Drivetrain Test Vss?

Discussion in 'Fox 5.0 Mustang Tech' started by 87MustangGT, Dec 21, 2013.

  1. Is there any way to test if my VSS has gone bad? A while back my speedo stopped working (or rather, nothing until 60mph, then LOTS of bouncing). I've replaced the speedometer itself, and the cable (and lubed it), and now I have no movement at all. I know my white drive gear (3.55s out back, T-5z spec rebuild) has some teeth chewed up, but everything I've read indicates that (1) the white gears get chewed up almost immediately, and (2) they will still work while chewed up. No?

    Before I keep throwing money at this problem, can I test the VSS? Or should I just replace the white gear?
  2. Vss has nothing to do with the speedometer.. It tells the computer how fast you are going. Helps to keep the car from stalling with abrupt stops
  3. Hmm... So likely culprit then would be the gear(s)?
  4. More than likely it could be the speedometer unit itself. Most Fords of that era have that issue. If you remove the speedometer head unit in the dash, what you'll probably find is the magnets on the head unit which spin with the cable are contacting the "sun" disk -which is directly connected to the needle. You can carefully try and bend the magnets away from the disk so they don't contact. Be careful as you can damage the speedo unit easily and it won't work at all. Or find a JY speedo, or send yours out for recalibration.

    Mine has done this since I bought it but I've learned to live with it . I replaced my trans speedo gear with the correct one when I swapped gears, lubed the cable, and my speedo is still off by 20mph or so and bounces.
  5. Pull the speedo cable out of the tran, shine a flashlit in the hole and make sure you can see a green or yellow worm gear inside.

    Very common for the clips that hold the speedo drive gear to break, especially soon after a rebuild, and the gear to walk down the shaft
  6. Speedometer cable replacement for 87-93 Mustangs

    Revised 1-Apr-2012 to clarify steps 11, 12, and 13, cable replacement inside the car.

    How the speedometer works:
    The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.

    Lubrication warning
    Use a graphite based lubricant for the speedo cable. It is available at most auto parts stores in a very small tube. Lubricate only the lower half of the cable. The reason for this is that if you use too much lubricant, it works its way up into the speedo head and gets between the rotating magnet and the disk. This causes the speedo to seize up and wring the indicator needle off the indicator spindle. You may be able to fix things up with non-flammable brake parts cleaner to clean the disk and magnet assembly. Plan on replacing the current cable and housing with a new cable and housing to prevent the excess lubricant from doing it all over again.

    Speedometer cable replacement.

    Note: All 89-93 cars have a VSS sensor even if they do not have cruse control. The 87-88 only have a VSS sensor if they have cruse control.

    Speedo cable housing assembly without cruse control:
    View attachment 121255

    The VSS equipped cars have a speedo cable with a different fitting on the transmission end of the cable. It is the fitting on the LH side of the following picture.

    Speedo cable housing assembly with cruse control
    View attachment 121256

    Preparation: if you are only going to replace the inner part of the speedo cable, get lots of newspaper or a painter’s drop cloth to cover the inside front of the car. About the time you have the dirty, oily speedo cable core all over your lap and the inside of the car, you will thank me for this suggestion.

    Replacing only the inner cable: see steps 1- 6, 12, 13, 17-21

    Replacing the housing and inner cable as an assembly: see steps 1-11, 13-21

    Inside the car:
    1.) Remove the shield around the steering column that covers the ignition switch & turn indicator switch.
    2.) You now have access to the two screws that hold the lower part of the cluster housing in place. Remove them and place them in a zip bag.
    3.) Use a stubby or an offset Philips screwdriver to remove the two screws on the top of the cluster housing. The screws are up close to the windshield, so they can be hard to get at.
    4.) The cluster housing will now slide forward: depending on your particular car, you may or may not have to disconnect the wiring for the headlights, hazard lights, or cluster wiring. All of the wiring uses plastic connectors with plastic spring clips on them. To release the connectors, lift the plastic clips and pull straight back.
    5.) The speedo cable is secured in the speedo head by a white plastic clip. Depress the clip or squeeze it and pull the cable out of the speedo head. This can be tricky, but it will come out if you have the white clip depressed enough.

    Speedo head cable clip

    Photos courtesy of Almost Stock


    6.) With speedo cable removed from the speedo head, try twisting the cable end with your fingers. If it turns more than 1/4 turn, the cable may either be broken or you have damage at the other end where the cable mates to the VSS sensor or speedo pickup gear in the transmission.

    Outside the car, replacing the cable housing assembly.
    The following steps are necessary only if you plan on replacing the cable & cable housing assembly.
    7.) If you are going to the replace the cable housing, the next step is important. Tie a study string or wire to the VSS sensor end of the cable housing. This string or wire is to be used to fish the cable housing back through the maze of wires that is under the dash. If all you are going to do is replace the inner cable, you can omit this step.
    8.) Jack up the car, all 4 tires must be off the ground. Place jackstands under the car for safety.
    9.) Locate the VSS sensor on the driver’s side of the transmission tailshaft housing. The speedo cable housing will be secured in the VSS sensor with a hairpin clip Do not remove the clip!!!: The hairpin clip stays in place. If you remove it, the odds are that you will not be able to get the cable to stay in place on re-installation. Pull firmly straight back on the cable housing and it will come out. A considerable amount of effort may be required to get the cable out of the VSS sensor, but it will pull out.
    10.) Release the cable housing from the clips that secure it to the car body.

    Inside the car:
    11.) If all you are going to do is replace the inner cable, you can omit this step.
    The housing assembly can then be pulled out and the fish string or wire can be removed from the old cable housing and secured to the new one.
    12.) You can omit this step if you are replacing the cable & cable housing assembly.
    The inner cable can be removed by pulling it out of the housing assembly. Watch out for the lubricant so that you don’t get it on the car’s interior.
    13.) You can omit this step if you are replacing the cable & cable housing assembly.
    Lubricate only the lower part of the new cable with speedometer lubricant or graphite. Don’t use too much lubricant, or it will work its way up into the speedo head unit and damage it. Thread the inner cable into the housing, turning it as you go. When you are all the way in with the new cable, it will engage the VSS sensor and stop turning.

    Outside the car, replacing the cable housing assembly.
    The following steps are necessary only if you plan on replacing the cable & cable housing assembly.
    14.) Use the fish string or wire to feed the cable housing assembly through the dash wiring and out the cable hole in the firewall.
    15.) Secure the cable in the body clips, making sure that the cable isn’t rubbing against the exhaust pipe.
    16.) Push the cable housing assembly into the VSS sensor until it snaps in past the hairpin clip. Connect VSS wiring connector back to VSS sensor.

    Inside the car:
    17.) Push the cable housing back into the speedo head unit. You should be able to feel the white clip click into place.
    18.) Reconnect all the wires & connectors on the speedo head unit.
    19.) Re-install the cluster unit in the dash & tighten the 4 screws that hold it in place.
    20.) Re-install the cover for the ignition switch & turn signal.
    21.) If the car is up on jackstands, start the car, place it in gear & watch the speedo to see if it works OK. If you didn’t jack the car up, take a test drive.