Electronic GPS Speedometer Conversion
#1
03-17-2017, 09:27 AM
Electronic GPS Speedometer Conversion
The Problem: I rebuilt my 1969 Jaguar E-type a number ofyears ago and replaced the original 3.54:1 differential with a 2.88:1 rear axle. I have been driving with a significantspeedometer error ever since and making mental corrections to the readings. The only alternative until recently has beento remove the speedometer head, roll the car in gear for a premeasureddistance, count the rotations of the cable core, and send off the speedometer tobe recalibrated for about $250 plus shipping.
Alternative Solution: Classic Automotive Innovations (https://www.classicautoinnovations.com)markets a cost effective $275 alternative that also allows for futureadjustments for any wheel or tire changes. It is an electronic GPS conversion unit that can be owner calibratedregardless of what rear axle ratio or tires are used. By disconnecting the speedometer cable fromthe gearbox and screwing it into the unit’s black box, an acquired GPS signalcontrols the speed of an electric motor that rotates the cable core, thustransmitting a corrected mechanical signal to the speedometer head. A separate remote plug-in “dongle” is used todial in an initial calibration reference point after which the car can be drivento verify the speedometer reading with a GPS navigator or smart phone.
Initial Checks: My speedometer gauge worked well, havingpreviously disassembled the head and checked and cleaned the internals, and thespeedometer cable had been replaced. Checkingthese items is essential to assure that the car’s speedometer system works welland that any preexisting problems will not be blamed on the GPS conversionunit. To test feasibility beforeordering the conversion kit, I made a cardboard mockup sized to the GPS unitand searched for an accessible location. The box fit neatly into a large empty cavity in the cowl behind theglove box inside the car. Although waterresistant, I wanted the black box mounted away from harmful elements outside inthe engine compartment.
How to Do It: I removed the center and radio consoles,folded down the tunnel carpets, and removed the gearbox access cover todisconnect the speedometer cable. Jaguarhas a starter bolt access cover in the passenger foot well that I removed,drilled a hole in the cover, added a cable grommet, rerouted the cable throughthe grommet, and then reattached the access cover. The conversion kit provides a plastic cover toscrew onto the gearbox to seal in the lubricant, but I elected to use analuminum cap with an internal rubber seal that I had. The carpets and center and radio consoles werethen reattached.
The rerouted speedometer cable’s stock length wassatisfactory, however Classic offers custom made speedometer cables of variouslengths if needed. Because of tightquarters in the gearbox tunnel, Jaguar used a right angle speedometer drivewhich has a 3/4"x26 fine thread screw cap that attaches to thegearbox. (Note that the cable end has a1/2"x26 fine thread cap which many other Brit cars screw directly into thegearbox without the right angle drive. Be sure to order the correct size GPS unit). Using the right angle drive on the GPS blackbox allowed for the cable to remain flat against the cowling and have the unitinstalled perpendicularly in the cowl cavity to avoid having to make any severebends in the cable.
The GPS antenna base is magnetic and was mounted in thecenter of the dash cap with the cable tucked in between the windscreen and dashcap, down under the right side corner, behind the glove box, and through anopening into the cowl cavity. The unit’sblack ground wire was attached to an existing ground screw at the startersolenoid relay on the A pillar while the red hot wire was fitted with a Lucasbullet connector and slipped into a 4-way connector from fuse # 7 feeding thechoke and reverse switches and lamps (all wires solid green). Excess wiring and antenna cable were bundledand tucked into a cavity in the side of the cowling.
The Results: After making the initial calibrationaccording to the manufacturer’s instructions, I prepared to test drive thecar. Turning on the ignition, the unitmade a grinding noise (similar to road-tire noise) and high pitched sound asthe unit initialized for about 15 seconds, and then the sounds stop. This is a normal. As soon as the car began moving, the speedometerneedle registers the speed. Using aseparate GPS navigator unit to verify speed, the speedometer’s calibrationaccuracy was about +2 mph at 70 mph. Itweaked the calibration a couple of times until I was satisfied with it.
