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
I believe Powerglide transmissions can also be push started.
Interesting! Looks to be the case!
I had a 1962 Studebaker GT Hawk with an automatic that could be push started. I had a few 1980’s Mercedes and never knew that this was possible.
I guess there’s still only one transmission type to go with… I never bump start in first, always second or reverse… seems to be less harsh in second *shrugging face* reverse is emergency start gear, where forwards is not an option
I wish I’d have thought of bump starting in reverse years ago. Specifically the time me and my brother pushed our car up a small incline, and then laboriously pushed it in a three point turn in order to bump start it. (There wasn’t enough room at the bottom of the slope to turn round). Even with two of us, a pretty slight incline, and a light car, I remember it being a lot of effort.
Still, if we’d had an auto we’d have been stuck.
Bump starting in reverse has saved my arse more times than I can count
my push start story.. used to go fishing at this remote mountain reservoir, borrowed my brother’s Alfa Giulia and parked high on the hill. Good day’s fishing, car didn’t start, tried 3 times rolling down that hill until the wheels were at the water’s edge. It was dark and cold and twenty miles to the nearest paved road, another twenty to town. Luckily a stray engineer came by to look at the waterworks and gave me a jump..
That Giulia was great fun to drive on the twisty mountain passes up to the lake, took about 3 hours. Later a friend had one of the first GTIs, dropped our time to 2h20min with better handling and way way more power..
great story! bet that felt like salvation, getting a jump start to get you out of there.
Neat, but I can see why this didn’t last long as pushing a heavy car up to 15-18 MPH cant be that easy. They mention towing to accomplish this too but why not just jump the battery if you have another running vehicle.
I feel like push starting a manual only takes 8-10 MPH but I guess every car could be different.
yeah, exactly. the case for tow starting is incredibly niche, pretty much only if you have a starter motor failure.
I’ve had to push and steer my contemporaneous Mercedes and I can’t imagine pushing it to 5mph, much less 18-30. Those bastards were heavy.
I miss bump starting my old shitboxes but the last 2 cars I have driven have had electric parking brakes that don’t release without power. I was parked at the top of a hill the last time I had a dead battery and had to wait for a jump. So much wasted potential.
I see what you did there. Physics humor!
hilariously frustrating
I wonder if this would have worked better or worse with the 80s diesel benzes with their high compression ratios… especially the less common naturally aspirated 5 cylinder models.
you’re not relying on powered spark ignition in that case, so could be easier!
Starter motor broke once on my W124 250D which is a naturally aspirated 5 cyl and push starting it was very easy and car started instantaneously after dumping clutch. Yeah, it was a manual but still.
My favorite push start was the one I observed in Santa Cruz maybe 15 years ago.
I was stopped in traffic and saw a dreadlocked wannabe hippie in the parking lot of a gas station pushing his ancient VW aircooled vanagon. For reasons only known to himself he was wearing no shirt, sandals and very loose fitting olive green army pants and no belt so he was pushing with one hand and holding his pants up with the other. I don’t remember if there was anyone else helping him as my attention was on him. This all happened very quickly.
The van suddenly started rolling backwards. Without thinking the hippie put both hands on the car to stop it from rolling him over…
His pants dropped…
He was going commando!
Commando just saves money on underwear. And that cash can be used for more drugs. But I do miss the hippie girls from about almost 60 years ago. As a kid I encountered a girlfriend going commando around 1972…It soon became a thing in our small town school. Sorry.
I forgot where I was, and just realized that once again was rambling on about stuff not Autopian approved.
Best push start memory is in school. Big old 1961 Chevy panel van with a push from same model Chevy truck. Popped the clutch at about 35mph, and the ancient U joints exploded, dropped the driveshaft into the road. Which my friend promptly ran over. The damage resulted in his folks buying him a new ride. Which turned out to be a clean 55 Chevy Nomad style panel. That thing was so awesome, and a great party car.
I grew up with a lot of crazy kids. When it comes to push start screw ups? Could spend an afternoon recounting those between the tears of laughter at the memories…
So this explains why pull-starting an old Concord worked: seems they used the 988 TorqueFlite transmission. I mean, we had to take it up to 30ish, and it felt really weird compared to old VWs I was used to, but it did happen and know I know why it could.
I sort of miss the old Forest Service green AMCs that were like cockroaches around here in the 80s. We even had a couple for Driver’s Ed cars.
Wow. Wish I had known that back in 1974. Even though it was a brand new car, our’s had a starter issue the local dealer seemed unable to resolve.
