Y Technical Document SS201111A Produced by the Electrical Rebuilder’s Association www.electricalrebuilders.org Lucas M50 Starters with Double-Contact Solenoids Farmers are an independent lot. They can fix anything and get by with next to nothing. As a result, there is a lot of vintage equipment out there still being used. Figure 1—Lucas M50 Starter Many older machines have the (in)famous Lucas M50 double-contact type starters. Some examples are Lester # 17646, 17072, 17071, and 17649. Every so often, as I rebuild one, I have to replace a burned field coil (see Figure 2), and it is always the same one of the four. Often, the solenoid coils are burned inside as well. Why? Coil that burns Notice the flag lead on the left. It connects to the M1 stud on the solenoid and goes to the coil that often individually burns. Figure 2—M5S0 Field Coils I’ve found that it’s because only one set of contacts (B+ to M71) closes. It might be that the second contact set (B+ to M3) is burned/fouled, or it may be that the pinion clearance is too tight, preventing the linkage from allowing the 43 contacts to close. Figure 3—Solenoid Terminals Thus, the current flows through only the first set of contacts, and therefore, flows through only the one shunt coil. The starter then cranks only very slowly. The temptation is to keep trying until the tractor starts “Just one more time”’. Also, the high current pull-in coil of the solenoid is not cancelled, because the contacts it is connected to don’t close, causing the solenoid to overheat. (More on this later) This is also hard on the ignition switch. But why use the double-contact solenoid design (also called two-stage) in the first place? The way I figure it, some time ago, some engineers were looking at chewed up starter drive gears and ring gears and decided that something had to be done about that. Did it have to do with the fact that in the instant when the solenoid contacts closed, the armature started turning at the same time, whether or not the drive was already meshed? They probably started to make changes to the linkage, introducing things like mechanical hysteresis, various springs offsetting each other, adjustable clearance, different spline angles on the armature, interrupted linkage, and other things engineers like to write calculus equations about. Then they came up with this wonder: “Soft Start’. 1 Created by Nathan Unger and Wes Grueninger Sr. for the Electrical Rebuilder’s Association. © 2011, Electrical Rebuilders Association—All rights reserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. Y Technical Document SS201111A Produced by the Electrical Rebuilder’s Association www.electricalrebuilders.org If the solenoid could throw out the drive and engage only one of the field coils, then the armature would accelerate only slowly. In the case where, at first attempt, the drive teeth didn’t mesh with the ring gear (called tooth abutment), then the drive would turn only slowly while the internal spring in the plunger would hold pressure on the drive. Once the drive engaged fully, it would allow the other set of contacts to connect the rest of the field coils for full cranking power. Take notice of the plunger (see Figure 4) — it is unique to the double-contact solenoid in that it has a hole in the bottom of it. This is so that it will not push on the second set of moveable contacts during tooth abutment. HOLE FOR MOVEABLE CONTACT ASSY. Figure 4—M50 Plunger Assembly This soft-start process would prevent the armature from being able to turn too fast before full engagement, thus preventing the gears from skipping and grinding. That’s how I figure they came up with this Lucas M50 soft-start set up with double contacts and double field-coil connections. But this design has its weaknesses, too. If the second set of contacts were fouled, or if the pinion clearance were set too tight to allow them to close, then only the single coil would get power. On the other hand, if the pinion clearance were too slack, then the gears would grind anyway. Picture the farmer out there on a cold moming, spraying ether into the air intake, all the while torturing his starter. Then imagine an old battery with greenish terminals and snaggly jumper cables running up somewhere under the hood of his pickup—until finally, the phone rings at your shop… Educate the Customer Advise the farmer to change his ignition switch and replace all bad connections, grounds, and wiring. Also, tell him that the pad area onto which the starter mounts must be clean and free of rust. Although the starter has a ground bolt at the back, many machines have no ground strap. Make sure he has a good, fully charged battery. Low voltage to the S-terminal causes chattering, which can burn out the contacts. In addition, low voltage to the starter causes it to draw excessive current, which also burns out the contacts prematurely. Since these starters are typically found on older machines, you can expect lots of voltage-drop problems, and those can be difficult to find if the farmer doesn’t know how to do voltage-drop testing. There is a great article by Wes Grueninger and Bob Thomas on voltage-drop testing on the ERA website in the Technical Articles Library’. You could even print it up and give a copy to the farmer. Service the Starter You can use a coil from another set (you know, make one good set out of two bad sets), first making sure it is wound in the same direction. Some suppliers may sell that one coil separately. SOLENOID COIL BALANCE TEST HOLD-IN CONNECT TO SOLENOID BODY @] OG PULL-IN CONNECT TO M3 STUD 12V When connected as shown above, the magnetic fields of the pull-in coil and the hold-in coil will cancel each other out. The plunger should slide in and out without any attraction. If you feel any magnetic pull on the plunger, then the coils are out of balance and the solenoid must be replaced. Figure 5—Solenoid Coil Balance Test The solenoid coils have to be checked for balance. Put 12v to M3, ground the case and slide a plunger in to see if it is magnetically attracted (see Figure 5). It shouldn’t be. If it passes that test, then you should still 2 Created by Nathan Unger and Wes Grueninger Sr. for the Electrical Rebuilder’s Association. © 2011, Electrical Rebuilder’s Association—All rights reserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association.