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When the 451 fortwo started exhibiting the same problems when selecting
reverse, people immediately assumed that smart hadn't learned their lesson
and that a faulty brake light switch was to blame as with the previous models.
Although it can cause the fault, it is more likely to be the clutch or the software.
Generally what you will get is the N flashing on the speedo.
by smart technician
"This is a fairly common fault, it occurs more frequently on our sales display smarts as
they are left for long periods of time. Smart did at one point advise us to replace the
clutches for ones with a modified teflon spline. This does fix the issue but, on all the
smarts I have had the issue on, I have found a later software version for the gearbox
that gives it a higher threshold before it gives up trying to push the clutch in."
"In all the instances that I have had this issue, it has rectified it every time."
"It is something that would incur somewhere around half an
hour labour time for anybody who has run out of warranty."
Take it to smart and have the gearbox controller software updated.
When you stop, hold the foot brake and select reverse, if it just flashes an N at you
on the speedo LCD instead of selecting reverse, try this. Reselect 1st gear, drive
forwards a few inches and try selecting reverse again. So far I have had the problem
appear about 5 times and on each time, this quick method allows you to select reverse.
Anon found that edging forwards didn't circumvent this fault. However, they did find that manually
pushing the car back a few inches with their leg out the door seemed to allow selection of reverse.
I have heard of 3 failed brake light switches on this model. Far far less than the previous models.
The 451 has a separate controller for the gearbox, it's underneath the carpet, underneath the left hand seat.
The cause of the leak is unknown but probably a leaking seal or windscreen however, the water pools
up around the gearbox controller, gets inside it (it's not waterproof) and causes major problems.
Feel under the carpets for water.
You would think that having the spiral gear of the clutch actuator snap would cause the 3 bars to show on the
speedo. Normally it is the case but on the odd occasion, the car will happily sit there in the gear it failed in.
Obviously it won't allow the car to change gear with no control over the clutch.
Remove the clutch actuator, remove the 6 torx screws from the top and remove the bottom plate.
You can then physically check the motor spiral drive gear for damage.
This is a fairly new one from mid 2017 but it is becoming more and more common.
It looks as though 1 of the 4 angle rotation sensors inside the gear change motors simply fails.
You have to remove the airbox and air inlet pipe to get to the 451 gear change motor.
You can bench test it or fit a borrowed/2nd hand unit to test.
When fitting a new unit, you will need to have it aligned with MB Star Diagnostics.
The short wiring loom from the gearbox controller (GDU) to the transmission ancillaries has been known to fail.
It could be rubbing, fatigued, pinched, trapped, moving which causes internal breakage or water ingress.
Continuity test each wire or ideally load test each wire. Replacement looms are available from smart.
Alex H had issues selecting any gears and after exhausting the above possibilities he checked to
see if the brake lights functioned correctly. They didn't. He assumed that it was the brake light switch
but decided to check the fuses as matter of course. This is when he found that fuse 24 had blown.
Fuse 24 powers the brake lights and upon replacing that fuse, the car functioned correctly.
How a combined 3.5 amps blew a 15 amp fuse is unknown but the do deteriorate over time.
Or it could have just been a faulty or badly made fuse.
On the heels of unlikely fuses blowing, I figured I may as well cover another possibilty, just in case.
If your car refuses to select gears and the radio doesn't function, it could be fuse 20.
Fuse 20 powers the SE Drive that selects the gears via the gearbox control unit.
However, this fault is more likely to bring up 3 bars on the speedo display.
This fuse blowing would definitely cause gear issues as it powers the gearbox control unit.
However, if there was no power, you'd almost certainly get the 3 bar error on the speedo display.