TPS and Tach Sensor

[Drew2010]

HELP! TPS and Tach Sensor

Well guys, I just got done throwing my new to me 94 engine in my 90. Well it runs and drives, but I have 2 problems.

The first is: I dont have a working tach. It moves a little when cranking but when it starts, it doesnt move at all. I switched the sensor from the motor I took out, but still no go.

Second is: I was going to set the TPS, and I am getting almost 12v at the center pin. When I move the throttle to WOT, the voltage goes down. What could be causing this?

 

[Drew2010]

Anyone? I would really love to have this on the road tomorrow or the next day. Im tired of getting 10 mpg in my 96.

 

[DragRag]

The tps voltage being to high is not related to the tach not working. I did the same switch as you about a month ago. I put a 94' serpentine belt engine in my 1991 F350, and the tach was not working after swapping out the tach sensor with known good ones I found an issue with the harness, that is where you need to be looking. I honestly can't exactly remember what we did, but I ended up using part of my old harness to get it working since it is a necessary item with an E40D trans. Not sure on your tps issue, make sure you are checking the correct wire, and is should obviously be much lower then 12 volts.

 

[Drew2010]

Thanks, that is a step in the right direction. Ill start tearing into the loom today. Ill try and find wiring diagrams for each year and determine if something is different in the pins or something.


Any one have any Idea on the TPS?

 

[DragRag]

Can you swap on your old tps to see if the tps is causing the voltage problem?

 

[Agnem]

Did you remember to connect the ground wire from the engine to the cab?

 

[basilbowman]

On the tach - I pulled mine from a junkyard after my first one went out and they swapped right up. If I recall, it was a 93 into my 92. Does the spedometer/odometer work? Those three are fused together and will throw your tranny into limp mode when that one fuse blows.

 

[Drew2010]

Well guys. i tore all the loom off of both harnesses. turns out the small round connector on the 90 harness runs the tach and has a 12v key on wire, and the larger round plug runs the TPS and A/C switches. On the 94 harness, the small plug runs the TPS and the larger plug does the rest.

I never would have thought this would be. The connectors are the same with the same amount of pins and the pins were in the same location in the plug. I spent two days screwing with this stuff, without tearing the loom off, I dont know if I would have figured it out.

So if anyone is ever swapping a newer motor... use the original harness to the truck for this. Only modification, was to use a heat shrink adhesive lined butt connector to hook up the key on power wire to the wire from the alternator. It was a green with a red stripe.

A big thanks to Dragrag for giving me the idea to tear into the harness.