I have only run into this two times and both times on KW's. Complaint is speedometer is way off, recording very slow MPH.
Before you check the magnetic pickup or replace it, check the alternator voltage. And don't go by what the volt gauge in the dash says.
Overcharging can cause the speedo to read wrong yet it may not effect any other gauge. An overcharging alternator will
effect the sine wave produced by the magnetic pickup for the speedometer.
Truck Shop
I too have seen some pretty weird stuff relating to alternator malfunction, Had an 06 pete , C15 cat with a replacement Alternator whose regulator was insane, or at least manic. it would under charge or over charge, intermittently. one day it overcharged a bit more than the ecm liked, confusing the ecm into believing the engine was overheating, and to add to the confusion showing the driver 10 volts on the voltmeter, I took another tractor to the pull out where he was waiting, I brought my temp gun and multi meter, Checked the temp, got 180 at most places, the cab guage showed 260, the driver had the fan on manually, but the guage didn't drop, I had told him if the coolant wasn't bubbling in the surge tank or overflow not to shut it down. the engine had severely de rated, I checked the alternator output, my multi meter showed 17 volts, same at batteries (AGMs under the seat) , we swapped tractors, parking the Pete behind the low bed so we could pull start it if necessary. I shut it down, unhooked the alternator, re fired it, the gauges showed normal. I hooked the alternator back up, it acted fine, in fact behaved until I was a mile from their yard, where it went manic again, Limped it to the yard, changed out the alternator ,end of problem.
