, speedometer signal and cruise control reference signal, if equipped. I believe the 1992+ clusters also have extra electronics that talk to the transmission computer. Not sure how the 1990 TCM will respond to the different environment.

Wish I could give you a better answer.
 

Thewespaul

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Thanks for the info, that’s my main concern with it being a different generation truck. Looking at the schematic the tach wiring looks indentical, so at the least I should be able to plug the spare confuser in and see if the tach works or if the unwanted voltage goes away. If not I’m thinking psom is really the only other component in that circuit that could be causing this issue, assuming wiring is good which the harness did check out okay.

Even if I can’t get a solid answer I’ll definitely give it a shot and report my findings.
 

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