Tach problems in MGB

MGBMarty · Post by **MGBMarty** » Mon May 24, 2010 11:21 am

I got the MJ2 up and running, and it works excellent! I have a tachometer issue ..

I have tried a few scenarios....

I have an MG RVC tachometer (from a 1977to 1980 MGB). It works connected to the Tach output on the MJ, and also works connected to the PIP output from the EDIS module.

The MG is a 1965 and the RVC tach doesn't match the early gauges.

I took an early MG TACH and fitted the guts from a 1989 Pontiac tachometer, using the Smiths movement. I calibrated it on my wave form generator.. All looks good.

This tach will not work connected to the TACH output from the MJ, but it will work connected to the PIP output, however it only works when the MJ is disconnected. When I plug the cable back in the MJ ( I have the cable split at the MJ end to wire the tach), it stops working? It also will not work connecting it to the Tach output on the EDSI.

Any ideas? I have yet to try pulling the signal from the coils with the diodes...

Post by **brentp** » Mon May 24, 2010 3:09 pm

It seems the PIP output is not able to feed both in your particular installation. I would try the diode circuit or thr IDM output on the edis module (we don't document this wire, but some google searching will help)

MGBMarty · Post by **MGBMarty** » Wed May 26, 2010 1:57 am

I did try the output from pin 2 on the EDIS module, but it behaved the same as the Tach OP on the MJ unit. I will try the diode setup.

MGBMarty · Post by **MGBMarty** » Sat Aug 07, 2010 11:39 am

So I tried the diode setup, but the tach was pulsing? I pretty sure I have the setup correctly? Any ideas?

dan97019 · Post by **dan97019** » Sun Aug 08, 2010 3:06 am

i guess while were on the subject I will mention my tach problem I have a 63 bug with an older style Sun tach. and I used the diode setup to get it working and it works fine up to 4k RPM and then it just stops climbing so I put in another Sun tach that I had and it did the same thing, also I should mention that I do not have the zeiner diode in the circuit because I was not able to find one, could this be causing the problem?

Elf · Post by **Elf** » Sun Aug 08, 2010 10:39 am

I just heard today about using shielded cable for the tacho - does that help any?

MGBMarty · Post by **MGBMarty** » Sun Aug 08, 2010 2:14 pm

my test was 'under the hood' with jumpers and twisted wires.. so I suppose I could have had some interference, but I don't believe a wire to a tach should be so sensitive that it requires a shielded cable! I did use a Zener, but I;m not sure what the theory is as to why the Zener is required. I can see the two diodes to kep the tow signals from the coils intefering with each other, but I'm not sure what the zener does.. I did read somewhere here someone who didn't use the zener and had no problems.