fasteddy
Web Wheeler
Reged: 01/30/01
Posts: 12750
Loc: Flat Creek, GA
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Re: Understanding your GenI Speedometer
10/15/04 10:21 PM
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Quote:
1. "I'm more than a little dence.."
2. "Wind resistance - That's what I thought was the difference."
3. "I always wondered what a Reedswitch was."
1. NO! See 2.
2. YES! Aerodynamic drag goes up by the square of the increase in speed, all other things being equal (humidity, temp, pressure). From 45 to 55, you'd have to have (55/44) squared times the 45mph hp to overcome the air resistance increase, or just a tiny tad under 1.5 times as much. From 55 to my assumed 70mph I-Five cruising speed needs an increase of (70/55) squared, or 1.62 times the 55mph cruise hp, and 2.42 times the 45mph cruise hp.
The purpose of all this math is that you have to burn a fixed amount of fuel per hp at steady cruise, because the engine is tuned to run at a certain fuel/air ratio by carb jetting and/or ecu fuel control. To rough in the assumed fuel economy savings, you'd divide the fuel burn rate in mpg at say, 70, and if you lower the cruise to 55, you'd divide the 70 mph fuel economy (assume 12.3469mpg) by 1.62, and estimate the expected 55mph economy at 20.0mpg. If you get 18, the trailer is costing you 2mpg AT 55 (and that 2mpg number will be different at any other speed and speed change range).
This ignores some other variables like the changing of the engine efficiency at different rpms (which is why it's a torque "curve") which happens when you run at different speeds, so if you ignore any of those that are significant, you just wasted about 5 minutes running those numbers, unless you are satisfied with decreasing your degree of ignorance. My mind works that way on some stuff, and on stuff like this my head likes to understand it ALL. Futile....
3. A reed switch has a set of magnets that spin (I imagine a disc with say, 4 at 90* intervals) past a switch that makes contact as each magnet passes. That's a 4 pole speed sensor. I guess it gets it's name from the way the switch contacts bend like a reed (I imagine one non-magnet attractable like brass/copper, and one in steel with a copper contact. Non-mag is closest to the spinnin' magnets, and steel one furthest. Magnet passes contacts on whirling wheel and yanks steel half of circuit leaves toward the non- mag one, making contact and sending a voltage to a solid state counter gizmo in the ecu or whatever magic box needs the vehicle speed. May work opposite, where passing magnet breaks instead of makes the electrical contact, but counting effect is the same - pulsed volts is pulsed volts, whether in distributor points, reluctors, Hall effect sensors, tachometers, Monty t-case output shaft pulse generators, or whatever else there is out there that measures the speed of a rotating shaft. I guess, in theory, you could yank the stock speedo guts (minus still-operating reed switch and cable) out of a Monty, tape an electronic speedo in the hole, wire it to the truck power/ground/dash lights, and take it's electronic speed signal from the reed switch instead of one on the tranny. Guess it would even give you esrever sdeeps...
Some aftermarket cruise controls take their vehicle speed signal from some balanced number of magnets strapped to the driveshaft plus a reed switch mounted close to the magnets on the shaft. Starion digital LED dashes used a tranny mounted pulse generator sender.
Wish you hadn't asked????? 
BTW, I concur that there is no electrical connection to the KM148, whether lockup torque converter or not, other that the overdrive hydraulic circuit valve solenoid that's controlled by the o/d relay controlled by the o/d switch on the shifter and the a/t temp overheat light sender. The entire rest of the tranny control circuits is the internal valving spring pressures, applied or unapplied valve counter pressures, governor pressures, and the very important throttle valve pressures which are controlled by your right foot ('les you are a rural contract mail delivery) via the throttle cable to the throttle arm to the throttle cable. A sticking cable makes the throttle pressure vary in a way that doesn't match the engine output, and you get weird shifts at weird speeds in extreme cases. A cable thats adjusted too loose gives you late downshifts and and early upshifts AND too low clamping pressures in the various tranny interal brakes and clutches, and chance of slippance, heat, and tranny death from same. One that's too tight gives slammy shifts, late upshifts, early downshifts. One that sticks usually sticks to tight, as it's a positive pull as the throttle opens, and an internal tranny spring return to loose for the cable.
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"If you can't be a good influence, don't worry, you can still be a horrid example."
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