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#1
07-20-2007, 08:00 PM
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Tach on speed?
Hi ya'll,
I'm new as a Pontiac owner, after having owned Chryslers and Fords in the past. I just purchased a 1991 Trans Sport 3.1 V6. Old car, I know, but in pretty nice shape for age and mileage. It's got about 300k km on the clock, but new engine and tranny with only 175k km on them. Considering the shape of the rest of the vehicle, I'd say it has many more miles to travel! One thing I noticed was that the car held the gears rather long. One small tug on the TV cable fixed that. Easy! The other thing I noticed was that the car seemed to rev very high all the time. Unreasonably high actually, considering the final drive ratio the transmission is supposed to have. So I hooked up my own tachometer and lo and behold, the dash tach was way off! I took readings at idle speed (700-800rpm), 2000 and 3000rpm. The same revs on the dash tach showed 1200, 3000 and 4500rpm respectively! That's a 50% increase!  I know that having a tach set to the wrong amount of cylinders can give this error. To my knowledge though, there is no switch on these stock GM tachometers, like you find on typical aftermarket ones. Or is there? I also called my local GM mechanic, and he was as as stumped as me. No manual or diagram has helped me either. Did the new drop in engine have a different tach filter, maybe? It's a 3.1 with the 3T40, just like the original. Did these parts change over the years? Well folks, there's your weekend nut to crack. Anyone got any ideas? 
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#2
07-23-2007, 07:07 PM
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Re: Tach on speed?
Obviously that is a good one... INdeed there may have been variations in the filter design/tach winding characteristics over the years.
In my world, since the signal fed to the tach is analog, I'd install a resistor to suit. I do not have values but measuring the tach resistance (disconnect tach at filter -mounted at ignition coil bracket - then measure between B white wire on tach side and ground) and then installing a resistor of twice the value in parallel (from B to ground) should down the reading close to normal. www.avigex.ca/xport/instrumentpanel.jpg
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'93 Ponty Transport 3.8; 2011 SUzuki SX4 AWD (wife's) ; 2015 Sorento 2.4 AWD 1928 Graham Paige Model 629 I don't believe in miracles. Last edited by LMP; 07-23-2007 at 07:50 PM.
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