Note: This issue and information is only applicable to the E-Series Van. The F-series trucks are different.

Hey guys, I'm back again. Seems like a get a new problem every week. - My vehicle is a 99 e350 van converted to 4x4 by some previous owner.
Latest issue is that a couple days ago my speedometer started jumping like crazy and now the van barely shifts. I replaced the VSS and that didn't help. I have a P0500 code reporting bad VSS from the PCM. I pulled up Forscan and if I look at the ABS module rear speed sensor, the sensor is reading just fine. If I pull up the PCM module and look at speed, the readings are all over the place. Based on the research I've done, I thought the ABS module is what decodes the VSS signal and reports it to the PCM.
So I'm trying to find a wiring diagram that shows how (and what wires) are used to send the current speed information from the ABS module to the PCM. But when I pull up the wiring diagram for my vehicle on Alldata and Mitchell1 it shows the rear diff VSS as going to just the ABS module, and there being a separate VSS on the transmission that goes straight to the PCM and to the dash. I cannot find any information on the connections between the ABS module and the PCM module. I've got a sensor on my transfer case that looks the same as the alternate VSS sold on auto sites, which has a plastic gear. However, the wires going to it have totally different coloring than what it says in the wiring diagram which makes everything even more confusing. Is the wiring diagram wrong?


Any ideas as to what wires connect the ABS module to the PCM so I can do continuity checks? Or does the PCM use a signal from the transmission mounted sensor instead of the rear differential sensor?

 

Have you chased the wiring from the sensor at the diff? I had a similar scenario last spring. Started with the speedo going haywire randomly, eventually quite all together. Had code for sensor so I replaced it, still no go. I found one of the wires had worn in two at the hanger. Repaired that and not a quiver in the needle since

 

I haven't done a full electrical continuity check, partly fed by the fact I have no idea where the ABS module harness actually is. But I didn't believe I needed to do that because of the results I got from Forscan. The ABS module is getting a good and accurate signal from the VSS. The PCM however, it isn't getting the signal cleanly. I've attached a picture of the data below.



What I'm trying to figure out is if the issue is a broken wire between ABS module and PCM, how can I figure out which wire, and which ABS module it's coming from? Apparently there are two ABS modules, a Rear ABS and a 4wheel? I can't seem to figure out which ABS module outputs to the PCM, and what wire it does that over. I've spent hours looking at electrical diagrams and there is no connection between the ABS module and the PCM according to any of the diagrams, which is the opposite of what I've read in tons of threads on here and other forums. I've read online that there is a "grey/black" wire that sends speedometer data from the ABS to the PCM, but not only are there no grey/black wires that even go to the ABS in the first place, the only grey/black wire I see in any electrical diagram goes straight from a mystical tranny VSS to the PCM and bypasses the ABS module altogether as seen below. Does the van use two separate speed sensors while the trucks only use one?



Tomorrow I'm going to replace the connector to the VSS and rewire a couple feet where the wire flexes the most - but per the troubleshooting I've done, this shouldn't be the issue. And According to the wiring diagrams, I should be able to disconnect the rear diff sensor altogether and not even notice a difference in behavior. Only unplugging the speed sensor on the transfer case/tranny should have an effect on what the PCM sees... or am I missing something here?

 

Brother came over and we did some more troubleshooting. I disconnected the rear diff VSS and it had no effect whatsoever. I pulled up forscan and it was showing zero for ABS module rear speed and the same normal erratic readings for PCM speed. I then disconnected the transfer case VSS and reconnected the rear diff VSS. It showed zero for PCM speed and showed ABS speed as increasing.

So this definitively shows that my vehicle has two speed sensors. One is only used by the ABS module and the other one is only used by the PCM. The modules do not share data/readings.

So I bought a new transfer case VSS and installed it. This greatly improved the shifting issues and the speedometer went from not working at all to just bouncing a lot (5mph constant fluttering) I followed the wires to the transfer case VSS and it looks like the previous owner extended the cabling by several feet to accommodate the sensor's new position with the addition of a transfer case. I'll be redoing his wiring in the hopes that does the trick. The plastic gear on the end of it has some wear, but nothing I would expect to have caused this level of flutter in the speedometer. I've attached an image of the new PCM speed signal, going from 0-40mph. The original signal would drop out completely from time to time, and then have random 50mph spikes, so at least this is better.



How much wear on the plastic gear is acceptable? How much flutter in the speedometer signal is normal?

 

Nice, sounds like you just about got her fixed.

 

It should be made known, for others that may do a search for a speedometer or shifting issue, that the vehicle in question is not a factory-built vehicle but modified.

A factory 99-03 F-series Superduty uses ONLY the vehicle speed sensor on the rear differential. This variable reluctance sensor uses a pair of wires to send an analog AC signal to the antilock brake module (either rear wheel ABS or four wheel ABS) located under the driver's side battery tray to be converted to a digital 8000 pulse per mile signal that is sent to various modules in the vehicle. No other inputs or outputs need to be present for this system to function (other than power and ground).

