1981 GS750L Charging Problem I did read the Posting Too

[cecchini]

Hello All,

My good old 1981 GS750L won't charge the battery. I have read the charging post listed in this forum and it seems to be in conflict with what my bike says and the manual for the 78/79 GS1000. My measurements are a whole lot more similar to the clymer manual.

I have a DVM. Here is what I have found.

Battery sitting there 12.85 volts. Ignition on, bike not started 11.99V. Engine running 11.99V no increase in voltage with increased rpm. Yep I have a charging problem.

I do not have a voltage drop from the battery connector to the regulator connector and do not have voltage across the battery ground and the black wire.

Resistance between between any of the stator legs is right around .7 ohms measuring any combo.

If I check resistance from any of the stator legs to the engine and they are all open.

Here is where the forum seems to stray. Change my meter to diode check and measure voltage? A diode check on a meter just gives it a little more voltage to push the diode nexus, but it is essentially still an ohm meter, so how the heck would I measure voltage? HOWEVER if I ohm out the regulator/rectifier and it ohms correctly, like the manual. around 1500 ohms in one direction and open in the other direction for each of the leads relative to ground and vice versa when relative to the red lead. Open in either direction between ground and red lead. Perhaps the forum nots meant to say 1500 ohms or 1.5kohms?

I check the AC output of the stator and I get 24 to 25 volts between any leg with it at 3000 to 5000 rpm. The manual says when pulling the blue/white or yellow lead from the reg/rectifier and measuring at the battery I should see 16VDC because the regulator is bypassed. I don't, just 11.9V

Conflict - the forum guide says I should read over 60VAC between any stator lead at 5000rpm. I get 24 to 25VAC. The 60VAC seems a might high to rectify and regulate down to 14 to 15 volts under load.

Checked the wire and connectors. No frays and all are really clean. Checked the ground to engine it is good also.

Given that my meter is working.

1) Why the big voltage conflict between the manual and the forum posting?
2) With the above information, what is bad? the rectifier/regulator box or the stator?

The guy who wants to buy the right part the first time.

Dick Cecchini

 

[denydog]

Conflict - the forum guide says I should read over 60VAC between any stator lead at 5000rpm. I get 24 to 25VAC. The 60VAC seems a might high to rectify and regulate down to 14 to 15 volts under load.

My stator reads 80 vac between any of the three leads at 5,000 rpm, which is what's called for in my Suzuki (GS1100) shop manual. Sorry, your stator does seem a little suspect.

Here is where the forum seems to stray. Change my meter to diode check and measure voltage?

On this portion of the test you are correct that you are checking continuity through the diodes of the rectifier (both ways). The yellow stator wires are the input, and the red wire is the output. Maybe the V is an error, I don't know off hand. My shop manual shows me a more detailed chart to check each diode individually, but it sounds like you have done the check correctly.

 

[Steve]

OK, I will try to clear up a couple of things that I see might be confusing you.

Diode test:
Yes, in diode test mode, the DVM is actually reading the voltage that it takes to get across the diode gate. Remove all wires from the Regulator/Rectifier (R/R). Clip your black meter lead to the black R/R wire, touch the red meter lead to each of the three input wires. You should get an open or infinite reading. Clip the red meter wire to the black R/R lead, do the same tests. You should get about .5 volts. (NOTE: Some meters read in millivolts, so your reading would be about 500.) Now clip the red meter lead to the red R/R wire and touch the black meter lead to the three inputs. Again, you should get open readings. Clip the black meter lead to the red R/R wire, touch the red meter lead to the input wires, you should get about .5 volts again. Note that this is 12 separate readings. If any one of them is not right, your R/R is probably toast.

Stator output:
You stated that the resistance on the three phases was all about .7 ohms, thats good. :-D
You also stated that the resistance from each lead to ground was open, that's also good. :-D
Now let's make sure you are measuring the AC output correctly. Make sure all three stator wires are disconnected from the R/R. With the engine running at 4-5000 rpm (sounds frighteningly fast, but don't worry about it), check the AC output between the three possible pairs. Think of the wires as being labeled A, B, and C. It does not matter which one is which. Measure A to B, B to C, and C to A. This is where you should have 60+ volts AC.

I am not sure when the change happened, but some of the older bikes, and the '78 and '79 might be included, had separate rectifiers and regulators. Their charging specs might also be different, but the steps I outlined above for you will deifinitely work on your '81. That might be why the manual that you have is confusing you.

Try these tests and let us know how they turned out.


.

 

[Boondocks]

...Here is where the forum seems to stray. Change my meter to diode check and measure voltage? A diode check on a meter just gives it a little more voltage to push the diode nexus, but it is essentially still an ohm meter, so how the heck would I measure voltage? ...

The point is that a good R/R should not show voltages in the indicated ranges, or otherwise the diodes are faulty. The part showing a reading of 1.5V or higher on all three tests is misleading, because that is a reading of a specific DVM. The reading of 1.5V or more is actually OL (over limit), or an open circuit. This may be indicated differently depending on the DVM.

I check the AC output of the stator and I get 24 to 25 volts between any leg with it at 3000 to 5000 rpm. The manual says when pulling the blue/white or yellow lead from the reg/rectifier and measuring at the battery I should see 16VDC because the regulator is bypassed. I don't, just 11.9V

Conflict - the forum guide says I should read over 60VAC between any stator lead at 5000rpm. I get 24 to 25VAC. The 60VAC seems a might high to rectify and regulate down to 14 to 15 volts under load.

Checked the wire and connectors. No frays and all are really clean. Checked the ground to engine it is good also.

Given that my meter is working.

1) Why the big voltage conflict between the manual and the forum posting?
2) With the above information, what is bad? the rectifier/regulator box or the stator?

The guy who wants to buy the right part the first time.

Dick Cecchini

1. There is no big voltage conflict.

2. With the stator wires disconnected and the engine running at 5,000 rpm, if you get a reading of 24-25V between legs then the stator is defective. 60VAC is not high at all. My bike's stator produces 80VAC at 5,000 rpm. Your Clymer manual may show a test voltage as high as 80VAC.

I don't think that the static bench test measurements are particulary reliable, and may fail under load even if they pass a static test. The dynamic tests are easy to perform and are the "proof of the pudding":
Disconnect the stator and test it at 5,000 rpm for the ACV indicated in the Clymer manual. If this test is passed, the connections are clean and correct and the bike still won't charge between about 13.9-14.5V, the R/R is bad.

 

[cecchini]

Alrighty then.

I have 24VAC coming out of any pair on the stator. All agreed that it should be over 60VAC coming out. I will shop for a stator.

Thank you all.

Dick