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Discussion Starter · #1 ·
I am :headbang: over this issue now. First it was only happening when it would rain. I replaced the tach sensor cause the wires going into the sensor were stretched and bare. Solved the problem for about a week and it came back.

Pulled all the grounds, cleaned or replaced as needed. That solved the problem for about 2 weeks.

Yesterday I replaced the TCM relay. It improved as now it only goes out momentarily when I hit a good size bump.

I can not duplicate the problem by moving any wires around, so trying to find a section of wire that may be corroded without ripping apart the entire wire harness is difficult.

Anybody got some information on what wires feed the TCM power so I can pull the plug and do a continuity test?

Note: When the tach goes out, the OD light does not flash nor does it come on when I push the button. OD system works fine when tach is working, so I am 99% sure the TCM is loosing power somewhere along the line.

As usual....thanks for your help guys!
 

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Check all the connections for corrosion. Pull the connector that goes thru the firewall and check that.

Does your speedometer still working when the tack goes out? Could be the tack connection on the instrument panel.

Pics are small, might be able to print them out.
 

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Discussion Starter · #4 ·
Check all the connections for corrosion. Pull the connector that goes thru the firewall and check that.

Does your speedometer still working when the tack goes out? Could be the tack connection on the instrument panel.

Pics are small, might be able to print them out.
Speedometer still works. OD switch stops working when tach is out though.
 

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I don't know if you fixed it yet but I had the same issue. Tach would die randomly on cold nights or rainy weather. Od button wouldn't work. What it was on mine was the eec relay connector and eec relay were both bad. I just bought a new connector on Amazon. What my issue was when it got cold or moist the connection would go bad till the engine warmed up enough for the contacts to barely set off the relay. My relay was also bad so I thought initially I fixed it but it came back later you have to change both and so far so good 4 months been working find no issues.

Did you backprobe the relay to make sure your getting voltage from the relay?
http://easyautodiagnostics.com/ford/4.9L-5.0L-5.8L/eec-power-relay-tests-1
Check this out its a simple way to test what part is bad cause I'm betting it's something to do with your relay
I went crazy trying to figure it out. Good luck
 

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By the way if you smack the eec relay your tach will die momentarily so you might want to mount your eec relay in a better spot if it's just dangling
 

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Discussion Starter · #7 ·
By the way if you smack the eec relay your tach will die momentarily so you might want to mount your eec relay in a better spot if it's just dangling
Thanks, but already did that. Still hunting through wires. Every time I stop the tach starts working so it is making hard to pin point cause shaking wires does not duplicate the problem and so far everything checks out with testing.
 

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I had the same problem and did every thing you already did also. finaly found the culprit when I pulled out the radiator out for replacement. wire harness goes infront of and towards the the top part of radiator and passes by the screw for the X strut on front of radiator. screw backed out and was rubbing on harness, rubbed the cover off of two wires in that harness and when ever i hit a bump or got into rain it would momentarily ground out and id loose my tach, thus looseing my torque coverter lock up.
 

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Discussion Starter · #9 ·
I had the same problem and did every thing you already did also. finaly found the culprit when I pulled out the radiator out for replacement. wire harness goes infront of and towards the the top part of radiator and passes by the screw for the X strut on front of radiator. screw backed out and was rubbing on harness, rubbed the cover off of two wires in that harness and when ever i hit a bump or got into rain it would momentarily ground out and id loose my tach, thus looseing my torque coverter lock up.
Really? I will check that tonight. I am still trying to figure this out. :headbang: Thank you!
 

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I tried to post a picture from standing infront of truck looking down at driver side of radiator
 

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