[khe]

The battery light on my 1988 318 will come on when the PTO is engaged and the headlights are on. If I turn off either the headlights or the PTO, the battery light goes off. The tractor always starts up and the battery does not get discharged.

I have the 1295 bulbs installed in the headlight sockets but the problem was there before I installed the 1295 bulbs.

The engine is the B suffix model. Below is some data I obtained:

Battery voltage with engine off: 12.25V
Battery voltage with PTO engaged and headlights on: 13.5V
Regulator output: 14V
Stator output: 54VAC at full throttle.
Stator resistance: 0.2 ohms
Resistance of PTO field coil: 5.3 ohms with engine hot.

I believe the stator and regulator are OK as well as the battery and am now leaning toward the PTO field coil.

I can't find the resistance spec. for the field coil in the technical manual - all it says to do is apply 12V to the coil and it should hold a bolt....

Thanks!

 

[knottyrope]

if you have 13.5 with both PTO and head lights on, then the sensing unit is at fault. You have plenty of current going to the battery if it is maintaining 13.5 volts.

 

[dean_mink]

Your battery is starting to go.
Battery voltage with engine off shouldn't go below 12.8.
PTO coil should be around 4.6 ohms but yours is fine drawing 2.5 amps.
Your stator and regulator are good but your battery is not doing well.

 

[khe]

The Onan engine service manual states the battery voltage with the engine not running should be 12-13 volts. I checked it just now and it was 12.5 volts.

The battery cranks the tractor over fine - if it were starting to puke, wouldn't it have trouble turning over?

Just for the heck of it, I measured the battery in both of my cars - one was 12.5 volts and the other was 12.2 volts without the engines running.

I'm going to have the battery load tested and see what happens.

 

[vinnie110]

A quick battery check is to measure terminal voltage while cranking the engine. Anything less than 10.5 volts means the battery needs to be replaced.

 

[dean_mink]

Thats odd, I just checked both my vehicles and they're 12.84-12.90, also checked my Dodge Charger that has been sitting for 3 weeks and its 12.82.

 

[kencombs]

With the info given, I would suspect both the sensing unit and your meter.
Wouldn't be the first time a meter was out of spec.

 

[khe]

I had the battery load tested Saturday and it was OK. The battery voltage was 12.75 so it looks like my meter is a little off calibration.

I am now thinking the PTO clutch coil is developing a short when it heats up. I am going to inspect the clutch coil to see if it shows signs of overheating.

 

[khe]

I removed the clutch last night and found the problem. The Coil has overheated and is blistered and cracked. I applied 12v to the coil and after a coupe of minutes, it went from drawing 6.5 amps to well over 10 amps. I then measured the resistance of the coil and it was 0.8 ohms.

The shop manual states to replace the rotor and armature if they are worn or grooved. The armature has grooves that are concentric with the circumference of the armature. The photo in the shop manual looks the same as mine so I believe I just need a new field coil.

 

[khe]

I bought a new field coil and installed it. I noticed the bearing seemed dry so I removed the snap ring and the bearing seal to find traces of dried grease. I repacked the bearing with bearing grease, installed the seal and the snap ring and it is like a new again. I was able to run the mower and the headlights and the battery light never went on. It's fixed!

The resistance of the new coil was 4 ohms. That may be helpful for someone troubleshooting the PTO system.