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91 F 250 with IDI diesel, start up no tach....a few minutes go by sometimes much as 30 then it start working. Transmission is in limp mode until the tach starts working. It has the e4od with 5000 miles on new rebuild. If I turn engine off and restart, the tach wont work again. I have replaced the tach sensor with a new one from ford...I have replaced the relay on the drivers fender as well. I have cleaned and used dieelectric grease on everything I can think of....what am I missing? I also pulled the tach out of the cluster and cleaned it and the contacts in back of it and ensured a good connection. It is really driving me nuts! I have no clue...please help! Someone said there was a 1"x1" box near the water neck under the fuel filter that has something to do with the tach.... I found it but not sure that is what is causing my issue... any gurus that can heelp me out???
If your sure you have no bad wires and all connections are clean, I would suspect the TECA (computer). The TECA takes the tach sensor and buffers it and sends it to the tachometer, so what I would do is unplug the TECA and jumper 2 sets of pins and start your truck up and see if the tachometer works that way. you will need 2 piece of wire nothing too long and follow the picture. This will NOT harm your tansmission as your not driving it, IF you do drive it the trans will be in limp mode, exactly what the TECA would do just so you know.
Ok thanks! Im not SURE all the wires are good but I've looked at everyrhing I can see and didnt find anything so far. Where does the TECA mount? I havent seen anyrhing with that kind of plug on it but I will look closer this evening and test it out
That verifies your tach sensor and tachometer work, I would check a few more things like is the TECA for sure getting power? does it have good grounds? stuff like that before getting a new one. Junkyards, ebay, rockauto all have them, if you get a new one try and get a 93-94 one as those ones had updated programing in them, they are plug and play compatible with your 91 (do NOT get one for a power stroke it will not work, 94 is the mix year here)
pin 60 and 40 are ground, black/white it grounds to the rad support.
pin 57 and 37 are powered by the relay which is turned on by the ignition switch
pin 1 is keep alive memory and is yellow
Does the cancel overdrive switch work when you press it and the light cycles on and off?
Is fuse 5 (15A) good in the under dash fuse panel?
Do you have 5 volts at both the map sensor on the firewall by the blower motor and the throttle position sensor on the side of the injection pump brown/white is the wire color (key must be on for this test)
When the rpm guage is working the OD switch does cycle on and off when you push it. When it isnt working the switch is dead and has no light. I will check on the rest tomorrow in the light to see what all Ihave on the map and tps sensor
When the rpm guage is working the OD switch does cycle on and off when you push it. When it isnt working the switch is dead and has no light. I will check on the rest tomorrow in the light to see what all Ihave on the map and tps sensor
That to me indicates either the TECA doesn't have power for the first bit for whatever reason like a bad connection or possibly an internal fault and either after worming up and driving the connection gets better or the TECA warms up and the internal fault goes away, usually heat related problems like the device work only when either warm or cold can be bad capacitors which theses units are known for. Tomorrow when you plug the TECA back in and it's not working I'm thinking you won't have the 5v reference at either of those sensors until it starts working again.
Ok, I checked voltage at the map sensor and have 5 volts there and also 5 volts at the TPS. The #5 fuse is good and I have voltage at that fuse too. Is there anything else I can do to troubleshoot and further help diagnose? I really appreciate all the help. I love this old truck and I would hate to give up on her.
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