Puzzler - Startdr running backwards

Today’s solution had a battery charged backwards (another discussion topic). It was suggesred that a starter would run backwards if the power polarity were reversed. As I recall, starters are series would motors, so both the field and rotor would reverse, resulting in the motor running the same direction. Series motors are also commonly used in vacuum cleaners, which run on AC, where the polarity is reversing continuously. If the motor had a permanent magnet field instead of an electomagnet field (these motors are commonly used in toys), the direction would reverse with reverse polarity. However, series motors have higher low speed torque, so I suspect starters are series wound. Has this changed?


I purchased a 1948 Dodge back in 1977 from a woman in her 80s that had given up driving. The ammeter gauge read backward and the radio didn’t play. When I checked it out, I found the polarity had been reversed. Chrysler products in those days had the positive post grounded. However, the car did start, so I am certain that the starter motor had to be series wound. I removed the battery and hooked a load to it to completely discharge it. I then brought it up slowly on a trickle charger. When I installed it in the car, I went through the procedure to re-polarize the generator. When I started the car, the ammeter read correctly and the radio played. I know from experience that the starter turns the same way if the polarity is reversed in old cars.

Interesting that the rafio did not work. An original radio would have been vacuum tube with an electromechanical vibrator to convert DC to AC to drive a transformer to generate the high voltage plate supply. This should be polarity indendent. However, some radios used another set of contacts on the vibrator to rectify the high voltage secondary voltage (similar to synchronous rectification in today’s high efficiency switching power supplies) instead of a separate rectifier (in those days, a selenium rectifier or another tube). If the radio used the vibrator for rectification, reversing the low voltage into the radio would cause a negative plate voltage, and the radio would not work! An aftermarket transistor radio would have just emitted smoke when hit with reverse polarity.

Thanks for the comment!


I have old tractors that have been converted from positive to negative ground, and the original starter still works.
I also belong to several other truck and tractor related forums. It has been stated numerous times that the starter does not care which battery terminal is grounded.
Hooking the battery up backwards might fry something else, or even make sparks, but the starter should still work.

@Hallikainen–this was the original radio and it had the vibrator and vacuum tubes. This was back in 1977 and it is possible that the radio might have worked. When I noticed right away that the ammeter worked backwards, I knew what had happened and I may not have tried to radio until I set things straight.
I do remember changing vibrators in car radios. The rapidly pulsating D.C. could be stepped up with a transformer to provide the plate voltage.
My parents had a 1960 Rambler and that radio had 12 volts on the plates of the tubes. These were a rather recent development in tubes and the radio did not need a vibrator or transformer. However, the transistor was so much better, that these tube type radios were phased out rather quickly.

After hearing this puzzler last week, I thought about sending a response warning that reversing polarity to the starter motor would be a b-o-o-o-g-u-s answer, for several reasons. Harold’s right; most starters in service today are series wound and will turn the same direction, regardless of polarity. But newer cars, especially the small ones, are using permanent-magnet starters nowadays to save weight. Permanent magnet DC motors do switch directions if the polarity is switched. And, according to their internet websites, some marine suppliers are selling this new style starter as a cheaper replacement for common series-wound starters. But all the models I saw used solenoid-operated pinion gears, so the pinion gear would have engaged the flywheel (solenoids don’t change direction if polarity is switched). The engine wouldn’t have turned because the pinion has an overunning clutch, but that clutch would have screeched to high heaven.

I worked on small aircraft a number of years; some of the small twins use engines that turn in opposited directions. It’s a safety feature that takes a while to explain. Both engines used the same part-number starter motors, but had different “Bendix” (inertial pinion drive) part numbers. If only the motor was being replaced, the mechanic had to be certain it would turn in the correct direction; if it needed to be changed, he would rotate the back-plate 90 degrees. That would change the brushes’ position on the commutator, reversing polarity only for the armature. Switching either the armature or field polarity will change a series-wound motor’s direction, but switching both at the same time (hooking up backwards, for example) will not change the direction.

Sometimes, a single-engine aircraft owner would would drag his airplane up to our door, complaining that the starter motor would spin, but not engage the engine. We could look at the starter and tell, in most cases, that the owner had taken it to a local auto electrical shop and had it repaired; that glossy, sloppy black spray-paint job on the starter was a dead give-away. The motors were designed by Prestolite, so were the same type as lots of farm equipment, so the shop wouldn’t have necessarily known it was an aircraft starter. But even so, they should have been careful to mark the backplate position before taking the starter apart. The aircraft owner, who had removed and reinstalled the starter himself, not knowing for sure what he was doing, really didn’t have an excuse either.

We would rotate the backplate (usually could do that without removing the starter), look over the installation and fix any other screw-ups, and give the owner a stern lecture that he must call one of us to check his work in the future, or we would turn him in to the feds. I don’t know if the lecture worked 100% of the time.

As Rich’s Toys noted, if the battery was, indeed, charged backwards, everything on electronic on the boat was at risk; the alternator diodes, for sure, would have smoked. If the bilge hadn’t been thoroughly cleared (wonder which direction the bilge fan turned), that alone would have been interesting and eventful. The puzzler submitter apparently didn’t mention how much this cost the marina, but if it really happened, it wasn’t cheap.

I doubt, however, that the battery would have been discharged deeply enough to allow reverse charging. I haven’t seen a battery charger in thirty years without reverse-charging protection; if the boat only had the navigation lights left on overnight and the battery was in decent shape, it would still register 5-8 volts the next day. No “quick-charge” battery charger would charge it in reverse. If the battery was, indeed, stone dead. sulfation would limit its charge rate on a constant-voltage charger; recharging (in reverse) would take 12 hours or more, I bet. It would likley take a constant-current pulse charger to get it started charging.

