1986 300d Auto (UK Car) Sudden no Crank
 

Hello everyone!

I purchased a 1986 300d from my father in law that had been stored in his garage for 22 years, I changed oil and filter, air filter, radiator, thermostat, fuel filter and all fuses and it was running (although a little lumpy). I booked it in with my local mechanic as the blowers weren't getting warm so I assumed my heater matrix was blocked. Last night I went to start her to get her ready for the journey to the mechanic and she would not crank.

I have looked through many old threads and have tried the following:

1. New battery
2. Check fuses
3. Check battery terminals
4. Used the middle plug on the firewall to positive battery terminal trick (it cranked but didn't start)
5. Tried to start in P & N
6. Wiggled the gear stick around the P & N areas still no start

My next job will be to try and jump the positive terminal plug screw to the purple wire screw in the engine bay and if it cranks I will check the harness above the accelerator peddle, is this harness behind the plastic trim to the side of the peddle as I have been having a hard time trying to find it.

Would not crank means engine would not turn over when key turned or engine spins over but wont continue to run when key is released?

Engine doesn’t turn over at all, the dash lights come on, dim as normal but the engine doesn’t crank/turn over.

If I connect the positive battery terminal to the middle pin on the harness in the engine bay it turns over but doesn’t start as per this thread:


https://forums.mercedesclub.org.uk/index.php?threads/w124-starting-troubles-probably-not-battery.157041/

If I try to connect the small terminal to the large terminal on the other side of the engine bay it doesn’t turn over at all.

I have had a multimeter on the starter motor and the top connection has constant 12v the lower one only has 6v when turning the key. Does that sound like a dodgy starter motor?

Not sure what is meant by upper and lower wires, I just can't picture it. If the starter cranks when putting 12V to the "middle pin" the starter is ok. It sounds more likely to be a connection or a bad ignition switch. Check and clean all fuses for good measure.

What happens if 12V is jumped to the wire that is getting 6V in the crank mode? Nothing should be 6V, there is a bad connection somewhere causing the voltage drop and it may be inside the ignition switch or the neutral safety switch.

Have you tried moving the shifter to neutral and retrying?

Thanks Sugar Bear, Sorry for not being clear. The upper is 'live' constant 12v the lower wire I meant is the one that then gets power once the key is turned. Hopefully that makes more sense. All fuses are brand new as the old ones had corroded.

I have gone through all the gears with the ignition turned and it does not crank at all, all that happens is the light dims slightly.

Nothing happens if the 6V is jumped with 12v

Did you also check/clean the ground wire connections between the transmission bell housing and the body?

What about the ground wire that runs from the battery to where it connects to the body?

And the positive connection, the big wire, that runs from the battery to the starter?

Is the battery charged sufficiently?

Can you hit the starter with a stick or something similar when putting 12 volt to where you got 6? If it works when you do that then starter solenoid is ng. You should have 12 volt on 6 volt wire when turning key. Time to get schematic and start testing.

Disconnect the wire with 6 volts on it and try jumping 12 v to that terminal. That will tell you if starter ok or not.

I can jump my 1981 300SD with two screwdrivers. There is a plastic box under the hood, right side (in USA) with four screws holding wires. The box has a cap on it and measures about 2 inches by 4 inches. Located just in front and below the antifreeze reservoir. Connect the screw closest to the dash (call it #1) and screw #3 using crossed screwdrivers. If the starter is OK, it will spin just like using the key. (Of course key must be in the on position.)

This is the SD but I would be surprised if the 300D does not have it too.

Good luck.

Finally found a pic of the jump box. It might be on the other side in UK.

It's usually one of three things:
Bad earth / ground
Dud solenoid
Stuck throw out on the starter motor - try hitting it with a lump of wood to free it off.

For reference it was the immobiliser, I replaced the starter and it didn’t fix the issue. Traced the wiring and just cut out the immobiliser and it sorted the issue

Thx for getting back with the solution. This could help forum members in the future.