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500SE AMG, E350 4matic, GL 450
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Discussion Starter #1
I'm working on my starting problems. I pulled out the Overload Voltage Relay and tested it. I used instructions from a search of this site.....set the multimeter to diode mode. Positive test probe to terminal 31 and neg to term 30...... I got a reading of 0.544 V. I understand the norminal value is 0.4V to 1.5V. Since my OVP read to close to the lower end of the nominal value, could this still be why my w126 is having such a hard time starting?????
As I quick recap: The car has a hard time starting whether it is warm or cold out. I have to pump the gas pedal before it finally starts. If I shut it off and restart, it starts right up. Also the idle will drop when I stop at a light etc. The car sort of bounces up and down.
I did look at the fuse in the OVP, the filament looks intact.
 

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1991 300 SE
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18,534 Posts
Since the OVP provides system power to the CIS control unit, the ABS control unit and the idle speed control unit, a failed or faulty relay can cause hard starting and rough idle. Typically a faulty or failed OVP relay will result in the ABS warning indicator on and rough idle due to no CIS management. Also there is a 10-amp fuse incorporated in the relay to protect the sensitive electronics associated with the ABS and CIS.

Hard starting can result when the throttle valve switch and microswitch on the throttle linkage do not work properly due to a failed/faulty OVP. Although the relay can be replaced or repaired one should remember that there may be an electrical system failure that caused the overload such as a charging system problem or a buss short to ground.

There may be mechanical problems with the cold start valve or its relay and the control valve in the fuel distributor.
 

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1988 300 SEL
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6 Posts
RE: Have you tested your water temp sensor

I have a cold start problem that sounds similar to yours. The engine will absolutely not start without giving it some gas with the pedal. Even then it won't normally start on the first try. After it does start I have to keep feeding it to keep it running. Then the idle smooths out and drops to ~750 rpm instead of the 1000 range it should be in during 30F outside weather.

On the Fastlane forum I found that the water temp sensor can go bad.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.mercedesshop.com/&cookieid=1GK0TQCS51GL027VXE&year=1988&make=MB&model=300-SEL-006&category=C&part=Water+Temp.+Sensor&appEngines=-12-093916

When I went to inspect mine this afternoon, I found that one of the terminal pieces was actually broken off!! No wonder it starts so horribly but purrs like a kitten once started. As I understand it, the temp sensor only interacts with the fuel injection system at startup, after that it patiently waits for the next start.
 

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500SE AMG, E350 4matic, GL 450
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Discussion Starter #4
Water temp sensor....I will check it today. Thanks for the info. Where is this sensor located in the engine???
 
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