rls@iham1.UUCP (Rick Schieve) (01/10/85)
Automobile instrumentation always has been and continues to be nightmare. A typical example: In this age of solid state devices, Ford, and some others, still use a mechanical device, called a constant voltage regulator to regulate the voltage to dash gauges (this does not apply to the speedometer). While driving, battery voltage may vary as much as 20%. In order to keep the gauges from tracking with battery fluctuations, a regulator is used. Soundes like it should be simple, a three pin semiconductor device, ground, battery and regulated steady voltage level output at a voltage the battery will not drop below (say 5-10 volts). Wrong! Instead you get a bimetal strip wrapped with resistance wire. It works like this: the current flow through the bimetal strip is controlled by a physical connection at the end of the strip. As the bimetal strip warms it opens the contact stopping the current flow through the bimetal strip, the strip cools and contact is made again starting the cycle again. The output is a pulsing battery voltage the the shop manual has the nerve to call 5 volts. The 5 voltage average is maintained as the time current flows is a function of the battery voltage. That is, as long as the contact doesn't get dirty or stick in which case the gauges all go up in smoke (I am speaking from experience). Of course the duty cycle is so slow, the gauges have to be slow or they fluctuate with the duty cycle of the regulator, but this is no great problem, just make the gauge deflection controlled by more bimetal strips wrapped with resistance wire. Worked great in the 60s, why not today@!@#&%?~. "Nice instrumentaion in that Mustange GT, huh!" Sorry for the digression, back to the Honda speedometer. 2.8% is probably more accurate than most cars on the road. Cruise at 60 mph and see if the mile markers come up once a minute and adjust you speed until they do, this is the real 60 mph. It may save you a speeding ticket sometime. To compensate for differental ratios and different tire sizes there is a set of gears that drive the speedometer cable. These gears may be ordered in different ratios to help correct inaccuracy. Don't use the formulas from the dealer. Figure your present inaccuracy, find the gear ratio of your present speedometer gears, and calculated the ratio you need. Don't be surprized if what is available from Honda is in 5% steps. I am not familiar with Hondas so when they tell you the transmission has to be removed to change the gears you may decide 2.8% and some mental calculations is not so bad after all. Rick Schieve ihnp4!iham1!rls
mrl@drutx.UUCP (LongoMR) (01/11/85)
What I used to do in the days when I cared about this problem (I honestly can't remember WHY!) is to simply remark the speedometer dial. This required some disassembly of the instrumentation panel and I don't think it would work too well with today's new digital dash boards. I was never concerned about the odometer reading because I felt that if the warrenty ran out at 12K and 12,150 miles were showing, any dealer worth *his* salt would honor the warrenty.