olmstead.jim.w

Hello, I have an 87 GT Convertible, 5.0L, 5-speed... The speedometer seems to be working only part of the time. Most times it will word when you first start out until you get up to about 45-50mph and then it drops to 0 and stops working. Sometimes it might start working again when I stop and restart and sometimes I can drive for miles and never have an issue. Other times it wont work at all even starting out, yet while driving it might suddenly show how fast I'm going and then drop right back to 0 again.

Im not quite sure what to make of this issue and am not sure if I should be looking at replacing the speedometer cable or the speedometer.

Also my gas gauge seems to read full all the time unless I'm getting low on gas, then it will go from full to reading empty within a mile. I doubt that's related but I thought id mention it just in case.

If anyone has any ideas I would appreciate the help.

Thank you,
Jim

wbrockstar

With as many miles/years as the cable probably has on it,it could be getting dry internally or the gear on the end of it or inside the transmission could be starting to get worn down,as in missing teeth or having areas where some teeth are worn down to where theyre not meshing properly with the drive gear.The cable has a driven gear on the end of it that gets inserted into a hole in the side of the transmission and the transmission itself has a drive gear mounted inside of it that meshes with the driven gear on the speedo cable.You can inspect the speedo cable driven gear by removing one bolt and the metal retaining tab at the driver side of the trans.If some of the teeth are worn or missing,replacing the gear should fix the issue,as long as the drive gear is good.If the gear is ok,you can try lubricating the cable.Ive always used graphite.It looks like powdered metal & comes in a small bottle.If lubricating it doesn't help and the gears are ok,the cable needs to be replaced or the cluster itself is suspect.

olmstead.jim.w

TrimDrip

I assume your mileage quits accumulating after this happens

wbrockstar

There are several parts used for the cruise control system on 87-88 models.Two fuses,speedo cable,vss (vehicle speed sensor) harness,cruise control amplifier (behind dash) servo & a vacuum line.A blown fuse (#1 or #6 ) defective speedo cable, defective vss or leaking vacuum line is probably the most likely cause,for the cruise to not function.The vacuum line runs from the vacuum tree port labeled S/C to the servo mounted inside the driver fender.I believe the vss was only used for cruise on 87-88 Speed Density models.It was also used for idle control on 89+ Mass Air models.However if your car was converted to Mass Air before you bought it,the vss is supposed to be wired into the ECU at pin #3 & #6.If not,surging idle and stalling when slowing to a stop (if its a T5 equipped car) can/will occur.On SD models,the vss was tied into the cruise control amplifier.Im not sure whether the vss was tied into the clutch safety & brake light switch (to cut the cruise off instantaneously) directly or through the cruise module?? The speedo cable is inserted into the vss and the vss is the part that the actual speedo gear attaches to and the part that gets inserted into & attaches to the transmission.After you check the fuses,I would pull the vss/speedo cable out of the transmission to inspect the gear and vss.A picture of the vss is shown below.The speedo cable gets inserted right below where the harness plugs into it.


​​​​​​​

olmstead.jim.w

ok, so I just typed a whole bunch of stuff, hit a wrong button and lost everything lol, so I am going to try to remember what all I said in the lost msg.

First off, I want to thank you for all the information. Gives me a lot to think about and a lot of places to look. I did check an notice that the odometer is only working when the speedometer is working. The car was converted to Mass Airflow before I got the car, however to be honest I don't know what the difference is. I have either had cars that had it or didn't have it but until I got this one I hadn't heard of anyone converting to it. I plan on doing some research about this but if you happen to know of a good reliable link on information about this I would be quite interested in learning the difference and also how to do it, so that I can check to make sure everything is hooked where it should be. I know the guy that had it before me mentioned that a wire that comes from the Mass Airflow to the fuel pump relay under the drivers seat came off and needed to be replaced but I haven't had the time to get in there and take a look at it yet. It does have a bit of fluctuation when idling but its only ever stalled on me once when I was first bringing it home and I just assumed it was an operator error learning the new car.

I did check the speedometer when it was working today against my GPS tp see if it was even accurate when it is working and when I was at 55mph (on the dash) my GPS said I was only doing 45mph. I know I have 16in tires rather than 15in, but I wouldn't think that would make it that far off. I also don't know if there has been any changes made to the rear gears either. Then again it has an 85mph speedometer in the dash and I have seen many other GT's with 120mph and 140mph speedometers so I'm not even sure how I would know if its the right speedometer for the car.

