Good morning all you fountains of knowledge.

I have been working on a persistent problem with my Bronco. I will try to lay out what I have observed and how I have approached solving it.

Vehicle is a 92 Bronco with a 302 (5.0 l) and E4OD transmission. There are 307k miles on the vehicle. The engine is a stock replacement for the original and was installed at 273k miles. The transmission is an OEM rebuild installed at 247k miles. Recent service includes replacement of both rear axles, bearings, seals and a new pinion seal (291k miles - 18 months ago). A while after doing the axle replacement (301k miles - 10 months ago) the ABS speed sensor on the rear differential and the Manual Lever Shift Position Sensor on the transmission were replaced with new sensors due to fluctuating or sticking speedometer readings and hard shifts while driving between 35 and 55 mph. The parts used were aftermarket parts (MasterPro MLPS 2-8825 or S9192; Standard STD ALS177 ABS sensor). I did not re-pin or replace the MLPS socket connector when I did this, only the sensor itself since the wire harness appeared in good shape. I also adjusted the shift cable in the steering column to correct the alignment of the gearshift pointer with the P-R-N-D-2-1 indicator since it was not correct.

After doing all this, the shift indicator worked normally, the transmission shifted normally, the speedometer functioned normally, and cruise control worked normally.

Over time, the problem with the sticking speedometer returned in the same fashion, sticking between 35-55 mph depending on how aggressively one accelerated. The transmission also began to hard shift into gear recently over the last thousand miles.

I just completed 30k service which included a drain of the transmission pan and refill. Took 9 qts. At this time due to the return of the shift and speedometer problem I replaced the MLPS with a part from the Ford house (F5TZ-7A247-AA) and this time I replaced the whole thing including re-pinning the new connector. I also replaced the ABS speed sensor with a new one since the MLPS change didn't fix the speedo or hard shift problem. With fresh fluid and the new sensors I no longer have a hard shift at 1-2 or 2-3. The transmission appears to operate normally. The ABS sensor was pretty clean when I pulled it since the fluid is nearly new because it was replaced when I did the axles.

The problem that I have now is that the cruise control no longer works. It was working before these changes even when the speedo problem was present.

I have checked #13 fuse and found it good.

I didn't disturb the vacuum lines to the servo (this is a vacuum cruise control edition mounted on the driver side inner fender above the ECM near the OBD-I port harness.)

I am looking for any information about fixing this cruise control and anything about what I may have missed in trying to get past the earlier hard shift - speedometer sticking problems.

I can provide pictures if needed.

EDIT: Checked codes and found KOEO - 452 - Vehicle Speed Sensor

Thanks for reading this far.

 

EDIT: I tested the operation of everything after swapping the old VSS into the rear diff. It doesn't matter which sensor is installed, the result is the same. Transmission shifts 1-2 slightly hard, 2-3 slightly hard, 3-4 normal. Speedometer functions normally through speed range with no sticking or fluctuating. The cruise control still does not work. I have replaced the fuse, first with another 15A from a different slot and then with a new one I had in the shop. No change since the fuse wasn't bad (obviously). Just laying out all the things I've done since the original post.

When I plug the Innova reader in and dump codes KOEO I get a 452 code.

Does anyone have the resistance that I should see on the VSS or any other info about the pinouts that I can check to narrow this down? What is the voltage or resistance I should see on the plug at the VSS with the key on or off?

Thanks

 

You either have a wiring or PSOM malfunction, the VSS signal goes through the speedo cluster(PSOM) before going to the PCM which controls both engine and trans.The "easy" thing to get at is the PSOM, you could try another to see if that cures the problem, if it doesn't then you have a wiring issue to rack down.

 

Thanks Conanski. I'll begin checking wiring connections and continuity tomorrow. I drove it today and like I mentioned earlier it shifts a little hard 1-2, but it also seems to drop down a little hard 2-1 on the downshift.

I recently replaced the hood release cable and it comes through the firewall near where the vacuum line comes to the brake pedal for the brake part of the cruise sensor circuit. I'm gonna check that out with my vacuum tester to make sure all those lines are still good since this cruise is the vacuum servo style. There was also a wire with a push-button switch installed into a hole in the old hood release cable. I think that was part of a security system installed right after we bought the Bronco in 1991 but I have to do some wire following to know for sure.

