1990 F350 No overdrive and no tachometer - Diesel Forum - TheDieselStop.com
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post #1 of 10 (permalink) Old 10-10-2007, 09:18 PM Thread Starter
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1990 F350 No overdrive and no tachometer

Hi All,

My tach is not working (stuck at 0) and Overdrive OFF light is constantly on (switch on dash). I cleaned up the switch and wiring harness in dash but no help. I also check all my fuses and they are good. Anyone have any suggestions as to where to start troubleshooting? Also which OBD1 reader would you suggest for this truck? Thanks.

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post #2 of 10 (permalink) Old 10-11-2007, 02:21 AM
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re: 1990 F350 No overdrive and no tachometer

A 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.

Check the wires where they exit the big nut on the oil filler housing , known to short out due to bare wire.....

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.

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post #3 of 10 (permalink) Old 10-11-2007, 05:29 PM
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re: 1990 F350 No overdrive and no tachometer

we had the same problem on my dads truck

it can be a sensor in the transmission

or the tack sensor it self

the computer

a relay

or a problem with one of the wires

company trucks
2002 ford F-350 powerstroke cc sb zf-6
1987 ford F-350 6.9L idi flat bed
1989 ford F-350 7.3L idi pick up

1978 ford F-350 custom
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post #4 of 10 (permalink) Old 01-21-2015, 02:59 PM
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1990 7.3D E4OD Trans Problem

My 1990 F-350 7.3 has a intermittent tach working. It doesn't work until about a half hour of driving. When it isn't working, the transmission goes into limp mode. When the tach starts working everything is fine. I have replaced the tach sensor and still have the same problem.

Any ideas?
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post #5 of 10 (permalink) Old 01-21-2015, 03:13 PM
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The problem is most likely in the wire between the tach sensor and the computer. Often the easiest way to fix this is to just run a new wire.

Former Automatic Transmission Engineer 1988-2007

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post #6 of 10 (permalink) Old 01-22-2015, 08:03 AM
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like Mark said the wire is likely broken.

Mine broke right where the two wires come out of the harness and go to the sensor. The two ends of the broken wire were close enough that it would be intermittent like you're describing.

It was easy to see once I looked for it. I just spliced in a short piece of wire with a nice loop so it could flex and not break again.

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post #7 of 10 (permalink) Old 01-22-2015, 11:07 AM
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But doesn't it sound like a moisture problem?
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post #8 of 10 (permalink) Old 01-27-2015, 11:04 AM
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Same component different problem.
My tach works most of the time, almost always at an idle it will drop to zero. Revving the the engine always re-activates it but sometimes I can also turn the headlights on or touch the brake pedal and it starts working. On one occasion it was erratic while running down the highway for a short distance kicking it in and out of over drive. I have tried different tach sensors. I am thinking corrosion somewhere. This problem started 5 years ago right before I parked it. It has been sitting for 5 years but as my '06 was stolen (see signature)I have to use this rig until I get back to work. Going back through all the connections again today but I can not find my wiring diagrams for pin assignments. Can anybody help with the pin assignments.

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post #9 of 10 (permalink) Old 01-27-2015, 11:53 PM
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I have same problem, what wire would best to splice into to run the new wire
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post #10 of 10 (permalink) Old 01-28-2015, 06:20 AM
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standerd I use on auto motive is no smaller then 16 guage, for a vss it should be twisted, how many twists per inch you'd need to look it up or measur the current signal wire

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