Coil circuit question(s).

[Guest]

While I have my 1980 GS750E down for carb tuning and waiting for a new battery and air filter I thought I would check over the coil circuit. What a convoluted course those little electrons take to get to the coils.

Sinice I haven't received the new battery for the bike yet I decided to check out the coil circuit with my trusty DMM. After dis-assemblying and cleaning the contacts in the handle bar kill switch (where I found a fair chunk of the resistance in the coil feed circuit) I am reading 1.4 ohms from the positive battery cable to the harness side of the coil connector, 4.1 ohms across each coil (is this okay? (someone posted that these bikes use 3.0 ohm coils) ), and .5 ohms on the ground side of the coil harness connector back to the battery ground cable.

If I remember my V = IR stuff, a total of 6 ohms in this circuit with a 12.8 volt battery means it carrys ~2.1 amps when loaded (no wonder the spark isn't fat and dark blue).

That means I'm droppin almost 3 volts just to get to the coils. So with a battery putting out 12.8 volts my coils are seeing maybe 10.8 volts (when the engine is NOT being cranked with the starter.

I have not attempted to take the ignition switch a part to see what shape those contacts are in. Heck, I was reading .2 ohms just across the glass ignition fuse! So, what is a realistic voltage at the coils with the coil circuit up to snuff?

And when folks add the coil relay circuit, what kind of resistance does it read between the battery and the coils?

Thanks in advance for any insight into this "invisible" electric stuff.

[Eric]

Many people (including myself) have installed a relay to power the coils:

http://members.dslextreme.com/users/bikecliff/images/coil_relay_mod.html

[eil]

Standard advice is to clean all connections and switches anyway. The coil mod will help deliver a full(er) 12V to the coils regardless, but ensuring all of the mechanical contacts are cleaned at least once every 30 years is still a good idea.

4.1 ohms across each coil (is this okay? (someone posted that these bikes use 3.0 ohm coils) ),

Factory manual specs for the primary side of the coils is 3-5 ohms.

So, what is a realistic voltage at the coils with the coil circuit up to snuff?

And when folks add the coil relay circuit, what kind of resistance does it read between the battery and the coils?

I didn't measure the resistance but before doing the coil relay mod on my bike, I was getting around 10.5V to the coils, after was closer to 12.5V. There was still a bit of a drop, but not enough to care about.

When inspecting the ignition system, also measure resistance across the secondaries of the coils. I don't remember what the value is supposed to be off-hand (you'd have to check the manual). If you get infinite resistance, that means there's a break in the coil, wiring, or caps. The engine can still run fine like this if it's a small break but it could get worse over time or eventually short out against the engine or when in the presence of moisture.

[pdqford]

Factory manual specs for the primary side of the coils is 3-5 ohms.

Thank You for that information -
Looks like my coils ( at least the primary side ) should not be an issue.

I didn't measure the resistance but before doing the coil relay mod on my bike, I was getting around 10.5V to the coils, after was closer to 12.5V. There was still a bit of a drop, but not enough to care about.

Again, Thank You for those measurments. Looks like the coil relay mod is on my tado list. That's nearly a 20% increase in spark energy!

When inspecting the ignition system, also measure resistance across the secondaries of the coils. I don't remember what the value is supposed to be off-hand (you'd have to check the manual). If you get infinite resistance, that means there's a break in the coil, wiring, or caps.

Another Thank You for that explanation. I was wondering how one determines if the caps need replacing. Checking the secondaries is now on my tado list.

This manual you refer to - is this a factory service manual?

[koolaid_kid]

It is probably on this site: BassCliff's website