Stewart Warner Tachometer

[kboyd]

The Stewart Warner tachometer in my 83 Avanti has started having random periods where the indicated RPMs move up or down even though the engine is running at a steady speed. It normally reads around 2,000 RPMs at 60 MPH, but occasionally it will read as high as 3,000 RPMs or as low as 1,500 RPMs at the same steady state speed. Usually this occurs after the car has been driven for over an hour. If my math is correct, the tachometer should be reading around 2,200 RPMs at 60 MPH assuming a 26" tire and an overall final drive ratio of 2.87 (contemporary road test) but I do not know exactly what differential ratio the car is equipped with. The fourth gear reduction for the GM 700-R4 transmission is 0.70 so if the car is equipped with a 4.10 rear end (which seems reasonable) the overall 2.87 reduction cited in the roadtest seems reasonable.

Suspect the electronic controller for the tachometer is breaking down when it heats up but what I am trying to find out is the controller a separate component in the Stewart Warner tachometer circuit or is it built into the tachometer. Has anyone else experienced a similar problem with a Stewart Warner tachometer and how did you fix it?

Edited November 14, 2012 by kboyd

[Devildog]

I replaced my tachometer on my 1975. It acted the same way. i do not think you have a separate controller...mine did not. I just replaced the unit in the dash and it works fine (the older units had controller). there is just a wire that goes to the distributor, power wire and dash light wire

I doubt you have a 4.10 differential, probably more like a 3.5 or 3.7

I have the original Dana 44 differential with (I think a 3.7 ???) and with my 700-R4 I get 2200 @70 and 2400 @80 mph (not sure about 60 mph rpms, I seldom drive 60).

Hope that helps

Joe

Easy way to determine gear ratio: juck up rear, chalk mark the drive shaft and chalk mark a tire. Assuming you have a limited slip so BOTH tires turn at same rate, count the drive shaft revolutions vs the tire revs.

[Charlie Kile]

My 84 Touring optioned coupe has 2.86 gears with positraction, 700-R4 transmission,and P215 70 15 tires.

According to the Arkansas and Missouri Highway Patrols my speedometer is almost exactly 10% slow.I/E when I'm showing 72 mph I'm actualy going 79. Neither gave me a ticket only warnings. When I'm showing 72 mph my tach is reading a fat 1700 rpm.

Charlie RQB-3921