[Second Series]

Progress is going slow this time of year, but little by little we’re moving right along. The ends of the ’47 frame are cut and being grafted to the donor frame. The rear frame extension is offset lower for two reasons. The body will be closer to the frame so the rear bumper will be lower relative to the frame, and there is a rear cross sill attached to the body that mounts on top of the frame. The front frame ends are narrower on the ‘47, we’ll move those out to match the donor and modify the bumper bracket. The bed wood is out and is in great condition, so I’ll probably keep it.
I have been researching how to run the original speedometer with the electronic system. I have a couple options. I could replace the tailhousing of the transfer case to use a speedometer cable. I would then use a mechanical to electric pass through adapter to get either 2000 pulse per mile, or 4000 pulse per mile. I would need to build a DRAC that works opposite of the original and get the 128ppm for the anti-lock, and 2kppm for ECM or 4kppm for cruise, but I don’t think I have cruise control, so 4kppm isn’t needed. The other option is to keep the system stock, DRAC is integral with the gauge cluster, so I’ll get a DRAC module from a ’92 to ‘95 and get the 2kppm signal to run a 12v motor to drive the mechanical speedometer, sure I could buy an electric to mechanical speedo drive, where’s the fun in that? I have found 3 different off the shelf solutions in the $300 range.
The mechanical speedometer can be adjusted to work with the rear gear ratio and the tire size. There are a few driven gear choices available for the np241, actually the 700R4 driven gear is used. There are 12 options with a range between 15 and 45 teeth. I may make a chart to compare driven gear, axle ratio, and tire size, unless anybody knows if one exists? If this isn’t right on and the ECM signal is being generated from it, there could be engine performance issues.
The electric speedometer is adjusted in the DRAC(digital ratio adapter controller). The tailshaft has a reluctor ring that generates 40 pulses per driveshaft revolution. The DRAC converts the 40 pulses to 128,000 pulses per mile, that’s 1:3,200 seems like a small margin for error. Before the 128k is generated, there is a circuit to set the diff/tire ratio. I’ll have to look at the charts on that. The 128k is divided by 32 to get 4kppm, and again ÷2 to get the 2kppm for the ECM. This just seems like a more precise way to generate the signal for the ECM, but I don’t know if it really matters or not how acurate the actual speed is compared to what the computer thinks?
The off the shelf electric to mechanical speedometer adapters have a way to calibrate the speedometer. This may be a redundant function, as it should be done in the DRAC. If the speed is not adjusted in the DRAC, the ECM may be getting the wrong information.
I took the motor out of a cordless drill, had some random transistors on hand and built a darlington pair for the supply. I have been playing around with motor controllers for awhile and have an arduino set up already, so I used that to for the speed control. The motor can run somewhat slow, but not slow enough. It works out to be at 2MPH the speedometer cable spins at 33.3rpm, and this motor looks like it is spinning faster than a record player. I’m looking for gears, or a different motor. Once I get the speed dialed in, I’ll work on the code to convert 2kppm to 1krpm.