Help figuring out my tachometer

[blau911]

I have a 69T with 2.4 S spec carb'd motor. The gauges were all rebuilt by NH Speedo.
Since the installing the motor, my tachometer has always showed rev numbers 3 times higher than the real numbers.
I found out that the 69 cars had a ballast resistor which would explain the problem. Instead of installing a ballast resistor, I had the original tach rebuilt again with an internal ballast resistor. After installing the tach, the reading is correct until I accelerate, and then the rev's are agin approx 3 times the correct number. However, if I lightly put my foot on the gas while accelerating, the tach reading drops down to the correct number. This seems really bizarre to me.
How could touching the brakes on my car bring down the tach reading?
Just to clarify, I am not letting off the gas and letting the car slow down (which of course would reduce the rev's). If I touch the brakes, while in gear and maintaining the accelerator, the tachometer drops down to a normal number. Is soon as I let off the brakes, the tachometer needle jumps back up...
Does anyone have any ideas what the correlation would be between the brakes on my 69T and an exaggerated reading on my tach??????

[jbell959]

That is a very strange problem... There should be no correlation between the brakes and the operation of the tach, unless there is some type of wiring issue related to the brake light switch.

I would reach out to Bob Ashlock and pick his brain. He knows the tach operation very well and might have some ideas.

[pors1968]

Do you remember if ground is Brown or Brown with yellow dot 69 are pain to take rpm off