Serious voltage drop when ignition is on / comming from the dynatek iginion system

[DKGS850]

Dear all,
I have been doing a complete rebuild of my GS850 GN 1979, and with a bike that should be mechanically sound, I am now ironing out the last wrinkles in the electrical system, and this issues has me puzzled.
Before I start describing the issue, I need to confess that it is very much a "resto mod" rebuild. I have installed a dynatek ignition, M-unit, Mosfet R/R (not connected yet) and done a complete from scratch wiring harness This of course complicates the problem solving, but I hope that something can be deducted from electrical theory and a lot of GS knowledge.

Measuring the battery voltage I am around:

12,4V when the battery is disconnected.
11,3V when the battery is connected and the ignition is off
7,8V when the battery is connected and the ignition is on
Climbing quickly back to 11,3V again when the ignition is turned off again.

I have tried disconnecting the power wire leading to the Dynatek ignition (replacing the points). With the power wire disconnected there still is a voltage drop, but a much smaller one. It only drops from 11,3V to 10,9V.

I am not sure which of these measurements I should be worried about. 5 months ago I had everything hooked up right, and the starter motor was working, but that was with an "under construction" wiring harness. Now the wiring harness is complete.... except for this minor snag.

Any ideas would be highly appreciated. Also any ways of testing my way to a conclusion.

Thank you.

 

[Steve]

Measuring the battery voltage I am around:

12,4V when the battery is disconnected.
11,3V when the battery is connected and the ignition is off
7,8V when the battery is connected and the ignition is on
Climbing quickly back to 11,3V again when the ignition is turned off again.

I am not sure which of these measurements I should be worried about.

Actually, you should be worried about ALL of those readings.

A fully charged wet-cell battery should be 12.6 volts. A sealed AGM battery should be 12.8 volts. You are already a bit low when you start.

The fact that it drops to 11.3 with the key OFF says that you have SOMETHING connected without benefit of a key. You will need to disconnect things from your M-unit until you find what is sucking power.

Dropping to 7.8 when the key is turned ON says that the coils are now working, but there won't be enough power to do anything with them.

The first place to start is with a new battery. If you want to prove that with minimal expense, push your bike over to your car and connect the two with jumper cables, but be SURE that the car is NOT RUNNING. When the car is connected, do your same tests that you did above to see if the numbers are better.

Ideally, you should see 12.6 to 12.8 with the key OFF.
With the key ON, voltage should still be over 12.
While cranking the electric starter, voltage should still be over 11.

There are many other voltages that can be checked, but let's get the bike running, first.

.

 

[Redman]

Battery voltage is a little low to start with, so... have you had the battery on a charger? IF not, try that.

The fact that voltage dropps as soon as connect to bike makes me suspect something, first suspect is the R/R. Try disconnecting that.

THe fact that it drops more when turn on the key is somewhat normal since adding more load (lights, ignition), but not that much of a drop, so, again, I suspect a marginal or bad battery. Try charging the battery first.

So, yah, pretty much, same as what Steve said.

 

[DKGS850]

Dear Steve and Redman,

Thank you very much for you help, insights and suggestions.

I did have the battery on a charger, but maybe I wasnt patient enough, and used the wrong charger setting. Apparently you can damage a gel battery, if you charge it on "car" instead of "Motorcycle". My battery is large enough to be charged as a car battery according to the charger manual, but not by much. Ill try charge it as an MC battery instead and hope that it isnt already ruined.

I am still a bit puzzled by what happens when I disconnect the ignition, and I am afraid that I might have used the wrong terms when explaining this. It is when i disconnect the power lead going to the points (or where the point now have been replaced by a Dynatek ignition) that the voltage only drops to 10,9 V, instead of 7,8 V with the points connected. There are power to the coils in both scenarios.

I'll give my battery a charge and hope the fixes the problem.

Thanks again, Gentlemen.

Best regards

Michael

 

[DKGS850]

Dear Steve and Redman,

Charging the battery did not do much, but replacing the battery made a world of difference. I was actually able to start the motorcycle for the first time in 25 years.

So much win Thanks Gentlemen,

Best Regards

Michael



Than

 

[Steve]

Thanks for the report.

How old was the battery?

.