I appreciate everyone's input, information and advice. I have a lot to learn about this car and I thank you for you patience, understanding and help in getting me properly educated.

wbrockstar

The following link is a detailed description of how to do a maf conversion.Since yours is done already,Id just check to make sure the vss wires (pin 3 & 6) are tied into the ecu, because those two can cause surging/ stalling if theyre not connected.
https://www.stangnet.com/tech/maf/ma...onversion.html

The following link will explain everything you wanna know about speed density & mass air + a ton of other relevant Mustang info.Its a long read though.
http://www.thehotrodgarage.net/mustang_efi.html

One way you can check what rear gear youve got is to: jack the rear of the car up so both wheels are off the ground.Use Witeout or chalk and draw a mark on the tire sidewall at the 12 o'clock position.Draw a mark on the side of the driveshaft.Slowly rotate the wheel in a CW direction while paying attention to the driveshaft mark.Turn the wheel 1 complete revolution while you count the number of rotations the driveshaft makes in that period of time.If the driveshaft rotates 3.5 times,youve got a 3.55 gear.2.75 times=2.73 gear.3.0 times=3.08 gear. Most 5 speed foxbodys came with a 2.73.

If the previous owner changed the rear gears to something different than the oem ratio,but he didn't replace the plastic gear at the end of the vss to match the new gear ratio,that could cause the speedo mph to be off too.

olmstead.jim.w

Thank you again for all the information, there is a lot to learn there and I am looking forward to seeing what I can accomplish. I apologize for not replying sooner, I was having some computer issues for a while and ended up needing to go get a new laptop. I plan on printing all this information and studying it before I go out to the garage. I have some medical procedures coming up that may delay my progress for a little bit but I will try to update how I'm making out when I'm able to. I am sure that I will have more question and I greatly appreciate all the help that I have received so far, You guys have been great.

Thank you again

olmstead.jim.w

Hello, I just wanted to update, I was finally able to spend some time in the garage and try to find out what the gears in the car are. I followed the instructions here and with 1 rotation of the tire I had just 3 rotations of the shaft so I am assuming that mine has the 3.08 gears.

I went on to LMR's site for a speedometer gear calculator ( https://lmr.com/products/Mustang-Spe...ear-Calculator ) and I assuming that my drive gear is a 7 tooth? Has anyone else used this calculator and if so was it fairly accurate? From what I can tell I should have a white 17 tooth speedometer gear?

I think I am going to replace the speed sensor and the gear and see what happens from there. I did check out the Mass Air connections and did find a broken wire. Fixed that and the idle smoothed out for the most part until you have driven it a while and then it will every now and then go back to the fluctuation and if you rev it up a little sometimes it will smooth back out and idle perfectly fine. I read that the speed sensor could cause this problem and since I already don't have the speedometer/odometer and cruise control working, I thought that might sound like a good place to start?

I will continue to update as I make progress. I have been having a lot of health issues so I don't get to get a lot done these days but every time I don accomplish something with it its a happy day

Thank you all again for your time and help.

olmstead.jim.w

I was finally able to replace my speed sensor and speedometer gear. Started down the road and the speedometer and odometer were finally working again. Checked it against my GPS however and I notice up until about 55mph I am about 5mph slower than the speedometer says. After 55mph though I seem to be anywhere from 10-15mph off. At 60mph (According to the GPS) I am doing 75mph (on the speedometer). The gear that I used was one that I chose from the speedometer gear calculator on LMR's website. I ended up getting a white 17 tooth gear and what was on the car was a yellow gear however, I have not counted the teeth on the old gear to see what that had in it yet. The inside of the old one was rounded out and wasn't turning the speed sensor otherwise I would just put that one on the new sensor and see what happens.

Is it possible that I just have the wrong new gear on there (maybe I input the wrong info somehow when looking up what to get) in which case I will try ordering one that matches what I took out or does it sound like there's just a problem with the speedometer itself. My speedometer is the 85mph speedometer that I am told was standard on the 87 mustangs if that makes any difference. Eventually I want to get the Dakota Digital dash for this car which sounds like it will fix these issues in the long run but until I have the money to do that I m hoping that I can get somewhat close for the time being.

Thank you for any help.