Window switch grounds tach signal???

[Skinny]

I connected my window switch tach wire to the tach feed to the instrument panel and the power wire to the master arm switch for my nitrous. Every time I switched the power switch on my tach went dead. I broke out the ohm meter and verified that the tach wire on the msd switch was grounding when I powered the switch. To double check I connected the window switch to the tach feed from the ignition harness to the computer. When I powered the window switch the engine died.

My first though was to check the ground on the switch. I noticed that it wasn't a good ground, so I moved it to another location. The results were the same.
Any ideas?

[12secondv6]

I'm not a electrical person but someone will know the answer to this, sorry

[DeathBuzzz]

Here you go. This is the same problem I had. I talked to MSD many times and did some research on my own too. The MSD switch wants a 12v square wave for the tach lead. Our cars have a 4v square wave. When the switch is armed, it draws too much current from tach wire the computer kills the car ( it gets no RPM reading ). MSD will convert the switch for about $35.

I never got it working. I looked around and found out that the newer cars ( I have a 99 ) have no single tach lead. Everything is multiplexed and then sent to the instrument cluster then de-multiplexed. This started when they went to a completely "drive-by-wire" system. One thing that may be possible with out converting the switch is to find the engine RPM sensor from the ignition coils. It is one of the white wires behind the coils ( I think there are 3 of them and I dont know which one is the RPM, you would have to use a multimeter to find out ).

To re-phrase: Using the switch as it is will not work on our V6 cars. On the older models ( I *think* earlier than 99 or maybe 97 ) MSD can convert the switch to a 4v wave and that *might* work. On the other hand it may be possible to pull a RPM lead from the back of the coils.

If you want some more info that i found and/or determined, e-mail me: BChappel@tulane.edu


~Brian