My "new" 72 Javelin SST

It was 111 degrees here yesterday, and since the garage is not cooled and I don't have a lift, I decided to call a local transmission shop and see what they would charge to remove the 998 and put the AW4 in. I hate not doing this myself, but for $350, I couldn't pass it up.  They're just going to do the grunt work, and I'll deal with the driveshaft, cables, wiring and shifter once I get the car back home. 

They won't start until Monday, so that gave me some time to put some helper springs on the car.  It had been a little high in the front since I put the new coils springs on (probably because the 45-year old rear springs were sagging a bit too) so I thought the $35 cost of a set of helper springs would be worth a try.   It raised the back of the car an inch or two, and the stance is now just where I want it.  Feels a little more taut on the road too without ruining the ride.

Check some of the custom gauge vendors like Dakota Digital with the ECD-100. They offer a "motor" that reads the VSS and then drives an analog gauge just like a cable would. Something also like the Speedhut Speedbox works which is a little spendy because it can also go GPS.

Today was the day to sort out the speedometer drive, and the trip to the Pick 'n Pull was a success.  I didn't find any 2WD Cherokees, but took a chance that the 4WD speedo gear housings would be the same. The 2WD drive mounts to the transmission while the 4WD mounts to the transfer case, but they are indeed identical.

Next issue to overcome was to see whether the mechanical speedo drive from a pre-92 AW4 (top photo) would work in place of the electric VSS drive (lower photo) that was in my '94 transmission.  I grabbed a mechanical drive from an '89 Cherokee today for $10, and indeed, it bolts right in! Another reason the AW4 is a great choice for our old AMCs.  

It takes just a minute or two to change the drives out-- and connecting the Javelin speedo should now be just a matter of finding the right speedometer cable to match the threads on the drive.  Possibly the stock Javelin cable will fit, but I haven't tried it yet.  

The mechanical drive shown in the top photo is obviously pictured without the gear.  While the drives themselves fit without modification, the gears are different.  They have different shaft lengths, so if you're doing this swap yourself you'll need to use the right gear for your drive.  AW4 speedo gears are readily available at most Jeep/Off-Road outlets, and many different ratios are available to match the tire diameter and rear axle ratio of your car.  I just went with the dark-green gear that came with my drive, and we'll see how that works.  Looking at the charts available on many of the Jeep sites I should be close.

The pre '97 (or '98) have a simple reed switch output to the transmission controller.  The reed switch is activated by a magnetic trigger inside the transmission. That's separate from the speedometer output, which uses a gear on the output shaft.  The pre-97 AW4 use a standalone controller and don't tie in to the vehicle's computer.  The later AW4 do, so things get more complicated then.

Here's a description I "borrowed" from one of the Jeep forums:

I'm using a '94 transmission with the mechanical speedometer drive from an '87-90.