Tach/Ballast Resistor Question

[tom1394racing]

I am about to do the first start and break-in run on the engine in my '70E project. I have spun the engine with the starter to ensure fuel and oil pressure and no leaks. I have also checked to ensure spark. However, the Tach shows no response when the engine spins on the starter. I would like to make sure the Tach works so that I can set the RPM at 2000 for the first 20 min break in run.

I am not surprised that the Tach does not respond as it is wired through the ballast resistor but is a later model Tach that should not require the ballast resistor to condition the signal (see photo). From what I have researched, I should be able to make Tach signal from the coil bypass the ballast resistor by connecting the blue/yellow wire from the coil directly to the black/purple wire to the Tach. I plan to jump these two wires underneath the ballast resistor (see photo) and leave the ballast resistor in place for appearances.

Is this the correct approach to make my existing Tach work?

[uai]

Hi, I think (think, not know) that this might only work with the newer 8 pin cdi.

[mac73s]

Tom - I would recommend an email to Bob Ashlock (makes the TACH-ADAPT products)...he really knows the ins and outs of getting a tach to work properly - particularly if using non-original configurations/components. His email is: bob@ashlocktech.com

[tom1394racing]

Mac

Thanks for your suggestion. I will try Bob. I assume, since the Tach has a date stamp of 12.72, it is the later '71-'73 "high impedance" Tach that does require the ballast resistor.

[NorthernThrux]

Mac

Thanks for your suggestion. I will try Bob. I assume, since the Tach has a date stamp of 12.72, it is the later '71-'73 "high impedance" Tach that does require the ballast resistor.

Tom, don't assume anything because of the date stamp. Tach innards get altered all the time. My 12.72 tach had been monkeyed with internally, probably to run with a non Bosch-CDI, and never worked properly with a correctly restored 3-pin CDI unit (also from Bob). Bob fixed it with a custom internal board that he builds, making the tach even more linear with engine speed than it came from the factory. He's the man.

Ravi

[mac73s]

Tom - Bob Ashlock works closely with John Bell in California who is very talented and creative with custom gauges...attached is a photo of my 72 gauges he converted to green dials (a bit washed out in the photo) and a 10K tach/volt meter combo. Another reason I'm enjoying my hot rod build...my car, my way.

[tom1394racing]

Thanks guys. Interesting that the Tach is the only gauge not original to the car. I sent all the gauges to N Hollywood for refurbishment. My CDI unit was rebuilt by Kurt at PartsKlassic with his electronics upgrade. I assumed that if N Hollywood restored the gauge to its original “high impedance” condition and if I bypassed the ballast resistor as shown in the ‘71-‘73 wiring diagram, the Tach should work as designed. I will see if I can call Bob Ashlock into this discussion.

[Ratech]

Hi Tom,

I sent you email on this subject. I think the problem is that you have a low-voltage triggered tach and you have connected it to the 400V coil discharge signal. It probably is not too happy about that.

[tom1394racing]

Hi Tom,

I sent you email on this subject. I think the problem is that you have a low-voltage triggered tach and you have connected it to the 400V coil discharge signal. It probably is not too happy about that.

Bob Ashlock has replied to me via email. He has made some really good suggestions and helped my understanding. Obviously my “plumbers” approached the plug and play will not help make my tach function. He suggested some alternatives and I will follow up here and let everyone know how things turn out. Thanks for referring me to him.

[tom1394racing]

An update on this....I found a '70E tach in my stash and connected it through the ballast resistor. Worked perfectly and allowed me to do the first 20 min 2000 RPM break in run on the motor.