So I have a long-term project that may involve upgrading to a newer engine on a 78 F250 4X4. Right now, just at the pondering stage but one thing I am pondering about is what everyone does for the rear axle speed sensor when they put in a newer EFI engine that didn't have a speedo cable.

1. Is this circuit used just for speed info (i.e. does it just go to the cluster?) or does it somehow go back to the PCM and used to control engine functions?

2. I can't imagine that you could feasibly add a tone ring to the rear axle that didn't have one so what do folks do about adding (or getting around adding) this circuit?

Sorry if this has been asked before but I haven't seen it come up in the "upgrade" swaps I've viewed (granted, just a few so far).

Ironically (or maybe "fortunately"), I work for a company that makes "encoders" and I could easily design a piece of tape and sensor that you could wrap around a drive shaft and simulate the signal that is coming out of the rear axle speed sensor (think it's called the VSS, right?). It would be a bolt on affair and the only challenge would be keeping the tape clean with either shields or wipers but I'm sure it could be done (probably best coming out of the tranny or xfer case and not down by the diff).

Any thoughts or am I just way over-complicating this?

Late add: I did see this: https://www.ford-trucks.com/forums/9...-question.html but it didn't seem to address the issue; the link buried in there appeared to be for some other vehicle-specific conversions and not Fords.

Thanks,
BB

 

From my research, it depends on what newer engine tranny you go with. Some of the older EFI engines, with say an AOD tranny, some of these have the VSS in the the tranny. Even on the ones with rear axle VSS, there is also the possibility that there is still the spot for the transmission one. I do not believe that the older EFI systems used the VSS for anything in ECU, but newer new ones use it for lots of stuff. You really need to figure out what you want for motor and tranny and go from there.

 

VSS needs will be dictated by the engine you choose.
It's easier to swap a "tone ring" into a diff than you'd probably think, i've seen it done onto 9" diffs with factory 8.8 rings, and the D60's that came in vans have a tone ring that can be adapted to older truck D60's. you have to really want to keep those rears, its easier to just swap an 8.8/9.75/10.25 axle in that already has one.

most ECM's will go into a limp mode if you have the VSS disabled and try to go anywhere. they don't like to see the engine/gears spinning and tires not going anywhere (since the VSS is generally driven off the ring gear, and thus corresponds to the tire speed).

there are also companies that will machine a transmission output flange to have notched teeth in it and then build a bracket to mount the sensor.

 

Okay, that all makes sense. I wasn't necessarily looking for an "invention" opportunity (already have plenty I'm working on); sounds like there's potentially some easy solutions. I mostly just need to decide what the heck I'm going to do with "the beast".

BB

 

It depends on which engine/trans you decide to go with. Most early efi systems only used the vss for transmission control. Even the control pack for the new coyote motors don't require a vss. Which engine transmission did you have in mind? There may be an alternative to reengineering something.