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old_nodaker

Onan starter problem

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old_nodaker
Another problem with my new to me 9020. When I turn the switch to start, I get a 20 amp draw. No click or anything. I would guess it's not the low voltage thing. Any advice or opinions? (I thought "no problem", I'll just grab the one off this engine I took out of the 4041. 2 hours later I finally get the starters out and discover they're differentC)

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D-17_Dave
"Different"?? There were a few different starters used aftermarket. Will the other starter bolt up? Does the bendix engage correctly and are the teeth spaced correctly for proper meshing with the flywheel? The "other" starters used on these engines sometimes did not work correctly and did more damage than good. As for the amp load your getting. You can trace the load by unplugging the connections in the start circuit one by one until the short is removed. The problem could be a wire rubbing against the metal frame somewhere or a switch or safety interlock switch shorted internally or the starter is worn out.

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Al
Hi, The 20 amp load you are seeing may be because of low voltage to the starter. If you have a solenoid actuated starter and sometimes it starts and sometimes it don't, may start cold and not warm or visa-versa. May start and the next time you have to hit the switch 2 or 3 times for it to kick in, the problem may not be the starter. These starters have 2 windings in the solenoid a pull in winding and a hold in winding. Both are connected to the small energize (start) terminal of the solenoid. The pull in winding is wound of larger wire and the other end is connected to the large terminal the starter motor connects to. This winding draws from 15 to 30 amps depending on the solenoid. The hold in winding is connected from the small energize terminal to ground and normally pulls from 10 to 20 amps. When you go to start, there is a current draw of around 40 amps until the starter is engaged when it drops to 10 to 20 amps. (Not the current the starter motor draws) Often the pull in winding is wound closer to the nose of the starter and the hold in winding may be wound closer to the solenoid cap. The reason for the 2 windings is that the hold in winding doesn't have enough power to pull the plunger in and engage the drive by it self. If it were wound to, it would have to draw way too much current. The plunger in the solenoid is the part that does the mechanical work. It is connected to the starter drive by a fork and at rest held out of the solenoid by a spring. When the starter is engaged the plunger is pulled into the solenoid and it pulls on the fork, that pushes the drive into the flywheel. When you turn on the switch to start, the energize terminal of the starter draws about 40 to 45 amps. The current path is constant through the hold in winding to ground. It also flows through the hold in winding to the starter motor. This performs 2 functions. The starter motor has such low resistance that it looks like a direct short to the pull in winding. This actually starts to magnatize the armature and the fields in the starter motor and start it to to turn slowly since the pull in winding limits the amount of current to the motor to 15 to 30 amps. This provides what is referred to as a soft start to the motor. The solenoid plunger is pulled in and as it is pulled in if the gear teeth on the drive don't mesh, the fork pushes the drive forward and compresses a spring ahead of the fork. This pushes the drive against the flywheel teeth. Since the motor is barely turning with the limited current through the pull in winding as it turns, the teeth that are "butted together" slip and the spring in the drive pushes the drive the rest of the way into the flywheel to mesh the gear. (Note: most modern starters have a spiral spline of helix the drive slide on to facilitate this) As this is happening the plunger in the solenoid comes on back toward the cap. Under the cap are the 2 large terminals, one that the battery connects to and the other that the motor connects to. Also under the cap is a round copper disk, (usually silver on 24 Volt units") This disk has a rod mounted to it (through a plastic insulator) with a spring on the end to push it away from the cap on the other side of the disk is another spring that is much stronger than the spring on the end. The rod goes through a hole in the end of the solenoid into the area where the plunger comes back. When the plunger is drawn back by the solenoid windings and the shift fork it is connected to engages the drive, it also hits the rod the copper disk is mounted on. This rod pushes the disk across the 2 large terminals in the cap. The rod pushes on the stronger spring under the disk that pushes the disk against the contacts. keeping the disk from breaking the plastic cap. This provides the direct connection from the battery to the starter motor. When this happens, the pull in winding also gets 12 volts applied as one end is connected to the starter motor terminal. With 12 volts applied to the pull in winding from the energize terminal and the other end getting 12 volts from the starter motor terminal, NO current flows through this winding any more. The hold in winding still has 12 volts applied through the energize terminal through it to ground. The plunger is already drawn into the magnetic field of the hold in winding which is strong enough to hold it in while drawing only 10 to 20 amps. The starter motor will continue to run until the voltage is removed from the energize terminal. After the key is removed from the start position, the voltage is removed from the energize terminal of the solenoid. There is a spring behind the plunger in the solenoid pushes it out of the solenoid to its resting position. This action pulls the drive out of the flywheel and also lets the copper disk disconnect the battery from the motor. The small spring on the cap side of the contact disk pushes it away from the large termianls. Everything goes back to the parked mode. One thing I might add, in the drive is an overrunning sprag clutch. This allows the engine to start and if the starter is engaged the drive gear can turn and not turn the armature. If this clutch fails when the engine starts and the clutch locks up it tries to turn the armature about 20 or more times the engine speed and the armature explodes from centrifugal force. We see this on large diesel starters when the starter switch fails and keeps the starter engaged. Now to finally answer what I said at the beginning. When the key goes to start, the solenoid needs about 10 volts with about 40 amps available to pull it in and start the pull in sequence. As the wiring gets older and switch contacts wear, strands of wire break from vibration etc. the voltage may drop to 9 volts or below. Often this is not enough to pull in the solenoid and you see the ammeter pegged and then starter not engaging. This because the wiring and switch is no longer capable of providing the power to reliably pull in the solenoid. You can determine if this is the problem if you can get to the solenoid. If you can jump from the battery terminal to the energize terminal of the starter and the unit starts, the problem is probably not in the starter. You can also do this by connecting a wire to the energize terminal and then touch it to the positive battery terminal. If the unit starts consistently, the problem is probably not the starter, but a switch and or wiring issue. We make a kit to fix these problems and have sold them to a number of members here to fix the 720 family starting issues. With our kit the ignition switch only has to provide about 1/10 th of an amp to operate the starter. It is much easier to install (4 wires) than to try to fix all of the wiring. Plus it works even better than the original wiring. It will not fix the problem if it is in the starter. Sorry this is so long, but it is hard for me to address in a brief post. Al Eden

