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Over the weekend I had a situation where a 2010 VW Golf Diesel had a flat battery. I had a nice long but gentle hill to let it roll down, so I attempted a bump start, which didn't work.

When starting normally, after you turn the key, the dashboard display says, "Depress clutch to start." So you have to have the clutch in, for the starter motor to turn the engine over.

So rolling down the hill, in gear, with the clutch in, when the car got up speed I simultaneously turned the key and let the clutch out to engage the gear. There were some lurches of the car, but the engine didn't turn over. I tried this several times until I was at the bottom of the hill.

Eventually I got the car started with jump leads. No waiting or revving the host engine. Started straight away (Which I mention to indicate the battery wasn't dead as such, just flat.)

Is the "Depress clutch to start" mechanism preventing the car from being bump-started, or did I just not do it right?

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The list for why the car did not start is long. Instead lets hit some basics.

First to successfully bump start a modern car the battery cannot be completely flat. (that is less than 8v or so but depends on car) This is because the alternator needs some electricity to excite it to generate power. The car may not have enough power to turn over the starter but still enough to run the rest of the electrical system. In this case the bump start takes place of the starter. The alternator then takes over and charges the battery.

If the battery is too flat to run electrical system it may be enough to excite the alternator to bring up the charging system while the engine is spinning. This will bring up the electrical system and start the engine.

If the battery is so flat that it can't excite the alternator and nothing will happen.

Old diesel engines which were all mechanical could be bump started any time. The stipulation being that the solenoid that cuts the fuel up can be bypassed manually. Also old carburated cars with magneto ignition systems could be bump started any time.

Finally the depress clutch to start mechanism is only to make sure that the starter does not engage while the car is in gear. This does not prevent the car from starting, it only disables cranking.

  • What is this "alternator excitement", exactly? It's a generator, what prevents it from generating current if it is spun? – I have no idea what I'm doing Jul 13 '16 at 12:07
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    @ihavenoideawhati'mdoing A classic generator spins a magnet inside some coils to make power. The problem is that the power output is unregulated and RPM dependant. With a car generator (alternator) the spinning magnets are replaced with a spinning coil. The coil acts as a magnet when current flows through it. By varying the current the output can be regulated. The problem is when there is not enough power to get the current flowing in the rotor to make it a magnet. Meaning you need juice too make more juice. – vini_i Jul 13 '16 at 12:38
  • Thank you, I looked into it a bit more and it really filled a void I had in my alternator knowledge. – I have no idea what I'm doing Jul 13 '16 at 13:36
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The "Depress clutch to start" gadget should be dealing only with a starter motor relay. Everything else should be working anyway, so if your "Bump-Start" was at low speed and 1st gear, then yea, it was a faulty "Bump-start". Normally it should be done at 2nd or even 3rd gear on diesel engines because of very high compression..

  • Ah. I didn't try 3rd gear. Could that be it? Needed to be going faster in 3rd fear? – Stewart Jul 12 '16 at 6:33
  • I believe your 2010 Golf is a Turbo Diesel, might be 1.6 or 2.0. That engine will have 16.5:1 - 18.5:1 of a compression, which is twice as more than a petrol (gasoline) engine, so it is hard to turn it over... So yea, you might be okay with 3rd gear :) – Arturs Bolsunovskis Jul 12 '16 at 17:35

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