[ROVERNET - UK] RE: Final Drive Issues

Glen Wilson rovercar at comcast.net
Wed Feb 20 20:16:42 GMT 2008

Adrian McDonald wrote:
> I should have said that I "inter"calibrated by tacho with a multimeter. I set the engine rpm at 1000, 2000, and 3000rpm on the tacho and read what the digital multimeter displayed

Are you saying you used a DMM that counted pulses or frequency? Or are 
you referring to an automotive DMM reading independently from the 
ignition system? I may be missing a detail or two from the discussion, 
but it might be a good idea to take a step back if you are approaching 
this problem from an electrical engineer's perspective. Check the engine 
rpm with some sort of simple automotive rpm counter based on readings 
from the engine coil and see what that reads. It's entirely possible 
that a counting DMM may give you inaccurate readings at the dash 
tachometer, especially if the dash tach functions more on the basis of 
current pulses than on voltage pulses.

Remember, a $20 automotive tune-up meter from Pep Boys is probably much 
more of a precision instrument than the tach in your car with all the 
hysteresis and needle bounce. Verify the actual engine rpm measured 
independently at the engine and then revisit the differential question. 
You can also just go out on the interstate and check the 
odometer/speedometer by watching the green mile markers by the side of 
the highway and running ten miles. You can even time it to check your 
speed. This will tell you if your tiresize/rearendratio/speedometer are 
giving reasonably accurate readings.


>  at the same time. The rpms on the tacho matched the rpms on the multimeter, hence I deduced that my rpms were right. That left me knowing with 99% certainty I was doing 3000rpm in 4th gear while driving along, and being 98% sure I was only doing 40 something mph instead of 58.5mph.
> Also, I am not sure how to reply to messages on this thing. Not only a new P6 fiddler, but a new forum-er too!
> Adrian
> _______

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