MSD w/ Tach Adapter - But Still no Tach reading?

[gruen911]

After experiencing alot of plug fouling, this Spring I switched my CD box over from the previous owners Permatune 6 pin unit to a new MSD 6AL with 8910 tach adapter on my 1967 2.0 911s engine. Since then, the engine has run problem free with no plug fouling issues whatsoever. However, I can't seem to get my tachometer to function since the changeover.
At ignition, the tach needle makes a little jump, and then drops again and doesn't move. I re-checked the wiring instructions, and went thru the wiring 3 times to make sure everything was correctly done. The only thing that might even be slightly suspect is that the switch I am using to trigger the MSD unit is spliced into the same wire which triggers the fuel pump from the ignition - rather than have a wire leading directly to the ignition. However, I can't see why that would make any difference, since this is how the previous unit was triggered and the tachometer worked when the Permatune unit was installed - and the new MSD works (but not the tach). Only option I can think of is to send the tach to Hollwood Speedo and have them recalibrate it for the MSD set-up. Anyone else have any other ideas?

[john_cramer]

Gruen,

If you will draw a picture of your wiring EXACTLY as it is on the car and post it here, we can take a look at it. Also, are you using a diode to filter out back EMF in the tacho drive circuit?

Now, I'm going to tell you the really easy way to fix this. The side of the MSD 6AL has a tach output terminal. Get yourself a nice long wire and run it from the tach output terminal to the /1 input on the back of your tach, where the black/purple wire goes. That should do it, no problems, no adapters, no pullup resistors, no diodes, etc.

[gruen911]

Hi John,
Thanks for the reply. I will try to post a diagram later today. As regards using a diode, I initially installed the tach adapter without it. When it didn't work, I spliced in the diode that was provided with the adapter without any effect.
I will try the direct line from the MSD unit to the tach tomorrow. However, I am wondering how the tach will respond up to 3k rev's with the multiple sparks.

[john_cramer]

Gruen,

That's just it-- the tacho drive signal from the MSD box is NOT in the output to the coil (which is what makes multiple sparks), it's in the input from the points. So the tach should work fine, and it's a +12v square wave output with a 20% duty cycle.

I would try it with a nice long wire first before you go snaking anything through the tunnel, just to be sure you are happy with it. Be VERY careful removing your tachometer, don't scratch the bezel, careful of the gasket, it will be a tight squeeze getting it out over the clamshells around the stalks, may require removal of the clamshells. Watch out for the wiring-- many electrical fires have been caused by old chafed wiring behind the dash-- if you see anything that looks like it's overheated, e.g. melted insulation, scorch marks, etc., I would replace it before completing the tach wire install.

A good path to snake the wire is right next to the main electrical harness in the tunnel- you may have to remove the shift coupler cover and the carpeting around the shifter and the drivers wooden floorboard, in order to snake the wire up and to the left where it exits the tunnel and into the trunk. Avoid the fuel line if possible, and zip-tie the wire frequently so it doesn't flop around and chafe on the body sheetmetal.