One of the benefits of having a car with a stick shift is that it can occasionally get you out of a bind. If you realize you’ve left your lights on and your battery doesn’t have enough juice, no matter! You can just push the car up to speed in neutral, dump the clutch in first gear, and get the engine ticking over in no time. That doesn’t normally work in automatic transmissions, except in a few special cases.
As it turns out, some automakers wanted to preserve the bump-start capability of a manual transmission in automatic vehicles. Chrysler did this very thing when it built its early automatic transmissions. Mercedes-Benz similarly offered this functionality on its 4G-Tronic transmission. The feature didn’t stick around, as automakers eventually deemed the bump-starting capability to be unnecessary.
To learn how this feature worked, first we have to understand why it doesn’t work in most automatic transmissions. Let’s learn about autos, how they work, and how the addition of one little pump enabled certain older auto cars to bump-start with the best of them.
The magic of the standard automatic gearbox is the torque converter. This consists of an impeller, driven by the engine, and a turbine, connected to the transmission’s input shaft. There’s also a stator in there aiding efficiency, but it’s not relevant to our concerns today. As the engine turns, it drives the impeller in the torque converter, which basically churns up the transmission fluid. The motion of the fluid then in turn drives the turbine, which turns the transmission. Basically, the engine turns an impeller, which then transfers energy through the transmission fluid to the turbine, which drives the transmission and thus, the wheels. Imagine turning on one fan and watching how the air it moves makes another fan spin in turn, and you’ve got the idea.
This fluid link between the engine and transmission is very useful. When the engine is turning, but the vehicle is stopped, the fluid churned up by the impeller can slush around the stationary turbine blade without issue. This means there’s no need for a clutch to separate the engine and transmission.
Also critical to the automatic gearbox is the pump. This is basically a small pump that sucks up fluid from the transmission pan and supplies it to the valve body, transmission cooler, and torque converter. It’s normally driven by the engine via the torque converter and generates hydraulic pressure for controlling all the aspects of the transmission. The valve body uses this pressure to control the bands and clutches that shift the transmission through its various gear ratios.
So, what would happen if you tried to bump-start a regular automatic? Well, very little. You might be able to get the car rolling down a hill or something in neutral, but that would be about it. You could put the shifter into Drive or another selection, but nothing will happen. That’s because with the engine stopped, the transmission’s pump isn’t running. Thus, there’s no hydraulic pressure to activate the valves to shift the transmission into gear.
To get around this, Chrysler included a bonus pump on the PowerFlite and early TorqueFlite transmissions. This pump was mounted on the rear of the transmission, and was turned by the motion of the output shaft. Thus, when the wheels turned, the pump turned, generating hydraulic pressure in the transmission. The PowerFlite stuck around until 1961, and the feature stuck around on the TorqueFlite until 1966, when the rear pump was eliminated to cut costs.
This video on leak points of the 722.3xx 4G Tronic transmissions gives us a look at the pumps on the Mercedes design.
This additional rear pump allowed cars with these transmissions to be bump started. The general idea was much the same as with a manual transmission. The vehicle needed to get rolling at some speed, with the transmission in neutral and the ignition in the on position. Then, it could be shifted into gear—usually a low gear was specified. This would connect the wheels through the transmission to the turbine, which would then back-drive the impeller connected to the engine and ideally, start the car.
Mercedes-Benz used the same solution on the 4G-Tronic automatic transmission, first released in 1979. It also featured a rear pump that could generate line pressure for a bump start. The 4G Tronic remained in production until 1996, though models built past 1990 eliminated the bonus pump.
Notably, inefficiencies in the fluid coupling of an automatic gearbox still make bump-starting harder compared to a manual. I’ve successfully bump-started a tiny Mazda 121 by pushing it myself at a running pace on a flat road, jumping into the driver’s seat, putting it gear, and dumping the clutch. I wouldn’t have had the thing going more than 10 mph, tops. Such a feat would not likely be practical in an automatic. You need to get the car up to a much higher speed due to the losses between the turbine and the impeller attached to the engine—there’s no direct mechanical connection. In an SAE journal article published by Chrysler on the original PowerFlite, it recommended getting the car up to 25 mph before shifting into gear.
Similarly, in the manual for the 1986 Mercedes-Benz 300E, it was recommended to get the car up to 18 mph with a cold transmission, or 30 mph with a warm transmission, to account for the different viscosity of the transmission fluid. Indeed, Mercedes referred to this as “tow-starting” the vehicle, as it’s clearly unlikely you’d be able to push the vehicle up to such a speed even with a few helpers on hand. You could always attempt such a feat rolling downhill, but you’d want to be careful—without the engine running, you’d have no power steering or power brakes, and so stopping could be difficult.
It really tells you why Mercedes eventually abandoned the feature. Towing your car to start it does seem incredibly fussy. It still requires another car, just like a simple jump-start, so you’d be better advised to just bust out the jumper leads if you had a flat battery. The towing method only seems to make sense in a situation where you’ve got no jumper leads, or if your starter motor itself is kaput. It’s hard to imagine too many owners relying on this around town, though it might have saved a few intrepid motorists out bush.