When I was a teenager, my buddy and I got stranded in a big box store parking lot because his VW Rabbit’s battery died. It was a scorching Midwestern July day. The sun above was oppressive and the heat rising up from the asphalt would have burned bare feet in milliseconds. Temps were high 90’s with humidity about the same. We pushed the Rabbit to the far end of the lot so we’d have plenty of runway to pop-start the car. From our starting position, I pushed from the back and he pushed from the driver side door jamb with the door open. We pushed – the car started rolling and he jumped in the driver’s seat. I kept pushing and pushing wondering why I didn’t hear the engine turn over. I only heard him laughing. I was hot, sweaty, tired and cranky and by this time we had covered more than half the lot. I was out of breath and quit pushing. The car slowed and finally stopped. My friend popped out of the car and sheepishly said the keys were in his pocket and he was laughing so hard he couldn’t get them out. Fucker.
82 Rabbit LS—the one year they had the absolutely stupid shoulder belt but no lap belt. Shoulder belt clicked into upper rear corner of door frame so it automatically got put on when you sat down & closed the door. But not motorized. Sorry: that’s irrelevant but it still pisses me off.
Push started it several times thinking the solenoid was going bad before I realized that that stupid shoulder belt had to be plugged into the door or the safety interlock prevented the solenoid from functioning. Boy, I still feel dumb
This was a very common thing at my school too. The stoners usually also forgot to get out their keys as well. Good times.
My friend popped out of the car and sheepishly said the keys were in his pocket and he was laughing so hard he couldn’t get them out. Fucker
And that’s when you forgive him and try again, this time with him pushing for all he’s worth and you with your foot ever so slightly on the brake.
Good stuff Lewin, thanks.
When I was a kid we had a lot of classmates with the same auto trans in their MOPARS. And being poor High School kids, dead batteries were very common, especially in the school parking lot. Several friends learned the “tow to start” method would result in a rear end collision once the MOPAR started. So we switched to the “push to start” method. That became an issue fast too. Several of the students would try to get it cranked, then hit Ice or snow in the parking lot and spin it into other parked cars.
After a few of these mishaps, our school made this practice grounds for suspension. Seriously. The Principal would actually look out his window at lunch time and look for the “offenders”…
But the real fun came from watching friends try this on cars with auto trans that did not have the push start trans. And almost everyone who tried this trick ended up with smashed rear bumpers and bodywork. Good times.
Thanks for the memories!
It’s a miracle any of us survived to adulthood.
That video linked in the article made me think that I watched a real life version of Bobby Hill. I don’t think that was Hank sitting shotgun though.
TH400’s through 1966 had a rear pump…
As others have mentioned it wasn’t only Chrysler that had a rear pump back in the early days of automatic transmissions, many if not most had a rear pump. It was there for two reasons, PUSH starting and so the vehcile could be towed with the rear wheels on the ground w/o having to disconnect the driveshaft, since wreckers of the time typically didn’t carry dollys to put the rear wheels on. Batteries back in the day had far lower cranking amps than those of today, didn’t last that long and thanks to generators could end up discharged from driving at slow speeds with the lights and heater on.
Note the recommendation was always to push start the car, manuals for the cars I’ve had specifically say NEVER to tow start it since the vehicle could run into the towing car, if using a short chain/rope, if the engine surged to life on the fast idle setting. AT vehicles were never able to be “bump” started since you needed a fair amount speed and time at that speed to build enough pressure to fill the torque converter and turn the engine.
I used to live in a house with a very steep driveway (while always having cars with manual transmissions) and I hardly ever used the starter motor to start the car. Unless I parked in the garage, just let off the brake, coast a few feet, and drop the clutch.
Same here.
I’m in the same boat. My driveway is steep but only 30 feet long or so. Even when parked in the driveway, I can roll start my Sportwagen TDI and my 1972 Super Beetle. I roll start the Jetta frequently, having never owned a diesel before (and with all the complicated tech on mine) I wasn’t sure if it would roll start, but it does. Guess the only time you couldn’t is if it was really cold outside and the battery was so dead it couldn’t heat up the glow plugs.
I was trying to figure out which battery to buy for my 90s kei car (very strict size restrictions, Japanese pencil connectors) and consequently had to push start it a few times – by myself. The joys of having a car that weighs 1499lbs! I have had my three-year-old push it, just for fun (not to start it, and not near a street).
Oh, I love that joyful old Mercedes Source guy from YouTube, he always makes me want to buy one more old Merc 🙂
I’d read this bit of trivia at some point about early-mid-’60s Mopars, but I never knew automatic ’80s Benzes could be push-started (edit: I’m sorry, of course that should only be “tow-started”, at least to a German, or to anyone living in a densely-populated area). Good to know, just in case!
One to add to the list: VW Super Beetles with the “Auto-Stick”. You could leave the transmission engaged in low gear and – with the aid of a second vehicle – push the Beetle up to 15-ish mph and the torque converter would drag just enough to spin the flywheel (these cars had both a torque converter and a vacuum actuated mechanical clutch) over and start the car. I had a ’74 back in the day and when the starter failed and I couldn’t afford the replacement part I used this method, a lot.
