I thought i would throw in my 2c to this thread too. Ive spent ages trying to figure out why my tacho booster and tacho wasnt working for my 1ggte and last night i finally got it working.

It all came down to being a bit lazy, i wanted to test my tacho booster to see if it worked before wiring the power up to the car. So i just grabbed a spare battery from the shed and ran the power to the booster from that. It didnt read at all, not even a blip.

Im not sure if its due to the extra voltage from a running car or if the other battery was somehow interfering but after wiring the booster power to the car (the same power from the rest of the dash cluster) it worked like a dream.

Also my relay was clicking really loud at low rpm so i just popped the cover off and removed the clicking contact.