aftermarket tach stops working when needle hits 1300 RPM

[stillsmokin]

I figured I'd take a shot in the dark and see if anyone has seen this problem. Otherwise I'll call stewart warner in the morning.

I put a stewart warner 114015 programmable/memory recall/shift light tach in the 92 dodge this winter. At first I had it tied into the signal wire on the factory add on tach port under the dash, it didn't like the signal that came from the computer at all. So I moved the signal wire out to the engine compartment ahead of the computer. Today I had a chance to try it out, it works, until 1300 RPM or so then it drops back to zero, cycle the ignition and it's working again.

I'm lucky enough to have a scanner so I plugged it in, the tach is right on with the scanner, but the RPM signal is still there after the tach shuts down. This isn't a connection problem and I wired it from the chrysler service manual for the truck. It appears the tach is shutting down to protect itself from the signal it is recieving from the crank sensor. I don't know if the voltage/amperage is too high or what? I wonder if I need to put a resistor or something along those lines to keep it from shutting down?

 

[92OLBLUE]

I have a factory tach from dodge that plugs in under the dash and every once in a while it will act the same say. It drives me up the wall

 

[DieselDreamerDM]

My Stewart Warner tach/shift light just did the same thing, im wired straight off the sensor that reads off the harmonic balancer it works great. both tachs work but at 1670 my new tach goes to zero and the other one keep s on climbing...

Just wondering if either of you found a solution????
Thanks

 

[joeservo]

It appears the tach is shutting down to protect itself from the signal it is recieving from the crank sensor. I don't know if the voltage/amperage is too high or what? I wonder if I need to put a resistor or something along those lines to keep it from shutting down?

not the case its 5volts at max and current is a function of the input reistance of the tach which is very high. the problem is the pulse is so small some tachs think its noise. i've fixed autometer tachs in the past by removing filter caps. if we could widen the notch it would work better. at 600 rpm it looks like a 300Hz signal, slow but thats 4500RPM on a normal v8 tach. at 1800rpm its 900Hz or 13500RPM on a v8, a little fast for most tachs. one option is a one-shot circuit before the tach. or mod the tach. adjusting the sensor distance might help if you have it set to far.