Sure, you can just rig a switch. The pressure switch just makes it automatic instead of manual.

Another option on the speedo is an '89 or older mechanical speedo.

Ace, I think I figured out why your speedo dies when you unplug the PCM. The only ground for the VSS is in the PCM. If you supply a ground to the VSS the speedo will work.

 

Excellent, thanks Dave I'll give that a try!

 

This is getting interesting again. I finally got around to it, and guess what? Still no go. There are three pins indicated as ground: "signal ground" and two more further down the row called "power ground". I tried grounding each one, one at a time, and still no speedo.

Not sure if I like the name "power ground." Sounds like an oxymoron to me. I was waiting for the dash to up in a big cloud of smoke.

 

How about grounding the VSS directly?

 

I thought about that, but I don't know which wire to ground and I wanted to test your theory before cutting into the VSS wires. If the speedometer (or VSS) is getting ground from the PCM, it has to be getting it on one of those pins, right?

 

Yep. The orange/white wire is the signal wire.

 

If memory serves, your A/C may have to be rewired too. Don't remember for sure off-hand.

 

Nope, HVAC controls are independent of the PCM. Here's the list of things for the PCM (91.5):

1) OD
2) Cruise
3) WIF
4) Grids
5) Speedometer/VSS ground (what pin??????????????)

There's more for the 92-3's (VR is all I am aware of). It's not really much on the 91.5, especially with the Getrag and no cruise.

It's easy to see how they were already planning the move to electronic everything at this point, with only about a dozen or so of the 50-pin interface being used. Then this particular box became obsolete before they filled it up a couple years later.

It wouldn't make much sense to replace it if you can manage to rewire a few things yourself. I think used ones are selling on eBay for like $3-$400.

 

Let me find my 1990 FSM, and look at the wiring in it, as I remember my nonIC 91 having no PCM (and eating half a dozen voltage regulators in the process), and an electronic speedo.

 

i removed the pcm when i did the getrag swap in my '92. The alternator is now externally regulated.

To be honest, i did away with 99% of the factory wiring under the hood. It had been hacked and butchered too may times by POs. My ignition is controlled by a switch in the cab. When i flip the switch, a relay is triggered, which activates the charging system, my battery voltage gauge, and an hour meter, as well as an indicator light telling me the ignition is on. A pull-cable, shuts her down.

A push button switch spins the starter through a typical "ford-type" starter solenoid. I now have a mechanical oil pressure, voltmeter, hourmeter, boost, and coolant temp gauges. I dont need any of the factory gauges to work. I also dont care if my speed works or not. In general, my truck doesnt go anywhere it needs to go over 50mph.

As of now, the truck is up and running, i am still trying to figure out some lighting issues. Headlights are not a problem, but i am having issues trying to get the directionals, wipers, and brake lights work without using the factory bulkhead. I think i can get around directions and 4-ways by using a generic "grote-style" directional switch. wipers are another story. I gotta study my FSM to see if i can still use the factory blinker stick/wiper control, but add my own wires to it somehow..........

it'll be a long winter