Garage Projects

The Mighty Merc – Moving the VSS and Tone Ring Math

In 1999 (and newer) Explorers and Mountaineers use a tone ring in the rear differential to give a signal for the VSS. This signal is used for the automatic transmission inputs as well as the ABS and speedometer. While my autotragic and ABS is going away, I still wanted to have a working speedometer and cruise control.

So what to do? Well move the tone ring and VSS to the transfer case of course! First a little background and Math. Ford explorers/mountaineers came with stock 29” tires with 108 teeth on the tone ring in the 8.8. So with 3.73’s (Stock V8 trucks), you need 29 Teeth on the tone ring at the Transfer case since it spins faster than the axles. (108/3.73 = 28.95). We can extrapolate that out for different rations. In order to keep yours speedometer accurate when swapping gear ratios (with 29” tires), you’re looking at:
– 5.13 = 21 Teeth
– 4.88 = 22 Teeth
– 4.56 = 24 Teeth
– 4.10 = 26 Teeth
– 3.55 = 31 Teeth
– 3.23 = 33 Teeth

In my case, I am going up in size from 29” to 37” tires which is a 22% Increase which will make the speedo read 22% slower, so you need to add 22% more teeth to the tone ring on the transfer case:
– 3.23 = 41 Teeth
– 3.55 = 37 Teeth
– 3.73 = 35 Teeth
– 4.10 = 32 Teeth
– 4.56 = 29 Teeth
– 4.88 = 27 Teeth
– 5.13 = 26 Teeth

This will get you pretty dang close, but it might be worthwhile to make a tone ring set +/- 5% as not all tires are created equal which can throw things off a tad. Or just measure the mounted tires to get a true measurement to fine tune the numbers.

In my case with 4.88’s and 37’s, I made a 27 tooth tone ring that was then welded to a 1350 CV Flange:
Sara L.: Driveline &emdash;

I turned it on the lathe to true things up:
Sara L.: Driveline &emdash;

Since the teeth were plasma cut, I had to do a little fine tuning with a file to get a little more of a square profile on the teeth:
Sara L.: Driveline &emdash;

For the bracket, I once again turned to my favorite tool, the plasma table:
Sara L.: Driveline &emdash;

Sara L.: Driveline &emdash;

I stuck the VSS in there, and checked the clearances. The spec listed for this sensor lists a 1mm air gap max (0.039”), so I set mine at about 0.025-.030” with some shim stock:
Sara L.: Driveline &emdash;

Leave a Reply