 

Cleatus - This isn't an F-series truck, but is an E-series van, and could be related to why I've had a heck of a time finding information. Although it was modified to 4x4, the wiring diagrams makes it clear that the original design had the VSS used for shifting and speedometer located on the transmission - NOT the differential like in the trucks. My little investigation basically just confirmed that the wiring diagram from Ford is accurate. In the E-series for what appears to be just this year and engine combo, the VSS in the differential is only used for ABS functions and if disconnected has no effect on the driving/speedometer/shifting of the van. Other E-series models/years do not have a double vss like mine, and neither do the trucks which leads me to believe it is specific for just this year with the 7.3l engine.

I had an argument with a parts store employee who was adamant that the mechanical-electric sensor being shown for my van in his system was wrong. But it's what I have installed and it is one of 2 different VSS that come up when I search for this year and model of Van - both of which I have and have replaced.

After looking into it deeper, it looks like the wiring isn't the cause of my jumping speedometer. It's the arcane design ford used on this sensor which uses a plastic gear to spin an encoder that sends pulses to the PCM. The problem is that the plastic gear is slightly worn, and it mates with another plastic gear in the transfer case that is worn too. So to really fix this I need to take off my transfer case to replace both worn gears. Yay....
Since I couldn't find any info about how this VSS worked in the 7.3 section, I had to lookup what Bronco owners do. This is because the Broncos use the same mechanical-to-electric VSS, and they have gone through this exercise many times to fix jumping speedos.

What I'm wondering is if there is a way to bypass this using the rear VSS and have that signal go to the PCM. I've been having a heck of a time with this weird 1998.5/e99 van being an odd duck, and I'm wondering if I should just swap it out for a 2000+ pcm and harness. Looks like other years use the rear VSS and not the transmission VSS.

 

I realize that but you made this post in a primarily F-Series section so that's why I interjected.




Regardless of what the problem turns out to be, you SHOULD only have one vehicle speed sensor in use at any time. Here's something to look into:

Any signal from a reluctor (analog AC output) should use wiring that is TWISTED. The factory wiring is simply a twisted pair only and it runs the entire length of the vehicle. What this does is creates an inherent shielding effect both keeping the signals from being bombarded with electrical interference as well as emitting RFI. The signal you have plotted is VERY noisy and no whiz-bang electronic device could ever decode that. For an example of how sensitive these types of signal generators and receivers are, one has only to damage ONE tooth on the 120-tooth reluctor in the rear axle to see how the speedometer/transmission function is affected. Also, and I'm not saying this as a factual or prophetic diagnosis, but does the speedometer care about an initial rising or falling edge signal when moving? Again, I don't know, but an EDIS system that I got rid of when going to a newer PCM on a Crown Vic sure didn't like reversed wires on the camshaft sensor until I reversed the wires.

 

Cleatus - I added a warning to my previous posts to make sure future readers don't get confused.

So I did some investigation that I thought maybe one person on the internets would find interesting. You are definitely correct in that an AC signal like what we would see would be sensitive to interference. But before I spent the effort on rewiring with a twisted pair, I wanted to demonstrate that the signal is bad because of the wiring and not the gearing or sensor. So I pulled the sensor, put it in the drill and gave it a whirl. Beautiful signal with no noise/interference. I then turned the engine on so that the alternator would be running and making things electrically noisy... and the signal was still great. So at this point I don't think it's wiring or the sensor. I tried the old and the new sensor - and I get a great clean signal no matter what I tried. The gear on the sensor has a tiny bit of wear, but not enough to make the signal as bad as what I've been seeing. So I snaked my phone up over the tcase and snapped a picture. The worm gear in the tcase is basically gone. I think that's probably why I have this issue. I can't seem to find anybody who carries these plastic gears in stock.

 

To finish the story: I went to a junkyard and pulled a transfer case speedo gear from an old bronco. The bronco looked like it had a nearly infinite number of miles on it, but the gear looked basically new. It was a PITA to get off and to get it on my van, but that fixed the problem alright. The oil in my transfer case didn't look like it had ever been changed, which I believe was the reason why the gear went bad in the first place.

With that fixed, the speedometer is steady as can be with a beautiful signal. The wires aren't twisted pair, but I'm not getting enough interference to be detectable. She shifts just fine now.

 

Well done Mikethezipper! It is these oddball problems that are somewhat intriguing to us and we appreciate you closing the loop on the issue.

Just a thought... If you do happen to start to receive interference, you could try a shielded cable of some sort. We would use shielded cable for security purposes and in order to keep interference out of the circuits as much as possible. It is easily acquired and fairly cheap now a days.