Having difficulty jump starting a larger vehicle...

You should probably be able to do it with the Polo battery, assuming it hasn't been replaced in the past with a smaller one than standard. In warm weather like this (pah) engines don't use the full capacity of the battery (reserved for cold starts down to ten-twenty degrees below). Your main issue will be the cheapy cables, they simply won't let enough current to start the Merc through. Borrow or buy some fat ones and you'll be set for life. My Grandad gave me a set you could start a jumbo jet with and nothing has yet phased them, and I've managed to start far larger vehicles than my old 1.4 petrol Golf with them.

Make sure you do one cable first, then the other. Don't connect both at one end and then both at the other, as you'll probably end up touching the jaws together and scaring yourself with the nice big resulting spark. Start the Polo and leave it running for five minutes, and then try starting the Merc. Get somebody to sit in the Polo and build the revs up until there's enough power to turn the Merc over, as alternators make exponentially more power, the more the engine's being revved.

If that really doesn't work, then they're fairly easy to bump-start. I've bump started cars on my own on flat ground before, so given a couple of mates you could bump start a van. Turn the key to the on position, stick it in second (maybe third, given the compression of a big diesel engine) and clutch down. Get it up to 5-10 mph and then dump the clutch. If you're on a hill leave the clutch and let the momentum of the van turn the engine over before giving it some throttle. If you're on the flat, as soon as the engine catches drop the clutch to the floor and give it some throttle. Should be enough to get it going.

You should probably be able to do it with the Polo battery, assuming it hasn't been replaced in the past with a smaller one than standard. In warm weather like this (pah) engines don't use the full capacity of the battery (reserved for cold starts down to ten-twenty degrees below). Your main issue will be the cheapy cables, they simply won't let enough current to start the Merc through. Borrow or buy some fat ones and you'll be set for life. My Grandad gave me a set you could start a jumbo jet with and nothing has yet phased them, and I've managed to start far larger vehicles than my old 1.4 petrol Golf with them.

Make sure you do one cable first, then the other. Don't connect both at one end and then both at the other, as you'll probably end up touching the jaws together and scaring yourself with the nice big resulting spark. Start the Polo and leave it running for five minutes, and then try starting the Merc. Get somebody to sit in the Polo and build the revs up until there's enough power to turn the Merc over, as alternators make exponentially more power, the more the engine's being revved.

If that really doesn't work, then they're fairly easy to bump-start. I've bump started cars on my own on flat ground before, so given a couple of mates you could bump start a van. Turn the key to the on position, stick it in second (maybe third, given the compression of a big diesel engine) and clutch down. Get it up to 5-10 mph and then dump the clutch. If you're on a hill leave the clutch and let the momentum of the van turn the engine over before giving it some throttle. If you're on the flat, as soon as the engine catches drop the clutch to the floor and give it some throttle. Should be enough to get it going.

Thought I was right with all the people saying it wouldn't lol.

I thought you weren't supposed to actually rev the 'donor' car as the dead one was trying to turn over, wrecks the alternator or something? Or have I just made that up...

Thanks for all the replies guys. I am going to go out and buy some new booster cables. I have looked up the ones I have got and they are 5mm and the ones I am going to buy are 25mm. The description for the 5mm says they are only suitable for 1000cc vehicles but the 25mm states it is OK for 3500cc both diesel and petrol. I am even considering going mad and getting the 35mm ones just so it bloody well works!

Why on earth would it damage the alternator? You're more likely to damage things by not revving the donor car as you're trying to draw current where there is none. Obviously don't redline it, but 2.5-3k is fine - enough to get the other car turning over. Alternators don't make anything like their maximum rated amps until you get to around 3k anyway, so there's no point having the donor car even started up and idling unless you're going to rev it.

Dude, normally what you say is good but that's painful! Remember V=IR.
If the donor battery is in good shape you're unlikely to damage the windings of the alternator, but you would if you tried to jump start using just the alternator. Having said that, I have heard of damage to rectification stages from jump starting so if I have to jump start I tend to use the good car to charge the dead one for a bit and then turn the good car off before attempting to start the other. If you want me to write a bit more say and I'll try and find some time a bit later.


OP, jump leads should be connected between the positive (red) terminals first and then between the negative terminals, and the negative terminals should be disconnected first.

Thought I was right with all the people saying it wouldn't lol.

I thought you weren't supposed to actually rev the 'donor' car as the dead one was trying to turn over, wrecks the alternator or something? Or have I just made that up...