I had a 64 1/2 Barracuda with the push-button shift. Occasionally, a conversation in the high school parking lot would come around to push starting automatics. Nothing to do but get a buddy to push, pile a bunch of people in both cars and try it out. Worked like a charm on my car. Then we would try various other cars to no success. Lots of scratched chrome and wrinkled trim from mis-matched bumpers. I loved high school.
My dad, the cheapest MFr who ever lived, did this EVERY DAMN DAY down our driveway hill. There were skid marks all over the stretch where he’d dump the clutch in 1st. Probably lead to his ’74 Dart’s early demise.
I had a Dasher that lacked a starter when I was young. But there was a hill at home, a hill at work, and a hill at my girlfriend’s house. For other situations I would just leave it running and lock it with an extra key. I have found that using 2nd gear and releasing the clutch slowly is more effective than dumping it hard in 1st.
Ahh, the good ‘ol days.
Gotta save the starter.
And no lights on until it’s completely pitch dark, I’d bet?
Yup. And when he finally got a nice car, a Cordoba with intermittent wipers, he’d leave them on the most intermittent setting to “save the wipers”.
I also saw him once kill the engine on a long downhill grade, just to save gas. The look on his face when he realized that “new” cars have steering wheel locks was priceless.
Ford transmissions did this as well in the very early 60s. I don’t think it was billed as a feature, but I know that some of the Falcons you’re advised to not flat-tow them in neutral as it ends up turning the motor over as well. (something like that)
Not sure how many vehicles it affected though.
The flat tow issue is more likely that the transmission would be spinning from input from the rear wheels via the driveshaft, but the fluid pump at the front of the trans would not be providing lubrication since the output and pump stator aren’t physically coupled unless hydraulic pressure is provided to the clutch packs under engine power.
Nope. FordoMatic Trans. Had a rear pump as well.
Page 6
https://dgv4.xr793.org/wp-content/uploads/2018/08/1951-Fordomatic-Booklet.pdf
I think the FoM trans was last in the Falcons, which is where I heard of it.
My sister had a 1960 Falcon in school. That thing looked like it had been in a demolition derby after about a year. Good times.
Does the second pump also mean that cars with these transmissions could be flat-towed? I seem to remember that most automatics were not suitable for flat-towing because there was no fluid being pumped, but this secondary pump would seem to alleviate that.
I remember that early Saturn cars were a big hit with the RV crowd who liked towing a car behind their motorhomes, because they offered one of the few automatic transmissions that would allow flat towing, but I don’t know why that was.
wondering that myself. I know my car can’t be flat towed because of this.
Yes being able to tow the vehicle with the rear wheels on the ground was one of the reasons that many if not most early automatics had the rear pump. The other of course was so that the vehicle could be push started.
Not to mention that bump-starting your car could damage your catalytic converter which is a lot more expensive than calling roadside assistance. So I assume after those became more widespread in the 70s car makers wouldn’t want to include a feature that’d help you torch your own converter. Probably a bit of a liability issue.
I’ve never heard this. Why?
I was told in driver’s ed in the early 2000s never to bump or push-start a modern car. Apparently turning over the engine without spark can cause unburnt fuel to enter the exhaust system, which would then burn off when the system gets hot again, thus damaging whatever is in the catalytic converter, those weird hair-like structures I assume.
Hrm.
With a fuel injected car I don’t think that would be an issue. If the car isn’t running the injectors aren’t opening. With a carbureted car the engine will be sucking fuel provided its turning over, but I don’t think you’d have to worry about it with fuel injected?
I may be missing something
This is surprisingly frustrating to google. Apparently a lot of car manuals still say not to push-start the car. I’m trying to see what my car’s manual has to say to that, hopefully they elaborate on *why*
Welp, all I can find, including wikipedia, says it’s mostly an issue in carbureted cars. Still in the context of the article possibly relevant since at that time period there would have been carbureted cars with catalytic converters around I guess.
Idk how likely push-starting a modern car would be to cause misfires, but that’s a mechanism by which damage to the cat could still presumably occur. Luckily I haven’t been in a situation where I had to bump-start a car, fingers crossed.
You don’t have it spinning the motor w/o the ignition switch on so no it won’t be dumping fuel into the cat.
Cats on gas engines are a grid structure. It’s easier to extrude and coat with metals to do the catalyzing. Source: family were engineers for a company that made the ceramic cores for cats and diesel particulate filters among many other glass-like things.
Huh? I’ve bump started my 2012 car a bunch. No cat issues. The injectors won’t fire unless the ECU senses air movement from the MAF/MAP and the crank sensor also reports movement.
No you won’t damage your cat push starting a car. The proper method is to turn the key on, get the vehicle up to speed, then shift into gear. The engine doesn’t spin w/o the ignition on so it is not sucking or spraying fuel w/o spark.
Neat! Never knew this existed!
Wow. This is Torch taillight level obscure info. Thanks!