OD light on, limp mode, think the wiring is burnt

[JSVD]

On our 89 f250 recently the OD light came on and stays on (dim). The tach stopped working and the tranny is in limp mode. I've researched it and know it could be the EEC relay, the tach sensor, or the ECU for the transmission. I suppose it could be the connections on the transmission, but I feel like the tach would work then?

I've tested the relay and it clicks. Also tested the tach sensor and it has good continuity. I then tested the hot wire comming into the EEC relay and it reads 0 with the truck off. I then tested all the slots where the relay sits for ground, and they all ground out. Never dealt with burnt out wiring, but that leads me to beleive I have a big short in those 4 wires. Am I on the right track?

Is anything else wired with those wires? I'd like to test and see if anything else is grounded out. Also if the ECU isn't getting power would my tach not work?

Anyone have a wiring diagram for an 89 f250 7.3 with the e40d?

I do not have a code reader... could rig up a test light but i'm pretty sure it's in the wiring.

Thanks,

Jared

 

[Black dawg]

I have had that relay fail twice on my truck, and both times acted just like that...no tach and od light on dim.

just because the relay clicks doesnt mean it is actually sending power.

 

[JSVD]

the relay clicking is it switching on.. but the realy itself is not getting voltage. The C terminal is actaully grounded out.. I'm pretty sure that means the wires are burnt somewhere.. I'm just trying to figure out where

I'm also wondering if the trans computer isn't getting power if the tach wouldn't work?

wiring diagram

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[trackspeeder]

Try swapping the relay with a known good one. They're cheap grab one from your parts store if you don't have one.

 

[JSVD]

I spent the afternoon working on it. Thought it was the fusable link so I replaced it with a circut breaker and it worked for a bit.. then back to the dim orange light and no tach.
After a while of fiddling and testing voltage I was back to my original idea - that the wires were burnt out somewhere. I jumped the relay directly from the battery and it all worked great. So rewired it all comming off the battery (through a circut breaker) to the original plug (the two yellow wires)

Now the light works and the tach works, but the trans still feels funny (was shifting strong before all this) Sometimes it seems like the tach isn't keeping up and its over revving. Possibly missing 2nd gear.

 

[skeeter72]

I had a similar issue with no tach and dim light. My fuse kept blowing. Found out the fuel heater in the fur filter was shorting it out. On the 94 the fuel heater is on the trans sensor, EEC/tcm, tach circuit go figure. Unplugged the fuel heater and have not had a blown fuse since. Hope thus helps a little.