I'll update with what I find later.

 

Code 452 is a Continuous Memory (CM) code, it is not displayed in the KOEO tests. You stated you "dumped the codes" and code reappears. Is this after driving the truck?

The speed control amplifier gets it's power from Fuse 18 as you have found. The Brake On/Off (BOO) signal comes from Fuse 13 through the brake switch. Pressing the brake pedal sends the BOO signal to the servo amplifier. The process of depressing the brake pedal activates the dump valve to release the vacuum from the speed control servo.

Diagram of the vacuum servo speed control system on your truck:

Speed Control Amplifier


Speed Control Servo



Speed Control T/S Tips

 

Thanks rla2005. I'm gonna make use of this today and hopefully have an answer for why it quit working so I can return this Bronco to daily driver status.

To answer your question - The 452 shows up in the first part of the diagnostics using that Innova reader. It is a Continuous Memory code as you noted. The Innova flags it with a C to help you see which part of diagnostics reported it.

I have found that code every time that I dumped the codes since installing the VSS and the MLPS. Since it is a stored code and I have not erased it nor have I removed the battery cable or used the shorting pins or anything to clear it then there is the possibility that it is only reported until the vehicle sees enough starts without detecting the problem causing it to be cleared. Maybe that's how it works.

Anyway, after replacing those parts above, the transmission shift problems and the speedometer sticking and fluctuating problems are no longer present. That leads me to believe that the 452 code may be an old code that will clear itself with enough start cycles.

I appreciate the diagrams especially. This is gonna make it easier to track to a specific component of part of the harness.

I have shop manuals for the Bronco including electrical schematics. Unfortunately they are stored right now after I pulled everything out of my shop to do structural repairs on it and haven't completed those repairs so I would have to dig to find any of them. I appreciate you supplying the diagrams.

I'll update with anything that I find on testing.

 

I interpret "dump codes" as clearing them. If you are using the term to mean reading the codes then the answer is you need to clear the codes from Continuous Memory. Your reader has a function to do that. Otherwise you have to run the vehicle through 40 or 80 drive cycles to have the computer do it. I can never remember if it is 40 or 80 cycles, I find clearing the codes is a lot easier.

In my experience if you find all of the inputs to the speed control amplifier good and there is power to module then the amplifier is bad.

 

I want to thank everyone who read the original post. I especially want to thank u/Conanski and u/rla2005 for their very useful suggestions and insight, including the invaluable wiring diagrams.

Since the original post I got caught up in several things so it took me a while to finish all the troubleshooting and component checks. Luckily, though the Bronco is a daily driver, it is not my only set of wheels so I was able to leave it idle while I found the time to check it out.

I was able to fix this problem so that the transmission shifts normally now, the speedometer needle has no wobbling and does not hang up at any speed, and the cruise control works normally.

To accomplish this, I ended up testing the various components involved in the speed control and shift timing and narrowed it to the PSOM. I have a spare dash from an 94 F150 on hand that I had bought to replace a busted dash in my 95 F150 so I swapped the PSOM from that dash into the Bronco and gave a test drive. Everything was back to normal function after the PSOM swap so I consider this problem solved.

There was a large mileage difference between the two PSOMs. I am the original owner and will likely never sell it but I am wondering whether I have to take it to a speedo shop to have the mileage dialed up to the actual vehicle mileage. I do register it for road use every year.

The Bronco had 307405 miles on it when the PSOM failed and the F150 only had 127536 miles on it.

I documented the mileage difference and tagged the two PSOMs so that anyone looking will know they need to add about 180k miles to the displayed mileage to get the actual vehicle mileage.

Thanks for all the assistance. I was very valuable and I appreciate it.

 

I also wanted to add that the PSOM problem that I have is documented in several posts that I found. The components that appear to have failed on mine (signs of corrosion, burned PCB trace) are the same as in this video:


If you are handy with a soldering iron this may fix your issue without the hassle of replacing the OEM PSOM.