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old_nodaker
Wow, everything I wanted to know about a starter but was afraid to ask...I've worked on starters in my youth, never realized what a complicated mechanism they were. Thanks for the explanation. The first time this happened to me, (after the purchase, still on the trailer) I tried the jump to the energize terminal. Assuming I was to the correct terminal, what I got was the starter would spin up, but the solenoid didn't engage. However, then the key switch worked, maybe just a coincidence though. It then worked okay 3 or 4 times and acted up again at which point I pulled it out. I have the starter out now so will re-test, but does that give you any more clues? Dave - the mountings are completely different, one mounts directly to the housing, the other mounts onto a spacer plate which mounts onto the housing. On that one I had to pull the flywheel to get at the last (of 3) bolt, the other one the bolts (2)were all accessible from the back side. I'll try to get a picture of the two side by side.

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bobjohn
Without giving away Al's method of solving the 'bad wiring' problem with older tractors, which is quite accurate. In brief, the solution is to reduce the resistance to the starter switch. Most tractors have several feet of wire from the battery to the starter switch. The solution is to shorten this distance. Placing a 12VDC relay near the starter and activating it with the start switch on the dash panel of the tractor will bypass the long wiring from the switch to the starter. The relay will use a few milliampers of current compared to several amperes of the starter switch, so the damaged wire is no longer a problem. Al's solution is industry standard and works great. bob

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old_nodaker
I had an old relay kicking around so not wanting to spend the money on an engine I'll probably replace next spring I wired it in. Not nearly as elegant as Al's, but hopefully it will get me by for now. Seems to have solved the problem by the way. (Al, standby, I'll be having a bunch of questions about re powers.)

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