I have a 1990 F-350 7.3L Diesel and I can't get it into overdrive. The switch light seems to stay on no matter the position and my transmission selector seems to be misaligned (P R N D 2 1). Also my tachometer is not working but I am not sure if this is coincidental. Anyone have any ideas on this? Thanks.

 

It doesn't sound like a coincidence. Have you checked your fuses?

 

I check the fuses and found non of them were blown. I did see that there are some that are not the correct rating so I am going to replace them all. I am thinking it may just be the switch. Dealer wants $102 for a switch!

 

Could it be the VSS ?

 

Well I check the fuses, cleaned up the OD lockout switch tested it and reinstalled but I still have the issue of the OD being disabled. As soon as I start her up the OFF light is on and pressing the button has no effect. I am waiting for service manuals but in the meantime I was wondering if anyone had any ideas. The tachometer does not work as well. Where is the ECM on a 1990 F350 diesel? Thanks.

 

Well I have electrical diagrams, ODB 1 reader and I am going to look into fixing my tach. I did find that there is a 3 conductor connector under the dash that is not connected to anything and I suspect that this is my tach sensor. The fact that it is not connected to anything is a good indicator that this is why it is not working. Does anyone know where this connector should go? It is not obvious looking under the dash. This wire comes right through the firewall down around where the big wire connector feeds through the firewall.

 

I have a simular problem with my f350, were you able to fix you tach and overdrive?

 

Yeah it ended up being the computer. I could not perfrom any of the computer test either. Once the computer was replaced tach and overdrive worked fine.