# Charging System trouble shooting '77 ST18



## chuckm51 (Dec 10, 2013)

So, I am brand new here, but I have been shade-tree maintaining and modifying the Sears/Roper 70's tractor for 15 years. For years, I helped a friend keep his '74 ss16 running more or less.
But back in April I bought a '77 ST18, with the original Onan engine. Somewhat of a pig in a poke. But with lots of help and advice and courage, I have it running pretty well. Carb had been full of water that evaporated over time, so new carb kit. 
I repainted it recently, and partially disassembled it to do so. Ever since then, it does not want to charge the battery. Without lights on it seems to keep the charge up, but with headlights on the battery discharges in under an hour.

I have seen mention several times of a "fuse" to check somewhere, but my schematic doesn't show one I don't think, and I don't really see one.
The factory manual for the Onan engine gives some diagnostic info on the charging system, but it assumes a knowledge set I don't have. The factory owner's manual says to check the fuse, too. 
Is there a primer somewhere I can d/l and study?

Thanks in advance.

chuck in texas


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## film495 (Nov 1, 2013)

don't know that machine, but could it be the battery is on its way out? it seems weird that it will charge the battery and run OK, but the issue is draining the battery when running, but only when the lights are on? Is this a 12 volt system? If the lights are similar to what I have they total about 70Watts which I think draws 4 or 5 amps at 12Volts. My first test woult be to make sure I'm running a known good battery. You could then just put a volt meter on the battery terminals under a few test cases - engine not running. engine running. engine running with lights on - and the results could tell you a lot. You should have more volts when running if the system is charging the battery. I'm not sure what the reading would do really if the lights are on, but it would be fun to know. Possibly someone with that machine can point you right to the fuse, but it would be curious if the machine can charge itself, just not with the lights on. I'd think a fuse would break the circuit and it would let the juice through or it woudn't.


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## Bill Kapaun (May 8, 2007)

ST 18? I thought ST's were Tecumsehs??

What's the Sears 917.xxxxx number? Maybe there's a schematic floating around that could be useful for troubleshooting??


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## chuckm51 (Dec 10, 2013)

It for sure is original with an Onan. I have the BG engine manual and the owner manual for this guy. It is 917.25191.
Gonna try some more testing today. When running in neutral, lights off, my meter shows 12.6 d.c. volts. With lights on, it's more like 10 volts.


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## Bill Kapaun (May 8, 2007)

There are 2 things that draw power-
Lights & ignition, since this is a breaker point system. (oops- and lift system)
You might take readings-
Key OFF
Key ON
Key ON w/lights
and compare.

IF you have the engine manual, there should be an electrical trouble shooting section that gives the correct *AC* output value from the alternator.
Disconnect the wiring plug from that and measure your *AC* output voltage with the engine running at high speed. The "third" wire is the *DC output* to the key switch.
ONAN's have different minimum voltages, depending on the specific charging system.
Off the top of my head, I think the lower ones are *around 40 VAC*.

EDIT-
My schematic shows a 30A fuse between Battery + and the B terminal of the key switch.
Also a 15A fuse between the lights and the key switch L terminal.


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## film495 (Nov 1, 2013)

What does the battery read when it is not running? If it is 12.6 you are not charging your battery when running at all. Is there an ammeter in the system? Even if you find the lead and a blown fuse one has to ask, why did the fuse blow? I'd certainly do a quick sanity check to look for wires that don't go anywhere, possibly a connection came loose or a previous owner just put a wire nut on the lead and never fixed it the right way. My Ariens was like this when I was figuring out the wiring, eventually I found the wire from the generator was just sitting there disconnected. All I had to do was reconnect it.


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## chuckm51 (Dec 10, 2013)

Hi,

Thanks for all the thoughtful advice and comments. I was showing 12.6 volts at the battery with the ignition off. About the same with the motor running and no load. When I turned the head lights on the volts dropped to 10.5 or so. My BG Onan engine manual says to check the a/c volts out of the generator/stator as measured at the voltage regulator, and the d/c volts out of the voltage regulator/rectifier.
I got a fairly steady 28 a/c volts in to the regulator, as specified in the manual. But the d/c volts out were totally scattered on my digital meter, from 0 to 18, jumping every nanosecond. Removed it and cleaned up the metal contacts without success. It is supposed to generate a steady 13.6 d/c volts more or less. Consensus is a bad voltage regulator. Found one on ebay for $28. Arrives in a couple days. We'll see.


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## Bill Kapaun (May 8, 2007)

You did switch the meter to DC for the output test?
Easy to forget when measuring both AC & DC voltages.


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## chuckm51 (Dec 10, 2013)

I did take care to use the right scale on the multimeter. The reading was so unexpected I checked everything. I did also note that if I put the meter on the air cleaner cover while testing, the meter went crazy...proximity to the ground current I guess. 
Waiting on a new regulator/rectifier. Also a new drive belt and seat cover.

Thanks for all the good advice.

chuck


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## chuckm51 (Dec 10, 2013)

Louise before and almost done.
Just ordered a seat cover to recover the original seat. Supposed to use the original trim to attach it. They seem totally cooperative. We'll see. Got a notice it is in the mail.

hoping the new voltage regulator/rectifier will solve charging issues. 
driving last night, heard unusual slapping sound underneath. Drive belt has big crack almost all the way through. Ordered one yesterday too.



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