Telling the whole story on a radio show would have taken a long time; but shortening it up will leave most folks with the mistaken idea that reversing polarity will make a starter motor turn backwards when, in most cases, it won’t.


I believe @dagosa had mentioned once that twin props on a boat turn in opposite directions so that you don’t get the torsion steer effect. I imagine it would be the same in a plane making it harder to stay straight and level with two engines.

I think you’re right on the counter rotating props on boats, but heard it called the torque effect. On twin prop airplanes, it has more to do with the minimum safe airspeed with one engine out. If you have both turning the same direction (clockwise in the US, viewed from behind) and the right engine fails, the plane is able to fly a bit slower at a higher angle of attack than if the left engine fails. In either case, the aircraft will roll inverted if it gets too slow. It’s related to torque, too. It’s because a descending prop blade produces more thrust than the other blade at high climb angles. Since the right side prop descending blade is further from the aircraft center of gravity, it will apply more torque, or yaw, than the left one will under the same conditions. It will require more rudder deflection to counteract. At low airspeed, the rudder is less effective, so it loses the battle if the plane gets too slow. But it can go slower on the left engine than the right.

I have done my best to avoid testing this principle.

If the engines turn in opposite directions and the descending blades are inboard, then it doesn’t matter which one fails.

At cruise, straight and level, both engines running, it makes no difference. That’s what I was taught, anyway.


Generators (not solid state rectifier alternators) could be polarized to charge either way, I have seen starter motors that do not seem to care about the polarity, or voltage for that matter (consider the series-parallel switch for two batteries), however, I have a B.S. in chemistry, although many years ago, but I am fairly well convinced that regardless of the charge level, the battery polarity cannot be reversed because the electricity is produced by a chemical reaction. Charging a battery simply restores the acid strength and plates lead back on the plates. I do not know about NiCad or the calcium technology batteries.

Now, what is really interesting is that last week when this puzzler was aired, I happened to be experiencing the exact problem with a 2001 Nissan Altima starter motor. It is my daughter’s car so I do not drive it. She came in and said her car would not start, the battery was dead. So I got my Solar ES-6000 (brag) and proceeded to jump her engine off. Well, to my confusion and surprise, the starter spun over but did not engage with the ring gear. In theory, short of a mechanical failure of be bell-crank fork, the starter motor should not run without engaging the ring gear - although, once I did have a car with a ring gear missing some teeth. Fortunately, it had a manual transmission, so on those rare occasions the missing teeth lined up with the starter, I just put it in high gear and rocked the car to kick the motor over enough to move the missing teeth away from the starter. So, I could not see how this was possible. Incidentally, the bell-crank fork unit is nylon in this starter instead of the usual steel. Still, when I took the starter apart, the crank was in place and showed no signs of failure or dislocation. I don’t think it was the culprit. Also, the planetary outer gear assembly/housing is also nylon but all of the teeth looked fine as well as the planetary gears themselves.

I removed the starter (a remanufactured unit); bench checked it and it seemed to work fine. The motor spun and the Bendix gear (what us old farts call it) kicked out to the end of its travel. The examination of the ring gear showed it to be essentially perfect. I disassembled the starter motor and could not see anything wrong with it. For what it is worth, this is a planetary gear reduction starter motor. I cleaned everything and re-lubricated all of the bearings and the Bendix drive shaft and reassembled it. It tested fine again on the bench test, so I reinstalled it in the car. I did not reassemble anything else until I tried the starter. The starter did not sound right the first couple of times I tried it, and then it went back to just free spinning.

I ordered a new starter from Rock Auto and with some trepidation, I installed it. I just bolted it in, connected the battery cable and starter wire and tried it. It worked just fine, phew!

Since the Puzzler basically presented the same problem, I waited with great anticipation until this past Saturday to find out the answer. What a disappointment. Anyway, I happened to buy a new starter motor for which there was no core charge or requirement. So, I still have it. I will take it back apart to see what the problem is. I’m guessing the gear mounted on the end of the motor shaft that drives the planetary gear system may have stripped whatever was holding it to the motor shaft so that the gear does not turn once the system has kicked the Bendix gear out and engages the ring gear. This pressure may be then causing the motor gear to slip on its shaft. There is another possibility that something between the planetary gears and the Bendix gear is stripped or slipping, but this seems less likely.

If any of you know the answer to my “Puzzler,” please let me know. If I find out what it was, I will let you know.


As far as counter rotating aircraft engines are concerned, our old Navy Grumman C-1As Trader had two Curtis-Wright R-1820, rotary 9-cylinder piston engines both of which rotated in the same direction, clockwise. This same engine was used on T-28 aircraft that had a single engine. It is true that many boats and ships used counter-rotating engines but the only one I can specifically recall is the YP (yard patrol) which had counter-rotating diesel engines to emulate its larger destroyer mentor.

As far as I can recall, the two C-1A engines were identical and interchangeable. The C-1A has been taken out of service in 1988 and was replaced by the Grumman C-2 Greyhound, still an old aircraft.


@ronkriel I had the same experience with a 1986 Ford Tempo: starter spun up but did not turn the engine. Like you, I could find nothing wrong with the starter on the bench. I’m remembering a direct drive (no gear reduction) so I concluded that the failure was the sprag one-way clutch in the Bendix. (However, even with pliers on the pinion, I couldn’t make the clutch slip.) Of course, a rebuilt starter fixed everything.