Mercedes C Class 200/202 Intermittent no crank

[cattach]

1999 MERCEDES Class C 200/202 Auto Elegance

Insert key turn on to position one
all lights light up and extinguish one by one leaving just the battery light the oil light on .
Turn key to position 2 the cranking position, fairly loud clunk as the starter motor engages and nearly turns the engine, but it suddenly release resulting in a no crank situation. However keep on holding the key in the crank position and sometimes you just get lucky and it suddenly crank over as normal and the engine fires up and runs perfect.
Fitted a new battery just to eliminate it.
Still no difference.
I wondered if the starter was getting 12 volts and holding 12 volts, So I tested it with my volt meter. yes 12.40 volts at the battery terminals and also under cranking conditions so no loss of power, then I went under the car and found the positive lead where it comes out of the body on the passenger side bulk head, still 12.40 volts both static and under cranking.

Power from the joint at the bulk head to the starter = 12.40 volts and while cranking still 12.40. volts tried to get a reading of the power source from the key to the starter which should also be 12 volts but can only get a flick on the meter as it releases so fast the meter can’t register it. but it does try to crank thus proving that even if it’s for only a split second it does receive power.
Replaced the starter just to eliminate it.
No difference
Is the engine earthed ? Well all earth straps were undone cleaned and refitted.
No difference.

I farmed out the fitting of the starter motor to a well respected good kind of guy as I do not have a lift to get the car high enough for me to get under and get them hard to remove torx bolts out while laying with the car body touching my nose 🙁 Anyway after listening to my story he said that he had an Auto wiring expert friend car diagnostics company that was good, and if the mechanic guy feels that he is wasting his time and the customers money chasing down a fault, that he is not sure of then, he calls on this guy to help him out. He seemingly checked so many things out first then they decided it may be the main computer E.C.U that is at fault and could he remove it to find the numbers on it to order one.
That diagnostic report cost me £150
I don’t feel confident that the main computer is at fault because if it was then why does it first click the relay on the left hand side wing/fender a loud click plus you hear the fuel rail pump up.

why put power to the starter enough to make it twitch.
However it was purchased but did not resolve the problem.
Then the Gear box safety switch was replaced.
No change.
Then the Conductor Plate in the gearbox was replaced
No change.
He said he even replaced the transmission control unit.
Still no change.
Any help in any direction would be so helpful thank you.

Topic

[Ole Eggers]

If you turn on ignition and then short the starter with a jumper wire, will it crank normally ?

You don´t state if the igition barrel has been checked. Sounds like it´s worn out. I´ld suspect the problem is in that circuit.

[cattach]

Hi Iznogood Yes if you put power to the starter it will crank over but will not start Engine lock out they call it. The ignition switch is an electronic one it was replaced along with the E.C.U but never sorted the problem.

[cattach]

Thanks guys for all you helpful advice.

[Author]

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