When I installed an older IRS center section, to get a 3:54 ratio in preparation for my five speed upgrade, I lost my speedometer as the older units have no provisions inside the diff for speedo sensing. I expected this, and I had a few different ideas on how I might regain speedo function. Swapping to an older transmission /electric driven speedo set up was the least attractive option.
My first thought was to have the speedo head rebuilt to be GPS driven. After some research, I learned that GPS speedo heads are nothing special. The respond to a fixed number of pulses per mile (ppm), usually 4000, 8000 or 16000, the same as many other electric speedos.
I know there are six trigger surfaces in the diff:
I multiplied 6 trigger surfaces x 768 tire rotations per mile (per Tire Rack for my tires) and got 4,608. Hmm. That’s kinda close to 4000.
I also knew that the “speed interface” box in the boot was between the diff sensor and the speedo, so I surmised that maybe it turns the diff signal into something close to a ppm standard for the speedo.
More research revealed a universal gps speedo sender from Summit for $99. I decided to see what happened if I tried to drive the Jag speedo with that unit.
Here are the instructions for the unit. It is a small black box with a remote antenna. My plan was to do everything right in the corner of the boot.
Looking at the electrical guide it appeared I could reuse the wires from the speed interface:
Blue to speedo, black to ground, green for switched 12v.
One issue was that the speedo output wire colors on the unit didn’t match the instructions. Instead of purple, I had white. Hmm. Maybe a typo or production change. I wired everything up so I could easily try different pulse rates, slapped the antenna on the fender and went for a drive. I selected the Orange wire since I thought the magic number would be 4000.
For the first minute, I got nothing, and I figured I was a failure. Then all of a sudden the speedometer started working. It worked fine, except that it read about half of my actual speed. OK, that’s cool, so to raise the speed on the speedometer, I would need more pulses per mile.
I swapped to the white wire, which I am guessing I get instead of purple, and drove some more… Now I’m at about 1/4 speed. This is not right. The speed reading should have increased. I might as will try the 16,000 pulses per mile wire, the blue one. Interestingly, the wire to the speedometer is also blue. I hook it up go for a drive. Success! The speedometer reads dead on. I’m using an app on my iPhone, and it seems perfectly aligned. Very exciting. I can only conclude that the instructions don’t accurately call out the wire colors for the 4000 and 8000 pulse outputs.
Now it’s time for a more aesthetically pleasing install. My goal is to mount the GPS antenna inside on top of the chmsl. It needs a view of the sky.
I find these convenient little vent holes located in what appears to be exactly the right place.
I discover that I can sneak a stiff plastic tube through, and it comes out right next to the chmsl.
The GPS antenna won’t quite fit through the vent I holes, so I use a blunt punch and a few gentle taps of the hammer to open one up just enough to allow it to clear. I tape it to the end of my pull straw, and pul it up inside the car.
As I am perusing the electrical guide, I am reminded that the ABS computer is located behind the panel, near the speed interface box. I’d like to remove it, since I no longer have ABS, and I’m hopeful this will cause my yellow dash warning light to go off. Checking the electrical guide, it appears that the dash warning light is indeed driven by the ABS computer.
Huh. Now I have a sweet spot to mount the gps black box.
Panel back in place and we are good to go!
I need to drive a bit more to confirm, but the only small quirk seems to be that if I come to a complete stop, it takes about 10 mph or 2-3 seconds for the speedo to work. Not sure if it’s a time thing or motion thing. It works fine down to 1-2 mph, but if I stop it takes a sec. More driving may reveal this to be a anomaly, Even if it isn’t it’s not a big deal.
I am pretty happy to have my speedo back with this little of effort. Now I can say “Why yes officer, I know EXACTLY how fast I was going!”