TOPIC:

hey everyone, I have been working on this 77 kz750twin for awhile. Rewired it based on a barebones diagram here on the site.
Trouble with battery not charging. I am in unfamiliar waters trying to daignose electrical issues, so please help.

battery is new. bike starts and runs when battery is charged. when running battery drops voltage at a rate of ~0.01v per 1-2 seconds. checked the wiring, seems to be wired correctly per diagram. could not find any shorts. checked resistance of three yellow wires coming out of generator, all in spec.

I put a voltmeter on the white positive lead coming out of the rectifier. it only read 11 volts and dropping with bike running. DOES THIS MEAN RECTIFIER IS BAD?

If not, where should I look next?

Thanks,

That diagram looks correct. I'm sure you probably double checked to make sure the bike is wired as you have it drawn, but of course it never hurts to check again. Make sure the there is a ground path using wires and not just the frame or engine as ground for the charging system and battery.

You should definitely get a manual.

How, exactly did you test the output wire from the stator? Where were the meter leads?

Do that test again, but this time, disconnect the green wire at the regulator and jumper it to the black wire at the regulator. This puts full power on the field coil so the alternator should have full magnetism. Don't rev the bike too much if the voltage gets near 14v. Try not to let the voltage go over 15 at all.

I thought I should mention...

The 1976 and 1977 KZ750B1/B2 twins use a regulator unique to only those models. Their charging systems use field coils, but are not wired like other KZ's with field coils.

Most KZ's with field coils use a mechanical regulator where the regulator controls the positive side of the field coil's voltage. The KZ750B1/B2 twins use a solid-state regulator which controls the negative side of the field coil's voltage. The regulator's can't be directly swapped unless the field coil wiring is modified accordingly.

I mention this because some suppliers don' recognize this difference.
I believe OregonMotorcycleParts.com is aware of it, in case you need to source a new regulator.

LoudVX thank you.

I disconnected the green wire coming from the field coils and jumped the wire coming from the coils to the black (negative).

I did not see anything change from normal running. it continued to run in the low 10volt range. I measured both at the battery and at the white (hot) wire coming from the rectifier.

Thoughts?

Reconnect everything.
Start bike.
Measure the voltage on the brown wire going to the field coil. If battery voltage, ok.
Measure the voltage on the green wire going to the field coil. If near zero, ok.
These measurements should be done with the black meter lead attached to the battery's negative post.


Disconnect the 3 yellow (maybe white?) stator wires at the rectifier.
Set the meter to AC voltage.
Start bike.
Connect the meter to two of the three wires from the stator.
Rev the bike to 4000. What do you get?
Connect the meter to each pair of the three wires and repeat the measurement. (Three measurements total.)

FOLLOW UP WITH SOLUTION - big thanks to LOUDHVX!

I know this thread is old, but winter had set in and work went on pause.

This winter I ordered a new combo regulator rectifier. I bought it from oregon motorcycle parts. Tony who builds the parts did a fantastic job, good price, and shipped quickly. very pleasant to work with. Hooked up the new reg/rec and getting just over 13 volts at the battery. So that was it!

If anyone comes across this thread with similar issues here was how i daignosed:
- motorcycle dies after running for about 5 minutes.
- recharged battery and ran the bike again. monitored battery voltage. observed voltage drop at a consistent rate until bike shuts off.
- diagnosed as 'not charging the battery' or 'battery not holding charge'
- checked generator leads by testing resistance and making sure it is in spec. follow user manual.
- checked battery to make sure it was still 'good'
- checked field coil voltage per loudhvx recommendation (in thread)
- thought about everything i had done to the bike while working on it. realized i may have welded some supports on the frame while the electrical system was hooked in. if so, potential to short the regulator and or rectifier.
- after all this figured my next step was to purchase a new reg/rec from oregonmotorcycleparts.com as this was likely the cause.
- installed new regulator/rectifier per wiring diagrams from this site and BOOM! runs like a charm!