Tacho not working properly on twin plug MSD set up

[amallagh]

I have been struggling to get my original electronic VDO Tacho ('75/'76) to work properly on my twin plug 2.8.
It is a twin plug set up with twin MSD 6AL boxes, Twin MSD Blaster 2 coils and a JB Racing 12 plug dissy with internal magnetic pick up.
I'm taking the tacho feed off one of the MSD boxes. They have a specific tacho signal output.
The tacho works OK up to about 4000/4500rpm then drops to zero.

I've tried another similar tacho - same things happens
I tried feeding off the other MSD box - same things happens
I've tried an MSD 8920 Tach Adaptor - surprisingly this left the tacho not working at all.
I bought another new MSD 8920 Tach Adaptor - It does the same but at about 3800rpm the needle shoots up , then by 4500rpm it drop to zero again, before going full scale at about 5500rpm !! (The first tach adaptor may have done the same thing but I never tested through the rev range)

I'm at a bit of a loss as to what to do.

Has anyone else tried an original VDO Tacho with twin MSD boxes ?

Cheers
Andrew

[Jonny Hart]

What’s your battery voltage at 4000 RPM?

[amallagh]

What’s your battery voltage at 4000 RPM?

Voltage about 14.2v anywhere above 1000rpm. Drops to about 14.1volts by about 2500/3000rpm and is much the same voltage at about 4500rpm where it drop to zero on the tacho.
I tried to hold it as 5000rpm as best I could with the tacho reading zero and it was 14.1 volts at the battery.

[amallagh]

Still no joy trying to sort out this problem.
I posted on one of the US website and someone suggested that I use a Tach-Adapt by Bob Ashlock.
I've bought several of these in the past. I hadn't thought that it would work when MSDs own device to resolve the specific issue appear to not.
I'd like to try one. Does anyone have one spare I could try before buying one in the US and getting it sent ?

[amallagh]

Fitted the Bob Ashlock Tach-Adapt and it works perfectly !!!!

This has been through several 'experts' and tried MSDs tach adaptor and nothing worked. Plug plan play with Tach-Adapt and it is perfect.
If anyone wants a Tach-Adapt I have a couple of spares. PM me if you want one.
Cheers
Andrew