by jspbtown » 09 Fri Jan, 2009 9:14 pm
I apologize for being vague. When I built the Bradley I found the Dolphin tach to be slighly erratic and it didn't look like it was accurate based on the pure sound of the engine. I know...not scientific. In doing some research I came across some posts about a mod that was done to some VDO tachs that made them work better with the electronic ignitions. Below is what I have saved via cut and paste for my future use:
"What I meant was just to test your gauge light temporarily by wiring it to your front parking light. Hook your connection up like Scott said so your light will dim like when you turn you headlight switch knob around to brighten or dim your instrument lights. Pyro said to set it for 6 cylinder, that's what made mine work, but also try what Syberia said. Also like what was mentioned, it could be your ground. Solder all your connections for best results.. The diode I used was from Radio Shack, part# 276-1104, IN4005 Rectifier Diode. Let us now what the end result is, I'm interested to find out. It's just a shame that VDO makes a gauge that is problematic right out of the box. If I ever get another tach, it'll be a SUN Super II, I NEVER had a problem with that one, although not the best, it was reliable and functioned with no probs
Will VDO tachs work with points ignition or Pertronix?
Answer: Yes, with diode in line on the signal lead. Instructions are as follows:
Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4†long. Crimp 1/4†female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4†female spade connector used in #3 above to terminal #4 on the back of the tachometer. Connect a ground (-) wire to terminal #3. Connect a switched 12-volt power wire to terminal #2. Set switches for appropriate amount of cylinders
The side of the diode with the silver band goes toward the tach. It will make sense when you follow those instructions in my previous post and hook up the spade connectors to the diode. Get the smallest gauge (18-20) 1/4" connectors you can find to hook onto the diode. I hooked mine up so the diode is sort of independant of everything else, like this:
-Crimp 1/4†female spade connector on the end of the diode with the silver band.
-Crimp 1/4" male spade connector to other end of diode.
-Crimp 1/4" female spade connector to the end of the wire that goes to your coil.
-Plug diode into appropriate terminal on tachometer.
-Plug wire from the coil into the diode."
I applied this to my Dolphin tach on my Manx and it made a HUGE difference. Much smoother and not as erratic. Same pointless system, same tach, just on a different motor and a different car.