Intermittent E40D Failsafe
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6th Gear
Joined: Oct 2021
Posts: 6
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Intermittent E40D Failsafe
Been lurking for years. Finally have something that might help others. Owned truck 15 years, early 1994 (IDI), E40D. Would go into failsafe sometimes twice in one day, sometimes once in a month. Sometimes at low speeds, esp at 1 - 2 upshift, sometimes on highway and would then go out of failsafe (OD light would change from flashing to out). .Multiple (seemingly unrelated) codes. Replaced: transmission, TPS, ECU, grounds, check connectors, no joy. Gave up, just drove it. Tach quit working. Replaced sensor. Problem fixed. Never saw a problem with the tachometer before, until it started drop to zero at idle. Replacing the sensor is easier if you loosen the three (10MM head) screws holding the vacuum pump and flip the belt over the top of the pulley.
Tuned
Joined: Dec 2013
Posts: 363
Likes: 37
From: Corinth, TX
glad to have you on board, I had to replace a tach sensor, it wouldn't register at idle when cold and would cause intermittent shifting issues,
a few years later I had the flashing O/D light return, I can't remember what the code was it threw but it was something generic, I ended up pulling the dash and cleaning the connectors on the speedometer plug, no issues in a year since then,
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a few years later I had the flashing O/D light return, I can't remember what the code was it threw but it was something generic, I ended up pulling the dash and cleaning the connectors on the speedometer plug, no issues in a year since then,
.
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6th Gear
Joined: Oct 2021
Posts: 6
Likes: 0
Tach issues
Yes, I think the point is that the tach sensor can cause transmission issues without noticeable problems with the tachometer itself. Apparently, a short "no tach" signal is enough for the transmission computer to go into failsafe without seeing anything wrong with the tachometer. Miss the days when there was just a throttle pressure cable and governor...
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