Overdrive light and no 4th gear -- comes and goes

My father in law has a 1994 GT that the overdrive light comes on once the vehicle has been driven a little while (30 mins sometimes) and then it won't shift up to fourth. Sometimes it will go off by itself, but most of the time, the car won't shift up from third gear. We didn't pull any codes off the computer and it's been to the dealer and several transmission shops with no solution.

I replaced the ECU from another car and the car ran great for about two weeks and then it went back to doing the same thing. I think it may be a sensor/module that gets warm and then malfunctions, but I am not sure if it is the one in the console (TCM) or in the trans itself (hope not!!). Is it possible to try a different TCM, reset the ECU or does he have to pull the trans out and see what modules inside the case are bad? Approx 130K on the current trans. A member suggested I look for any stored codes, but any additional info would be greatly appreciated.

Ted, Chicago

Hi, RE your probe with o/d lamp and no 4th gear, I am supprised that a reputable dealer did not find your problem as there is a TSB out on it. Your problem will not at times emit a trouble code, BUT the TSB should if I remember correctly reccomend replaceing the vehicle speed sensor, Which by the wa is a real pain to replace, I just hope you have the correct tools, no special tools necessary, it just cannot be replaced with plires or end wrenches................GOOD LUCK, Clay.....

You are correct about the VSS needing replaced. This is what's causing the tranny to not shift properly. The ATX relies heavily on the VSS and TPS to do it's job.

The VSS is not an easy thing to replace if you're not mechanically inclined. It is not very easy to get to either, as it's in a PITA location. The VSS sensor is located on top of the differential housing. It is also very difficult to see. You are going to have to remove the airbox to gain access to it. The VSS will have a 2-wire connector going to it.

Thank you for your response. I haven't tried to pull the codes myself (dealer is a joke), but I wanted to try the VSS to see if the connections are dirty or if the sensor is completely blown.

You mentioned that it is underneath the airbox on top of the differential. This evening, I pulled the entire airbox ducting out and can see the shift linkage, plenty of harness wiring, but no VSS. Do I have to remove the shift linkage cable and battery to see the sensor? I checked the Haynes manual, but it doesn;t give a good picture of its location. Is it more towards the firewall, near the starter?

Please let me know if anyone has a way to pinpoint its location on the differential housing. I was going to jack up the car and see if I can locate it that way.

Thanks!!!

Ted, Chicago

Hi, RE, The vss location, Your best bet is from underneath the car, Even from this access point it still is a tough one to remove, Very tight squeeze, The steering rack almost obstucts the removal, but you still can get it out, If the connector is ok, just replace the vss, as there will probably be no visual way to tell if it is bad....Lot's of experience with these.....Good Luck...Clay...

Hi Clay:

Thank you for your info -- found it by removing the fuel filter bracket aside. I was able to get the bolt out and then from the underside of the car, unplug it. I cleaned out the connector and tried to clean the sensor. I then reinstalled it and the light went off for about five minutes and then started flashing again. When I pulled the code (paperclip method) it came up for that sensor. So, it will definitely need to be replaced.

I have been looking for a good used sensor from a low mileage car because Ford discontinued this part and there are only one company (Wells) that seems to make this part aftermarket. I am weary about installing an aftermarket part because it seems like all the Ford sensors are quirky. Have you had any experience with replacing this sensor with an aftermarkt and if so, where did you purchase yours?

Any additional members feel free to chime in as well. Thanks!!

Ted, Chicago

Update!!

Well, after using the good 'ol Internet, I was able to locate a factory VSS at a dealer who has had two on the shelf for a long time. He is willing to sell me the sensor at cost ($96.23), so he can move his inventory. Considering these were listed at approx $236.00 when they were readily available, I am going to purchase this Ford part instead of taking my chances with an aftermarket one.

Hopefully this thread and its replies have helped out a fellow member with the infamous transmission problems. If there is anyone out there who needs to find a brand new factory VSS, please contact me and I will be glad to give you the name of the dealership and the man to talk to. As you may know, these are impossible to find.........

Keeping my fingers crossed that this part solves my father in law's shifting issues.

Ted, Chicago

Hi, Ted, Read your msg, I am going to try and find you a picture, I have to get into an old shop manual if I can find one...Be patient, I will give you a call....See Ya, Clay..

Hi, Just could not resist a post here....GLAD WE SOLVED THE PROBLEM...............Clay.....

I was going to do an update, but Clay beat me to it! Yes, after countless hours of research and the good 'ol trial and error method, we diagnosed the problem as the VSS and PSS. Despite what you DIY guys are thinking, if you simply purchase a Haynes manual or the equivalent, you will not be able to work on your PGT very well. Even going straight to the dealer can often lead you in the wrong direction and end up costing you $$ and time in the way of wrong parts and confused looking service and parts technicians.

