Battery takes charge with charger but Ammeter in negative when running

I have a 2006 Turf Tiger 35 HP BS water cooled been a great machine.
Was mowing and stopped to take a bathroom break. Came back 10 minutes and battery was very week and then would not turn over engine to start. I used a battery charger with a start mode to get running again and finished my mowing job. While I was mowing the Ammeter was reading in negative -8. parked mower and put charger back on battery. Next day took volt meter and checked battery and had strong 12 volts DC. Hopped on mower and it stated right up, but Ammeter is reading negative 8. I am thinking what ever is superposed to generate voltage when running is not working - not my strong suit. Can anyone point me in the right direction? I can certainly replace a part if I knew what to replace.

You start by noting the model & serial numbers off the ID tag .
Write them in the space provided on your owners manual ( no one does ) so they will be handy.
Next go to the scag web site then download the parts list.
From memory the wiring diagram will be at the back of the parts book.
Post the wiring diagram , or the whole book if you don't know how to pull a single page out of a pdf.

The test to see if the alternator is charging the battery is to check the voltage across the battery
1) ignition off
2) ignition on
3) engine on and running full speed
4) engine on running full speed, blades running.

That will confirm what we currently suspect
The next step is to work out why and for that we need the wiring schematic so we can all be singing from the same song sheet.

A bad clutch will give you a neg. reading on the meter when applied. Check yellow fuse holder and fuse as well and after doing the test that was posted and if the motor is charging then look at the clutch prob drawing to many amps for the charging to keep up with and giving you the - reading on the meter.

bertsmobile1 said:

You start by noting the model & serial numbers off the ID tag .
Write them in the space provided on your owners manual ( no one does ) so they will be handy.
Next go to the scag web site then download the parts list.
From memory the wiring diagram will be at the back of the parts book.
Post the wiring diagram , or the whole book if you don't know how to pull a single page out of a pdf.

The test to see if the alternator is charging the battery is to check the voltage across the battery
1) ignition off
2) ignition on
3) engine on and running full speed
4) engine on running full speed, blades running.

That will confirm what we currently suspect
The next step is to work out why and for that we need the wiring schematic so we can all be singing from the same song sheet.

Click to expand...

Thanks for your reply. My model is STT72A-35BV-SS The only tag on the mower I can find is on the mower deck so I fear that is the mower dec model and S/N but this is it - SMST-72A C9000111. The Engine tag is Mod -613177 Type -0114-E1 Code -05122612. Prior to seeing your reply I Found The 20 amp Fuse Blown. (yellow Wires) After replacing the fuse with the engine running the Volts at the Battery could not be read with a DC meter - would no register as if AC voltage. The Ammeter was spiked way high above the +10 highest reading. At times the reading would drop down to +8 but then would go way high again. stayed way high most the time. I came to the conclusion that the Alternator was kicking out plenty of Juice but the voltage regulator must have failed. So I ordered a new voltage regulator from Briggs & Stratton but will not have it for 6 to 10 days.
Then I see your reply to my post today and I put the voltage regulator back in to try and get the reading you asked for but the fuse blew again so I think I am in a wait mode. I have tried to attached the PDF of the Manual with the wire diagram though, but it does not look right. View attachment 2007 STT IPL Manual 06152 Rev 1 Complete.pdf P.S I replaced the PTO clutch 2 months ago.

The most usual reason for an instant fuse blow is the battery hooked up backwards.
The batteries are handed and the terminals are not as well marked as they used to be or different sizes as on a can so it is very easy to just hook the battery cables to the nearest terminal

Also unlike your car when you hook it it back wards nothing happens till you turn the key on whereas on your car you get a healthy "wrong way idiot " spark.

If the fuse blows without the ignition turned on then the main power feed is shorting smaller wire on the power in post on the solenoid.

bertsmobile1 said:

The most usual reason for an instant fuse blow is the battery hooked up backwards.
The batteries are handed and the terminals are not as well marked as they used to be or different sizes as on a can so it is very easy to just hook the battery cables to the nearest terminal

Also unlike your car when you hook it it back wards nothing happens till you turn the key on whereas on your car you get a healthy "wrong way idiot " spark.

If the fuse blows without the ignition turned on then the main power feed is shorting smaller wire on the power in post on the solenoid.

Click to expand...

The battery has not been removed and has been in the mower for better than 5 years. This started when I turned the mower off for a bathroom break and when I came back the mower would not start so I jumped it with a battery charger with a quick start mode. While finishing mowing I noticed the Ammeter was reading -8 (Negative). When done mowing thinking the battery might be bad, bad cell or what ever, I put a charger on it to see if it would take a charge. Was going to buy a battery if it didn't take a charge but next day battery was fully charged and read 12.63 volts in off mode. That"s when I thought to check the fuses. Found the one with the yellow wires blown. After replacing the fuse I started the mower and the reading on the Ammeter was sky high.
And took volt reading at battery but meter did not like what it was seeing as if AC voltage. I of coarse was taking my reading in a DC mode. I had read that the alternator puts out AC voltage but is converted to DC by the voltage regulator. So i assumed the voltage regulator must have gone bad and I was getting AC voltage at the battery. So I quite and ordered a voltage regulator from Briggs & Stratton. 6-10 day delivery. That's when I went back to the lawn world forum and saw your reply. So I went back to the mower to try and get all the voltage reading you requested in your reply but the fuse blew again and I decided I would wait for the new voltage regulator. If the problem is not the voltage regulator I am really lost.

Switch your multi meter to AC then take a reading from the odd coloured wire on the rectiifier to groud or battery neg.
If you get an AC reading then the rectifier is in fact bad.
The two wires that are the same colour should read double battery voltage ( AC ) between them , or a bit higher and battery voltage between either on and ground.

bertsmobile1 said:

Switch your multi meter to AC then take a reading from the odd coloured wire on the rectiifier to groud or battery neg.
If you get an AC reading then the rectifier is in fact bad.
The two wires that are the same colour should read double battery voltage ( AC ) between them , or a bit higher and battery voltage between either on and ground.

Click to expand...

This is what I got. 1.35 volts AC. Not sure i had a good connection at odd colored wire (red) (the other two are yellow) so I also took reading across battery in AC mode and got 56 volts AC. Blew two more fuses doing this. Ammeter reading again was normal +5 volts, but also Jumped way high, and then zero which is what I get when fuse blows.