Tacho not working properly on twin plug MSD set up
 

I have been struggling to get my original electronic VDO Tacho 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've tried feeding off the other MSD box - same thing happens
I've tried an MSD 8920 Tach Adaptor (wired as per voltage triggered tacho) - 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 and encountered this problem ?

As the best of a bad job I've been running the tacho straight off one of the MSD boxes so I've at least got a reading up to just over 4000rpm.

I'd really like to get this sorted and working with the original tacho.

Cheers
Andrew

What year tach are you trying to use?

I think it is '74 or '75. Other one I tried was a bit later ('76/'77)

There has been a lot of discussion on tachs here in the past. Seems to me that what you have should work. Perhaps a wiring or ground issue?

Maybe someone who has more experience with this exact issue will chime in.

I have the JB Racing distributor and MSD ignition in my ‘71.
I had to use the Tach Adapt from this company. TACH-ADAPT
Works great now. Easy to install.

I have used this device before to convert 4 cylinder to 6 cylinder tacho.
If I was to try this then it would feel like a bit of a stab in the dark having tried 2 off MSDs own Tach Adaptors that are specifically designed for the job with MSD 6AL ignition boxes activated by a magnetic trigger.

I am hoping there is someone out there that has used twin MSD 6AL ignition boxes magnetically triggered with an original early electronic VDO Tacho that can offer some first hand advice, or someone who is a technical wizard who understands the engineering behind the issue and how to resolve it, and crucially why the MSD TACH Adaptor has not helped- in fact it has made the tacho response worse and less predictable.

Here's hoping
Andrew

Buy the ashlock tach adapter for 60 bucks.. Works unlike the msd set up.

Andy, some tachos are sensitive to pulse width.

Hi Jonny, so does pulse width change with increased RPM ?
Tacho is working perfectly up to 4500rpm and another identical tacho was exactly the same.

I was surprised the Tach Adaptor gave no tacho response until high rpm.

What to do ?

I have the exact same setup. I’m sure my ‘71 tach is the same as yours. Anyway it solved my tach issues.

Thankyou, can I just clarify - are you saying you used a MSD 8920 Tach Adaptor with a twin plug, twin MSD 6AL box set up and it didn't work, but you then used the Ashlock Tach Adaptor and it worked perfectly ?

Maybe. Early '71 tachs get their signal off the coil and use an "intermediate" device. Later '71 tachs should be the same as the '74 taking the signal off the points.

Send the tach to North Hollywood They'll fix it right up they have a MSD conversion

I have the twin 6al ignition boxes. JB twin plug distributor. I used the grey tach wire off the MSD boxes to mate with the factory Purple/Black tach wire. My tach didn’t do anything at all. I installed the tach adapter and it only partially worked. Then I read the instructions and internally I had to switch a contact. Then it worked great.
I had some great conversations with the owner of the tach adapt company. Give him a shout he will sort it out for you.

http://forums.pelicanparts.com/uploa...1558354672.jpg

The later tachs should trigger just fine off the MSD tach output. You could try a generic modern tachometer to compare or it could be that MSD weakened the tach output signal. It's also possible that a leaky capacitor or dying capacitor in your tach(s) is making them less sensitive.

With respect to seafeye's picture, (not the OP), I did notice the picture you posted of the two MSD boxes. Unless they are potted, which I doubt, the boxes should not be mounted upside down. Engine heat might drive any accumulated moisture off (and probably lots of heat in that location which is potentially another problem waiting to happen), but upside down is generally not a good way to mount those boxes, unless they are potted. Condensation can and will accumulate unless you live in a very arid climate. It needs to have a way out and MSD provided that by not sealing the base plate. Fred

Thank you all for the compliments about my TACH-ADAPT product. It commonly solves this problem. Pelican sells it under part number PEL-TA-V3. This product became mass-produced due to this forum!

TACH-ADAPT ,,,,, period

does anyone know if a tach adaptor is required to get the proper signal . from MSD6AL to a brand new version of the AIM MXG DASH/LOGGER? I am having same issues with the tach signal registering on the DASH... its and in the video ( on a SCHD) . its not smooth, and dies after . 3-4500 rpm..

Hello Bob,
you may recall I bought several of your Tach-Adapt several years ago.
Good to hear from you. I've sent you an email
Regards
Andrew