Hi all, as this is my first post, I wanted to start out by thanking all the regular contributors for their wisdom and insight. So far, all of you guys have been incredibly helpful in the previous advice you've given to others, and helped me avoid a tranny rebuild or replacement when it just came down to a few sensors. I've been lurking and scanning through thread after thread trying to find the answers to my problems, and while they've helped up till now, I've finally hit a stumbling block...

I bought an F-350 7.3 crew cab just over a month ago, with what seemed like a shot transmission (hard and late shifting, felt like in neutral while in 1st or 2nd, then drives fine in 3rd and 4th). Since, I've been able to diagnose the issues down to a couple shot sensors and or bad connections, though one is still eluding me.

I just replaced the MLPS and VSS, and seemed to solve half the problem, now just hard and late shifting. I scanned the codes and still came up with only PO500 (VSS input).. I've read in many other threads on the VSS issue that it could be shorted wires, the connectors, or the sensor. Trying to troubleshoot from the back up (while keeping it cheap and simple) I bought a new sensor and connector and soldered it on to the existing wiring, then plugged it in to the new sensor I had just installed. No change. Now the wires? My next plan is to just re-wire the VSS up to the PCM, but I don't have a manual on hand and cant find the PCM, though I can tell you that the ICM is not the PCM

So, where is the PCM on a 96 f-350 7.3? I do have a wiring diagram but can't find the module. The wires coming from the VSS are Red and Green, though the wiring diagram says they should be pink and grey. Obviously I haven't seen the PCM yet to determine if this is right or not, but I do know where on the PCM they should plug in. Do I solder the wires in? Would I be bypassing any other necessary circuit/connection? Should I not do this at all??? If this doesn't work, where's my next target? Another piece of info, the Cruise control and odometer don't work at all, as well as the speedo. I've been attributing all this to the VSS, connector, wiring, but haven't ruled out the PSOM, though I don't even know what that is, just read it in another VSS diagnostic/troubleshooting thread.

I'm hoping I can get some more good advice from all of you, and be it known that while my mechanical skills are usually sufficient to solve most problems, I'm a true novice when it comes to electrical and this is my first ford diesel.

Thanks in advance!

 

The way this system works is that the VSS in the rear axle sends a raw signal (it's a Hall effect inductive pickup, so it outputs an AC sine wave) directly to the PSOM input. The PSOM then scales the signal per it's (programmable) logic to a scaled value. This scaled speed signal is used to drive the stepper motor which moves the needle on the speedometer face. This same signal is also sent to the PCM, cruise control, and RABS module (anti-skid brake control) so they each know how fast the truck is moving.

When the EEC loses the speed reference from the PSOM, it puts the transmission in limp mode, where it operates within a safe set of fixed parameters as far as shift points and line pressure go. You feel this as hard, late shifts. The computer reports it as no VSS signal, although this is the "standard" definition for that code and is technically a lie in the case of our trucks. What it's actually missing is the PSOM signal.

Now I don't have a schematic handy, but I can tell you how to troubleshoot it. First we'll want to put the truck up on jackstands. Then reconnect everything at the VSS. Remove the gauge cluster and check your schematics to see which wires go from the VSS to the PSOM input. We're going to look for the signal from the VSS, so you'll need a multimeter set on AC volts, and make that an analog meter if at all possible. If you have fancy tools like a o'scope, all the better, but I don't and can tell you that an analog multimeter works best. Set it for 5V input, or whatever it's lowest range is.

Now just make sure the rear end of the truck's safely off the ground (make sure it ain't in 4 wheel drive!), start the engine up and put the trans in gear.
As the rear axle starts turning, you should see pulses coming into the PSOM from the VSS. As the speed increases, these pulses should increase in frequency as well. The meter might not be able to react fast enough and if so it'll just show voltage. I can't give you a known good value, but it's probably around a couple volts. We're more concerned with a signal being present than the actual value of it though. As long as it's there, and follow smoothly along with axle speed, the VSS in the rear axle and the wiring up to the PSOM is okay.

Now, check the schematic again and find the PSOM signal output. Measure this the same way we did the input signal.

If you have no signal into the PSOM, you know where to start looking.
No signal out of the PSOM, then PSOM is dead.

Everything after the PSOM is tied to it's output, and I doubt you have 3 failed circuits and/or modules (PCM, RABS, Cruise).

Also... I've done this same test on my truck for the same reasons. I don't know what the factory manual recommends but this is how I did it. What I found was that there was a signal being output from the PSOM, but... it was a bit unsteady. I looked at the schematic and started unplugging things that share the signal... when I pulled the connector on the RABS module, everything started working normally. Something in the RABS was injecting noise into the speed reference, even though it appeared to be working fine otherwise (aside from the slight unstability). Might give that a shot also.
RABS module is behind the glovebox, straight back, about 4" x 6".

Good luck and let us know what happens.

 

I know this is way late and I found it by searching google (couldnt find anything i needed on the forum search function) but this cleared up a lot of Questions I had thanks for the dissertation on VSS signals. My truck a 96 F350 PSD seems to be searching for gears when its unloaded (box truck portion empty) when she's heavy loaded it shifts fine. No codes no flashing OD light, Spedo is solid showing concurrence or close to it with GPS speed.
However this truck saw severe duty in Fairbanks Ak, the ARCO shop mechanics put at least two tubes of RTV on the rear diff VSS. I made the mistake of pulling it and of course the connector failed, honestly think it broke tabs off in Fairbanks thus all the RTV. Anyway i have a spare sensor (BWD) but I measured my OEM installed sensor at 1.3K ohms I read somewhere that is the correct value. Im just reviewing things before I head her back up to Arkansas.