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Discussion Starter · #1 ·
For years I've been laboring under the misconception that only one leg of the CDI unit operates the tach. The other operates the fuel pump relay.
Thanks to a post by casetractor, I did some investigating and found that the tach actually uses an input from both legs of the CDI as it fires the coils.
I was trying to find a good way for him to diagnose his problem, and ran into an often complained about discrepancy in the repair manuals.
The book shows a four pin connector at the LCD meter assy. This plug only exists on the '84 as far as I can tell. The later models use larger multiple connectors.
Below is a schematic of how the tach is fed. Please note that on one leg for the tach drive, the wiring color CHANGES.


Since the signal operates the coils, it will most likely be too fast to pick up with a conventional DVOM. You'd probably need something like an oscilloscope or a pulse/duration meter.
The only other option is to check for continuity between the coil blu/yel and the LCD yel/green. Repeat the same test between coil yel/blu and the LCD yel/blu. Note the LCD connector is not a four pin as per the manual. It's the thirteen pin white connector at the back of the meter.
If your bike is firing correctly, and you have continuity on the wires you've just tested, the LCD is N.G. If you don't have continuity on one or both of those legs, you have an open circuit that needs to be back tracked. (Broken wire, pin backed out of connector, bad crimp joint).
 

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Discussion Starter · #2 ·
The four pin white connector they seem to be refering to is the actual connector on the CDI itself. Measure continuity between the CDI and the LCD as described.
 

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GlHonda, you've given us the tools to track down this problem. Electronics are not my forte, so, just to make sure, one question:

How do I check for continuity between CDI/ LCD wires you've identified in the posting?
 

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Discussion Starter · #5 ·
GlHonda, you've given us the tools to track down this problem. Electronics are not my forte, so, just to make sure, one question:

How do I check for continuity between CDI/ LCD wires you've identified in the posting?
All it takes is a lot of work. Remove the faux tank. To the left of the air filter are multiple connectors. The white four pin connector (c121) toward the front will house the yellow/blue and the blue/yellow coming from the CDI.
Disconnect it.
Remove the windshield front garnish. The windshield may need to be removed as well, but you will find access holes in the front of the fairing.
Locate the 13 pin white connector (c18). You're are looking for yellow/blue and a yellow/green. Disconnect it.
Measure the resistance of the wires in the harness between the y/b (c121) and y/b (c18), then between b/y (c121) and y/g (c18). Continuity should exist.
Remember you measuring the harness, not the components. Also check the condition of the connectors and the pins.
Good luck.
 
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