First off, I started this project to try and help my future father in law fix up his Probe and get it road worthy again. After too many months of only being able to drive it to the local stores to run errands, I embarked on a quest to not only diagnose the affectionately named "Anal Probe" but also to fix it. Those of you who know me, understand that you have to send me kicking and screaming to the dealer before I will surrend to their price gouging and 1/2 assed repairs. That said, he is what to look out for.

After pulling the codes of the infamous OBDI PGT with my trusty paperclip, it gave me a VSS (vehicle speed sensor) code. As you will learn in this post, there is no code for the TSS (turbine speed sensor) or PSS (Pulse Speed Sensor). I then went about trying to locate it. According to my Haynes and a fellow ex-Ford Tech, I believed the VSS was located on the transmission housing right under the fuel filter bracket. I was able to pull it out, clean it and the connector and reinstall it. The flashing O/D light went out for about five minutes and then came back. The description of the sensor was "mounts in the transmission case, has a two pin plug and is secured with one 10mm bolt." I thought I located the correct sensor, but after receiving the correct sensor in the mail, it didn't look anything like the one I had removed. And for obvious reasons; I had removed the PSS, not the VSS or TSS.

After finally discovering where the VSS was located and becoming a contortionist for the evening, I was able to remove the VSS and install the new one (which was correctly received from the above dealer). Plugged it in, still got the flashing O/D light. Pulled the code and it still came up VSS. Called the dealer and talked with a helpful tech who offered to run a diagnostic on the car for $90 so he could tell me it was the VSS -- No thank you!! Then, I got a hold of a private transmission tech who actually does work on the PGTs. He advised that there is supposed to be an "air gap" between where the edge of the sensor meets the housing of the trans case. If there is ANY dirt, etc. in the gap, the sensor won't read the signal. Who in the hell designed an air gap sensor on an engine that is bound to get dirt and road grime on everything is beyond me. (Man, I love my Corvette!!)

Anyway, removed the sensor again and cleaned the gap with my trusty mini wire brush and a 1/2 can of brake cleaner (make sure you put a paper towel, etc in the sensor hole to prevent anything getting flushed into your trans). Cleaned out the VSS connector and started the engine. Still the flashing O/D light and VSS code. WTF? I drove the car around the block to make sure the speedo was working. Check. RPMs? Check. Then, I figured the ONLY time the O/D light went out was when I pulled the TSS (so I was told that is what it was called) and cleaned it. As you may know, the TSS and PSS are magneto sensors, so I lightly tapped the sensor tip on the workbench and reinstalled. Then light went out. Checked my information on pulling codes and looked for the TSS. It is not listed!! BTW, the PSS is not listed either. To this day, this doesn't make any sense, but I called the dealer and was told I needed a TSS for my 1994 PGT 2.5 ltr V6. Simple enough, right? WRONG!! The parts tech said Ford still made this sensor, but he would have to order it. After waiting 4 days, they called and said they had it. I ran down to the dealer on my lunchbreak and opened the box at the parts counter. It was the wrong sensor!! I even had the tech look it up on the computer and he found the exact same sensor listed. I got the "Probes came with a lot of different sensors, but they only give us this unclear image in the computer." WTF again!!!

After calling two managers, Lee Iaccoca and Henry Ford's great aunt, we discovered that Ford doesn't list the sensor I needed on the computer. FYI -- it's called a Pulse Speed Sensor. It was actually in a diagram on one of the archived pages in the dealer's computer!! Go figure. To make a long story short, I found a PSS at a nearby dealer and installed it that evening. Crossed my fingers and fired it up. Two mins, no light. Five mins, no light. Ten mins light returns. AHHHHHHHHHHHHH!!!!!!!

After a smoke treat, I erased the codes and tried again. I figured the connector may still have some electrical cleaner in the plug that had to dry out. No code for about 20 mins. I turned the key, but didn't start it right away and the light went out, but came back about 20 mins later. I did this again about two or three times and then A MIRACLE!! The light went out and hasn't been on for about 200 miles now. We drove the car and bonus -- 4th gear returned!!

The moral of the story is do as much research as you can and ask as many people as you can, because I got so many different stories it was ridiculous. And even when the actual dealers tell you what part you need, you will have to go outside those dealers to double check which part/sensor is the right one to buy. No wonder the PGTs had a lot of problems. The friggin dealers couldn't locate the right sensor to replace. BTW, the info about the TSS came from not one, but three dealers!! Not one of them mentioned the PSS, which is the part that ultimately solved my problem.

So far, 200+ miles and going strong. Thanks again Clay for all your help and hope this helps those frustrated with the Probe, which I have renamed Barney because of its purple color.

Ted, Chicago

Hi, Ted,,,Hey, You did all the work....A lot of people would have given up on that problem, simply because a lot of the info and parts are not available any more, let alone a tech with experience on the vehicle...and most of all the labor involved......See Ya Around,,,,,,Clay.....