It’s also a feature that would likely be of lesser utility on more modern vehicles. Early automatic gearboxes were just mechanically and hydraulically controlled. Thus, if you got the pump turning and the fluid flowing, the transmission could do its thing. If you then get the engine spinning, the alternator can then generate enough power to run the ignition and start it up. Contrast that with modern automatic gearboxes, which are almost universally electronically controlled. Even if you had a rear pump to get the fluid flowing, you’d still need enough electrical power in your battery to run the ECU for the gearbox and power its solenoid valves to get the thing to actually shift into gear. Thus, it’d only be useful if your battery was only just too flat to run the starter, or if you starter itself was out of commission.
It’s actually funny to think that this ever became a thing at all. Indeed, it seems to be a relic of an era when cars were simply less reliable, both mechanically and electronically. Bump-starting was common enough way back when that engineers found a need to include this feature with automatic transmissions. It’s worth noting that the rear pump has no other value beyond enabling a bump-starting capability. Including one required additional mechanical and hydraulic complexity, both for the pump itself and to ensure its pressure output didn’t interfere with that of the main front pump in the transmission during normal use. Once it became clear it was a largely irrelevant part, it’s no surprise it was dropped in short order.
The bump-startable automatic transmission is a rare automotive feature, and an interesting one at that. It should give you plenty of conversation material next time you’re hanging out with vintage Mopar heads or your local classic Mercedes club. You might even convince someone to give it a try in their own old-school automatic that was bestowed with the magical rear pump. Happy motoring out there, and if your starter does go out, we hope you find a way to get rolling again soon!
Image credits: Chrysler via SAE Transactions 1954, Daddy’s Money Garage via YouTube screenshot, Chrysler & Mercedes via FavCars.com
Push started a friends automatic 1962 Plymouth Barracuda once, the kind with the huge rear window. Not that hard. He’d said it was the last year you could do that, this was in the early 1980s.
Way back in HS a friends Vega which was always hard to start or the battery was dead
regularly needed a push start. It was spring and slushy so it took the two of us to get it moving. The thing wasn’t cooperating so he dumped the clutch and hopped out to help with pushing. The engine caught, door closed and car took off down the street pilotless.
It ending up embedding itself in a snowbank. Once we stopped laughing we did it all again.
I’ve push started my Austin Healey Sprite a handful of times myself. Just get it moving across some flat ground, jump in and kick it into gear. Super easy.
This is intriguing that someone put effort into making an automatic able to be push started. It also means that it can probably be flat towed without worrying about transmission lubrication ad there is a pump for the output side as well.
This place is crawling with engineers who write well and I’m here for it.
Oddly, having owned several vehicles with Torqueflites, I have never tried pushstarting one.
But I have handcranked my old Bedford truck, just to see how hard it was (had electric start but had a backup hand crank).
And I’ve helped a friend tow start his Mazda RX3, which was a bit dicey since it wouldn’t generate enough compression to start unless it was towed to about 50km/h.
I’ve also pushstarted an old Leyland double decker bus, fortunately on a perfectly level and smooth road – because of its weight it barely needed to be moving at all, so after pushing it from the back by hand to get it moving, there was time to walk up to the front, climb up into the drivers seat and put it in gear long before it ran out of momentum.
Conversely, trying to push start my old Datsun 610 was much harder, since it had a rollcage and race seats, so after pushing it to get it moving, it was very hard to climb in fast enough to get it in gear and let out the clutch before it lost forward momentum.
Hey,ive also crank started an old Bedford.You still have yours?Which model?
It was a TJ-J1 model (1963 from memory) with the 214″ 6 cylinder petrol engine. It had originally been fitted with a van body on the back with a single narrow door at the rear, for use as a food delivery van. Some time later, probably in the late 70s, the van body was converted into a camper body. I owned it for maybe 10 years, but never got around to giving it the attention it needed, and sold it a couple of years ago.
Interesting.Are you in Britain?
I just went down the internet rabbit hole trying to find the model my dad owned.Probably a late 60s J5.I’m sure it had the 300ci petrol engine.It had a two speed diff which i -as a 17yo learning to drive – thought was super cool!
Ours was an erratic starter until we finally tracked down the issue.The carb was draining overnight! Once we dealt with that it would start quite easily, even with the crank.
I’m in Australia.
Push it backwards, reverse is often shorter, so you don’t need to go as fast, and in the cars I’ve done it in, it’s easier to flop into the drivers seat from a pushing position. Probably easier to run yourself over as well, so be careful.
Pushing the Datsun in reverse would have been made a lot harder because it was a SSS coupe, with LONG, heavy doors – hard to push in reverse with the door open, and hard to get the door open quickly and jump in with the car rolling without risking getting knocked over by the door on the way in!
My technique was door open, pull on the steering wheel, with it turned a little to the left (so it’s not trying to run you over). Then drop into the seat, clutch down, reverse, and clutch up for a moment to start it.
Holy crap! In college I worked for Kent Bergsma, back when MercedesSource was still Bergsma Motorwerks. I’m glad to see he is doing well.
During that time I roll started several Mercedes sedans with automatics, but unless you are facing down a long hill you will need a tow car to get up enough speed. You not only need the car going much faster than with a manual transmission, but you have to overcome the surprising amount of resistance created by the hydraulic pump.