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Matt
Matt, Engineer
Category: Mercedes
Satisfied Customers: 21107
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Marty I have a 1999 ml 320 with 170,000 miles my problem it

Customer Question

Hi Marty I have a 1999 ml 320 with 170,000 miles my problem it doesn't crank when the temp is below 40 degrees otherwise starts and runs fine replaced crank angle sensor, battery ,fuel filter, fuel pump, gas cap and air filter with no luck any advise steve
Submitted: 1 year ago.
Category: Mercedes
Expert:  Matt replied 1 year ago.

Hello

if you haven't alreaady then I'd replace the coolant temperature sensor

as this has a strong effect on starting the engine

also check the condition of the wiring and the connector for any damage / corrosion

Customer: replied 1 year ago.
Hi marty i checked all wires and connectors all are fine i changed the coolant temperature sensor but i still have the problem starting the vehicle when below 40 degrees ( it appers that there is a command sent to the computer to not start the vehicle ) any ideas ? Thanks steve
Expert:  Matt replied 1 year ago.

Hello Steve

there's no special code generated by the ECU to not enable a start below 40 deg F , so there is something wrong

just to clarify that you say its not cranking?

so the motor isn't actually turning over on the starter at all? if this is the case then check that the main battery cables are on tight and the terminals are clean and bright. Do the same for the engine / gearbox earth strap.

Then follow the main positive lead back to the engine and down to the starter motor, and check the connection at this end as well.

There should also be a small wire to the motor which should have 12V when you're turning the ignition switch to crank. If the 12V isn't there, check back along this wire for damage to the insulation or corroded connections

This 12V feed comes directly from the ignition switch so this may be faulty and need replacing

Check the resistance of the main positive lead that runs down to the motor as anything more than a few ohms and it should be replaced

Don't forget to try a jump start from another car / battery as its possible the battery is poor, if you have a voltmeter a healthy battery should read 12.7+V

also try substituting the battery for another as I've known a very flat battery to drag down the system voltage too far even with jump cables

one other thing to consider is that the immobiliser is faulty and preventing the starter from being enabled - however if the dash / ignition lights are on then this usually means that its off

If this is all OK then its possible the car needs a new starter motor

Expert:  Matt replied 1 year ago.

Hi

do you still need help?

Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. so if my answers have been helpful please rate my answer.

Customer: replied 1 year ago.
Hi matt i replaced the battery so thats not it. Checked for corroded wiring - none the sequence goes like this - if the temp. Is below 40 degrees the car will not crank if u turn the key to the number 2 position and try to crank it does nothing however if u turn the key rapidly to crank , the car will crank and start but 2 sec later will stall if you repeatedly try to start car rapidly it will eventually start after 5-10 times and will stay running ( when the car is warm runs and starts perfectly) it seems that maybe not enough fuel pressure or there is a signal going to the ECM causing it to not crank any ideas? Thanks steve
Expert:  Matt replied 1 year ago.

Hello Steve

thanks for the extra information

I'd say this sounds like a problem with the ignition switch? they can be effected by temperature due to contacts expanding / contracting so I'd say the next step is to strip it down and clean it if possible but often these aren't serviceable and must be replaced

Customer: replied 1 year ago.
Hi matt i replaced the entire ignition switch and this morning the temperature was 30 degrees and again a no crank situation. I feel a sensor is giving a signal to the ecm or the ignition to not crank. What sensors control or monitor the fuel and spark?
Expert:  Matt replied 1 year ago.

Hello

the main sensor for fuel and spark is the crank / engine speed sensor

this is located at the back of the engine , pointing at the flywheel

check the wiring and connector for damage and corrosion too

Customer: replied 1 year ago.
Crank angle sensor was already replaced
Expert:  Matt replied 1 year ago.

Hmm

OK well the other sensor that can have a strong effect is the cam position sensor so check that over too

Customer: replied 1 year ago.
Hmm what sensor are u referring to?
Expert:  Matt replied 1 year ago.

Hello

the cam sensor is located on the back of the engine on the LH cam cover