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tach question??
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Junior User
Joined: Aug 2006
Posts: 96
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tach question??
i have a 94 f350 superduty that says on the body it is a turbo but engine is not a turbo,had for two weeks now and working thru the trucks problems my newest problem is the tach falls off to zero at idle but the truck is running fine, stepping on the gas and the tach jumps up to what appears to be correct rpm, judging by the rpm/speed from before tach went nuts, as a side note i noticed that only twice since it started this that the little "off" light on the overdrive will blink on and off till the tach comes back up.any ideas or help would be great,doesnt seem to affect operation of the truck by dont like things doing strange stuff
thank you
thank you
Post Fiend
Joined: Nov 2004
Posts: 5,638
Likes: 1
From: Manitoba
Ok your trans/PCM/TCIL is telling you it is not getting a good signal from the tack sensor. It is located on the oil filler/timing gear tower. Check the wires where they enter the 1" nut, quite often the wire coating will pull back and they will short out. Fan wind can separate them enough to work.
The dead tach sender should/would send the automatic trans into a "failure management mode" (limp mode) with harsh engagements and firm shift feel, and an abnormal shift schedule. It has no effect on a manual trans only the Tach readings.
Replace the tach sender on the oil fill housing a big 1" nut with 2 wires. Ford only part Engine RPM sensor E5TZ-17B384-A about $50. You can remove and clean it but usually changed later too. You can test it too, hold it in the air away from ferrous metal, using an ohm meter lead on each wire you need DC resistance between 2000-3000 ohms.
The dead tach sender should/would send the automatic trans into a "failure management mode" (limp mode) with harsh engagements and firm shift feel, and an abnormal shift schedule. It has no effect on a manual trans only the Tach readings.
Replace the tach sender on the oil fill housing a big 1" nut with 2 wires. Ford only part Engine RPM sensor E5TZ-17B384-A about $50. You can remove and clean it but usually changed later too. You can test it too, hold it in the air away from ferrous metal, using an ohm meter lead on each wire you need DC resistance between 2000-3000 ohms.
New User
Joined: Aug 2006
Posts: 1
Likes: 0
From: College Place, Wa.
I have a 92 crew w/94' 7.3 and a Banks Turbo. I'm having the same problem- tach being erratic and od light blinking, I'm going to try changing the tach rpm sensor like you said. Be nice if this finally fixes it.
Thanks, Rex N.
Thanks, Rex N.
Post Fiend
Joined: Nov 2004
Posts: 5,638
Likes: 1
From: Manitoba
You can disconnect the batteries for 10 minutes then reconnect and go for a drive. The trans will learn the stratagy required to operate correctly.
There are some codes that will disappear after a few starts and some require being eliminated by a code reader after the repair is done..
There are some codes that will disappear after a few starts and some require being eliminated by a code reader after the repair is done..
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
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Posting Guru
Joined: Aug 2006
Posts: 1,049
Likes: 0
From: Perry Georgia
Is there any way to find out if the sender unit is bad other than just removing it and replacing it? I have a 1987 f250 ext cab Diesel automatic.
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