I fit a foam box around the GPS unit andreinstalled the trim panels which quieted the GPS unit’s sounds which actuallyare only noticeable at startup in the garage before the unit has acquiredsatellites. Once the car is moving,there are enough squeaks and groans, wind, engine, gearbox, axle, exhaust, andtire noise … you know, the typicalnoises of vintage cars … that the unit’s sounds are not noticeable. And when I crank up The Eagles or Mo-Town onthe stereo, I cannot hear anything but the ‘70’s.
Alternative Solution: Classic Automotive Innovations (https://www.classicautoinnovations.com)markets a cost effective $275 alternative that also allows for futureadjustments for any wheel or tire changes. It is an electronic GPS conversion unit that can be owner calibratedregardless of what rear axle ratio or tires are used. By disconnecting the speedometer cable fromthe gearbox and screwing it into the unit’s black box, an acquired GPS signalcontrols the speed of an electric motor that rotates the cable core, thustransmitting a corrected mechanical signal to the speedometer head. A separate remote plug-in “dongle” is used todial in an initial calibration reference point after which the car can be drivento verify the speedometer reading with a GPS navigator or smart phone.
Initial Checks: My speedometer gauge worked well, havingpreviously disassembled the head and checked and cleaned the internals, and thespeedometer cable had been replaced. Checkingthese items is essential to assure that the car’s speedometer system works welland that any preexisting problems will not be blamed on the GPS conversionunit. To test feasibility beforeordering the conversion kit, I made a cardboard mockup sized to the GPS unitand searched for an accessible location. The box fit neatly into a large empty cavity in the cowl behind theglove box inside the car. Although waterresistant, I wanted the black box mounted away from harmful elements outside inthe engine compartment.
How to Do It: I removed the center and radio consoles,folded down the tunnel carpets, and removed the gearbox access cover todisconnect the speedometer cable. Jaguarhas a starter bolt access cover in the passenger foot well that I removed,drilled a hole in the cover, added a cable grommet, rerouted the cable throughthe grommet, and then reattached the access cover. The conversion kit provides a plastic cover toscrew onto the gearbox to seal in the lubricant, but I elected to use analuminum cap with an internal rubber seal that I had. The carpets and center and radio consoles werethen reattached.
The rerouted speedometer cable’s stock length wassatisfactory, however Classic offers custom made speedometer cables of variouslengths if needed. Because of tightquarters in the gearbox tunnel, Jaguar used a right angle speedometer drivewhich has a 3/4"x26 fine thread screw cap that attaches to thegearbox. (Note that the cable end has a1/2"x26 fine thread cap which many other Brit cars screw directly into thegearbox without the right angle drive. Be sure to order the correct size GPS unit). Using the right angle drive on the GPS blackbox allowed for the cable to remain flat against the cowling and have the unitinstalled perpendicularly in the cowl cavity to avoid having to make any severebends in the cable.
The GPS antenna base is magnetic and was mounted in thecenter of the dash cap with the cable tucked in between the windscreen and dashcap, down under the right side corner, behind the glove box, and through anopening into the cowl cavity. The unit’sblack ground wire was attached to an existing ground screw at the startersolenoid relay on the A pillar while the red hot wire was fitted with a Lucasbullet connector and slipped into a 4-way connector from fuse # 7 feeding thechoke and reverse switches and lamps (all wires solid green). Excess wiring and antenna cable were bundledand tucked into a cavity in the side of the cowling.
The Results: After making the initial calibrationaccording to the manufacturer’s instructions, I prepared to test drive thecar. Turning on the ignition, the unitmade a grinding noise (similar to road-tire noise) and high pitched sound asthe unit initialized for about 15 seconds, and then the sounds stop. This is a normal. As soon as the car began moving, the speedometerneedle registers the speed. Using aseparate GPS navigator unit to verify speed, the speedometer’s calibrationaccuracy was about +2 mph at 70 mph. Itweaked the calibration a couple of times until I was satisfied with it.
I fit a foam box around the GPS unit andreinstalled the trim panels which quieted the GPS unit’s sounds which actuallyare only noticeable at startup in the garage before the unit has acquiredsatellites. Once the car is moving,there are enough squeaks and groans, wind, engine, gearbox, axle, exhaust, andtire noise … you know, the typicalnoises of vintage cars … that the unit’s sounds are not noticeable. And when I crank up The Eagles or Mo-Town onthe stereo, I cannot hear anything but the ‘70’s.