Finishing the Serial Cable and Testing the Fuel Gauge

I had a bit more time today to work on some other small projects.


Here's the 914 patiently waiting for its rebuilt transmission from Rennsport Systems. It'll be awhile since Rennsport won't start on the rebuild until November.


Up until this point, the serial cable from the DMOC445 AC controller to the passenger seat was routed along the outside of the car. I cleaned this up today by taking out the two seats, pulling forward the upholstery in front of the firewall and drilling a hole for the serial cable. For some reason, I had an extra cap-plug left over from the kit, so I used it to protect the cable from the sharp metal edges. The cable now runs neatly behind the seats and into the central armrest.

I got really upset when putting the seats back in because I dinged the paint near the door handles with the sharp edges from the seat rails. I'll patch this up with touch-up paint, but I'm still frustrated with my clumsiness. Working in this small single-car garage is a challenge.


I also started researching how to drive the fuel gauge with the voltage on the batteries. The fuel gauge reads the resistance of the fuel sensor which varies from zero ohms (full) to 75 ohms (empty). I verified this by shorting out the sensor circuit and putting 75 ohms in the circuit. The system pulls 100 milliamps at zero ohms and 50 milliamps at 75 ohms.

For you electrical engineers out there, I'll probably implement a circuit with a 4N25 optoisolator to measure the voltage on the main battery pack and drive the fuel gauge with an op-amp controlling current through a transistor in emitter-follower configuration. I'll post the schematic here after I get it up and running. There's also a fuel-empty light that I can drive if the battery voltage gets too low or the voltage droops because I've accelerated too long and drawn down the main pack voltage. Any suggestions on how to make this circuit work are welcome.

I started getting a sore throat today, so I'm going to bed early and will probably take it easy tomorrow by working on the fuel gauge circuit. I'd really like to get the hair dryers installed soon so I can drive in the colder weather.


Cheers and good night...