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MHD Belt Failure

Mod Description
Again, the MHD engine proves it is badly designed and should be avoided.
Mod Details
PremiumNo Difficulty Mod ID886 Creditevilution Link Copy to Clipboard


Micro Hybrid Drive, a nonsense acronym that has nothing to do with a real hybrid engine. It is just a moniker that means that the engine has stop/start technology. When the car stops and your foot is on the brake pedal, the engine turns off. When you release the brake pedal, the engine immediately restarts.

It was an option (that no one specced) from October 2007. It was standard fitment from October 2008.

Sounds Great, Should I Buy One?

No, definitely not. Avoid them like sick people on the train. Check out the whole topic on this page.

Why Not? People Say They Are OK If…

These people own an MHD that hasn’t had an issue yet. Admitting that there could be a problem would be them admitting that they were stupid to buy a car without doing any actual research. People don’t admit to being stupid. Also, if they admitted there was an issue, they’d have to pull their head out of the sand/arsehole and come to the realisation that their car could let them down at any moment.

Belt Failure

In 2010 the first recorded case of a belt snapping on the smart fortwo 451 lead the way to this becoming a new common problem. Unlike other common problems on this car, it is serious, can destroy the engine in minutes and has been reported to happen as early as 6000 miles from new.

The problem is the auxiliary belt driven by the crank shaft pulley. If this belt snaps then you are in trouble.

Where Is This Belt?

Open and remove the engine bay lid. Look over to the far right and you should see the belt looping over the alternator pulley near the back of the engine. The alternator pulley is arrowed in the picture below.

(photo of a non turbo 451 engine, not an MHD but the belt is in the same place).

That is the belt that snaps. You can inspect and pull the belt very easily to check it is still on (with the engine off). A visual inspection of the belt whilst the engine is running will show if the belt is moving and if there is any obvious damage to it. Do not place any fingers, genitals, pets or your tie near the pullies and belt whilst they are rotating.

Why Does It Snap?

There are 2 reasons and 1 aggravates the other. For a start, the belt has to be incredibly tight so it can transfer the power from the starter motor (same part as the alternator in the MHD) to the crank shaft without the belt slipping over the pulleys. The power required to turn an engine is immense and it’s even harder when the engine is cold.

Secondly it would seem that a slight misalignment in the starter/alternator (starternator?) pulley is causing wear on one side of the belt. The wear obviously increases over time and eventually the belt snaps. The starter motor doesn’t start out misaligned. The tightness of the belt causes the starter motor to get pulled in and the hole through which the starter motor is bolted to engine starts to elongate which misaligns the starter motor.

What Happens When It Snaps?

There are 2 issues when the belt snaps. Firstly the alternator stops turning so the car is no longer being charged. Depending on the state of your battery and what electrical items you are using, you may be able to drive the car for 20 minutes before the battery died. The 2nd problem is far more serious and much quicker at stopping the car. Let’s look at the picture again.

At the bottom is the crank shaft pulley, this is connected directly to the engine and is the reason the belt is driven. Clockwise around the belt we get to the tensioner that maintains the high tension required to stop the belt slipping.

At the top we have the starter/alternator that starts the car and charges the battery. Clockwise around the belt again and we reach the water pump pulley. If the auxiliary belt snaps, the water pump stops turning and therefore stops pumping the coolant around the engine. Without the coolant keeping the engine temperature down, the engine will overheat in under 2 minutes.

The coolant boils, increases the pressure in the system and usually forces its way out either through the expansion tank lid, through the radiator, a burst pipe or through the head gasket. The engine head warps, the head gasket fails, oil and water mix in the cylinders and the engine fails.

What Are The Symptoms?

The 1st thing you will get is the battery light come on to tell you the battery is no longer being charged. 2nd up you will hear an alarm coming from the speedo and the water temperature light will come on.

Pull over as soon as you can and contact smart. Don’t try and make it home. Any driving after the battery light came on is causing damage and smart will not cover any warranty or free repairs if you didn’t pull over straight away.

If you didn’t notice the battery light and water temperature light and your music was so loud you didn’t hear the alarm, for fuck sake don’t tell smart that. Lie through your teeth and say you pulled over immediately.

What Can I Do?

The main thing to do is part exchange it. Don’t sell it privately as the next owners will cop the problems and karma will kick you in the balls. At least if the dealer sells it on, the new owner has a warranty.

Since 2009 I have (correctly) stated that there is absolutely no reason to buy an MHD 451. The non turbo 451 is a soulless, underpowered piece of shit that drains your life as you drive it. It is unenjoyable and tedious. If you want a car that’s good on fuel then buy the 54kw diesel 451 (or 45kw diesel and have it remapped) as it feels just as underpowered but the engine has soul. If you want a fun car then buy the 451 turbo as that is a hoot to drive but if you drive it carefully you can get the same MPG as the MHD. (if not more).

If you really want to punish yourself and keep this automotive catastrophe then I’d recommend visually inspecting the belt on a weekly basis with and without the engine running.

If you aren’t the sort of person who can maintain a car then for God’s sake make sure you have membership to a breakdown service and ensure that you pay the extra so in the event of a breakdown they will take your car to where YOU want it to go, not the nearest backstreet rip off merchant who the recovery driver is friends with. When your belt does snap, pull over as soon as it is safe to do so and get the car shipped straight to a smart dealer. They are currently repairing these free of charge due to it being a design fault.

Will Smart Fixing It Make It OK?

Smart did roll out the following repair instructions:

1 – An updated, stronger, carbon fibre laminated belt. (2010)
2 – The alternator was shimmed to straighten the pulley out to stop the wear. (2011-2012)
3 – Drill out the elongated alternator hole in the aluminium engine and replace it with a steel insert. (2013)
4 – New design of crankshaft pully that was less likely to damage the belt when fitted. (2014)

Issues With These Fixes

1 – the belt was made stronger longitudinally but the problem was wear causing lateral stress.
2 – the alternator might be straight but the forces on the belt just start bending it again.
3 – the aluminium around the steel insert started elongating instead.
4 – fitting of the belt was only a very small problem causing belts breaking.


None of these fixes are a reliable way to stop the belt from breaking.

Will Turning The MHD Function Off Help?

No. The belt spinning or not spinning is not the issue, the problem is how tight the belt has to be to be able to transfer the power to the engine without slipping. Even when the car is stationary and the engine is off, the belt is still very tight. This pulls on the pulley wheels, tensioners and alternator/starter and elongates the bolt holes.

How Much Will It Cost?

If you ignored the warning lights and alarm and cook the engine it will cost you over £3000 for a new engine and smart won’t help you in any way with the bill. If you pull over immediately and get the car shipped to smart it should cost you nothing at all. If they try to charge you, kick up a fuss as this is a manufacturers fault.

If your belt hasn’t snapped yet and you want smart to perform the fix then they will charge you about 4 hours labour. So probably over £350 plus parts. It’s a rip off but they will say that the belt hasn’t snapped so might not.

If your belt is looking bad then take it to smart to look at, they might do it for nothing. I can in no way condone driving your smart to a dealer and cutting the belt off to get it fixed free.

How Can I Tell If The Car Is An MHD?

Have a look at this page for more information.

Owner’s Documentation

In the UK, the owner’s document is called the V5C. It contains all the information about the car including the VIN code. Also on this document is the CO2 output in g/km. If the car has a petrol engine and the g/km number is 100 or below, it’s an MHD.