Bosch Starter Solenoid Rebuild

Inside a Dual Coil, Pull-in and Hold-In Starter Solenoid

Copyright 2012 by Morris Rosenthal All Rights Reserved

The Omni Project

If It Jams Home

My rebuilt Bosch starter for the Omni from Rock Auto failed 90 days after I bought it, but as a close-out, it only had a 30 day warranty so I was out of luck. With the solenoid removed, the starter would spin if powered directly, whereas all the solenoid would do when I hit it with power was spark. My Dodge Omni shop manual gave a test for the three terminal solenoid, checking for continuity between the solenoid terminal (the thin wire that runs to the starter relay on the strut tower near the battery) and ground (the case), and then checking continuity between the solenoid terminal and the starter terminal (as opposed to the battery terminal). The solenoid passed the test, with low resistance for each, but I was convinced it had to be bad so I took it apart. I had tried ordering a replacement Bosch solenoid, but the one they sent had two holes rather than three, the type for the original Chrysler Nippondenso starter (which lasted 26 years and 280,000 miles), so I gave up and bought a rebuilt starter from Napa to get the car running again. Then I took the solenoid apart to figure out how it was supposed to work.The picture to the right shows the solenoid can after I peeled it open. I just used a thin blade screwdriver to worry the crimped edge until it was free, and aside from stabbing myself in the hand a few times, it wasn't that bad. It would have been better if I had the sense to wear a glove on my holding hand from the start. Peeled can of Bosch starter solenoid
Dissassembled Bosch Starter Solenoid The picture to the left is an exploded view of the solenoid parts. Clockwise from the snap ring at 11:00, we have the iron plunger and spring at 1:30, which gets sucked in by the magnetic field when the solenoid is powered and serves the dual function of throwing out the starter gear to engage the flywheel and closing the switch that connects the starter windings to the battery. At 3:00 is the can with the three threaded holes for attaching to the starter, and a 6:30 is the solenoid coil with the two windings, that will be clear later. Note the iron plate with the copper wires coming through. On initial disassembly, we pried it out of the can, leaving the coil behind and breaking the copper wire welded to the iron plate, which also serves as a magnetic pole. At 7:30 is the bottom view of the lid with the three terminals, showing the large copper contact pads, and just above it at 9:00 is the switch with a copper contact plate and spring. Below we show a close-up of the copper plate contact for the switch and the two copper pads on the heavy solenoid terminals. The spring on the back of the contact plate sits in the hole between the two contact pads, and the brass switch plunger goes right into the middle of the coils and sticks out into the the interior cylinder where the iron plunger is pulled in.
When the iron plunger is pulled in by the magnetic force of the current in the windings, it depresses the brass plunger and the copper plate down onto the contact pads, which completes the circuit for the heavy battery power lead to flow current through the starter windings. Starter solenoid are actually designed with two copper coils. The pull-in winding is the inner low resistance coil of heavy magnetic wire to draw the maximum current for a strong magnetic force to pull the iron plunger in against the spring force even as the other end of the plunger is throwing out the starter gear (Bendix) to engage in the flywheel. The outer hold-in winding is a thinner wire, higher resistance, that need only keep the magnetic field energized to hold the plunger in place during starting. The picture below shows the nylon bobbin with the brass core back in the can. You should be able to see that there are two different gauges of copper magnet wire used, with the heavy wire to the inside and the thinner wire to the outside. The picture to the lower right shows the broken wire (in the right-hand slot) where the end of the hold-in coil is welded to the iron plate, which contacts the case for ground. The thicker pull-in coil wire is connected (through the lid) to the starter terminal. We soldered the broken wire to the piece still attached to the iron plate. Switch contact for starter power from battery
Windings for pull-in and plunger contact hold Hold-in winding soldered to ground (broken)
The two windings are connected to the solenoid terminal on the other side, soldered together. So when you turn the ignition key (or short the solenoid terminal of the starter relay to the battery), the current flows through both windings. Since the two windings are in parallel, the low resistance pull-in winding will draw most of the power, since it is grounded through the starter windings, which are also low resistance. This pulls in the plunger. When the plunger closes the contact between the battery terminal and the starter terminal, there is no longer a potential difference between the solenoid terminal and the starter terminal (both are at battery potential) so current stops flowing through the low resistance pull-in winding. Current continues to flow through the higher resistance hold-in winding, but at a lower level, leaving the maximum amount of current available to flow through the starter windings and crank the starter. It's an elegant design because it also won't attempt to crank the starter at high speed unless the gear is engaged in the flywheel. The picture below shows the inside of the solenoid housing on the starter, with the lever that turns into a fork on the other end to throw the starter gear into the flywheel. The picture to the bottom right shows the iron plunger with its rectangular cut out end that grapples the lever in the housing. The can and phenolic lid are factory marked for alignment. Coil leads through solenoid contact cover
Starter solenoid housing with Bendix throw-out lever Solenoid plunger with lever opening

If It Jams Home | The Omni Project | Contact