Every time I've jumped cars I've done it in the way I've described and had no problems at all. There have been times at which the dead car won't even crank until I give the donor car some revs. I'd be interested in seeing documentation saying otherwise. When you crank the dead car, you're increasing the load on the electrical system of the donor car. How is that different to having say a 4x4 with a couple of big spotlights and a winch running off the stock battery drawing a higher current through the system?

Sorry, looking back at that it comes across ruder than I intended - I just didn't like the way you were describing current!
The way you've described will, as a rule work, but there is a theoretical risk, at least. I have also heard of people blowing rectifiers when running the engine during a jump start though, which is why I don't - I've never needed to either, given a few minutes charging.

So why might running a starter motor off an alternator damage the alternator..
- Voltage, current and resistance are related by the equation V=IR. You can't change one without changing at least one of the others
- An alternator has a variable output, governed by a voltage regulator which modulates an excitation current to control the voltage of the output. The rated current is simply the max current that it will output at a nominal voltage (around 14.4V). The current itself isn't regulated
- Starter motors draw huge amounts of current for a very short time. Not many power sources can provide this, which is why cars still use lead acid batteries; there are plenty of other battery types which can store as much energy in a smaller package, but they can't deliver it quickly. The current will spike and start to drop off when it starts spinning: an effect called back EMF means the current drawn by a motor is greatest when it's stalled
So why does this matter? Well a big starter motor might draw 300 amps at 12V. That's fine if you've got a perfect power source (one which can provide infinite power) but a small alternator might only be capable of providing 40 amps at 12V. Ohm's law (V=IR) still has to be satisfied, we can't do anything about the resistance and the source can't provide enough current, so the voltage has to fall (that's why a battery might show 12V but when you try and crank it drops to 10V and won't start). Now the rated output of the alternator is meaningless; it's running at less than rated voltage so the current will go up (P=I^2R) if you're still providing the same amount of power. As the voltage drops one of a few things will happen: the starter motor will start to spin (unlikely - it's not getting enough power), the windings of the alternator will get very hot and possibly burn out, or the voltage regulator, if it's smart, will detect that the voltage has dropped too low and will cut the output from the alternator by cutting the exciter voltage.
To add another stage of complexity to this, the output of the alternator itself isn't DC but 3 phase AC. This means it needs to go through a rectifier to give the DC that's needed to charge the battery and run the car's electrical systems. Chances are that you'll blow the diodes in the rectifier before damaging the windings. The current output then drops and hopefully you stop at that point. If you still have at least one of your rectifiers intact you probably won't notice a problem at first - if you put a voltmeter on the terminals you'll probably still see the same voltage. However, you lose 1/3rd of the output of your alternator for every rectifier stage that's out, which means the voltage output will drop under heavy load. Worse, the phases that are still working have to work harder, and you're now much more likely to overheat the remaining working windings (alternators are not designed for a 100% duty cycle). You might not notice a problem until winter and you have heated screens, lights etc. on and the alternator can no longer keep up. At this stage it's unlikely you'll link it to the jump start, but there are people who have posted that they've had to replace the alternator within a few weeks of giving a jump start. This is also why an alternator should only be used to top off a battery, and not to try and charge a very badly discharged one.
So why doesn't this happen all the time? Well you're not just using your alternator to provide current; hopefully your battery is in a decent condition and that will start to provide current as the alternator output drops. If you're not trying to start a 6 litre diesel with a Micra you'll probably be OK, as the battery is designed to put out that kind of current and so will protect the alternator. However, if the mismatch is too great or the donor battery is suspect you could very easily damage the alternator; it just might not be immediately obvious. You're right, it's no different to a 4x4 running a winch - which is why most 4x4s running a winch have an extra battery and an uprated alternator. Bosch, who manufacture both alternators and batteries, recommend the donor engine is off when jump starting.
Hope that makes some sense - I tried to be brief but it's still turned into an essay!

Also a general point: it's worth checking where the jump leads should be attached, particularly on newer cars. Apparently connecting straight to the battery can blow the ECU on some cars - I've no idea why, but a friend of mine (who has a masters' degree in engineering and has been helping out in his dad's commercial garage all his life) told me he's seen it happen. My dad's Merc actually has terminals specifically for jump starting, possibly because it actually has 2 batteries in it! As ever, the best option, if possible, is to charge the battery using a proper charger.

It didn't work guys! I bought the 35mm cables too! I am going to buy another battery. Now I have a different issue however: one of the bolts that keeps the battery in place is rusted beyond belief so I can't get the bloody thing out. FML.