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  #1  
Old 05-02-2006, 11:30 AM
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1973 450SL Air Temp Sensor

My 1973 450SL has suddenly become a rough runner. Rough idle and misses a lot. In the course of trouble shooting, I discovered the air temp sensor on the horn of the air cleaner can. When disconnected, all seems fine. But reconnecting the sensor brings back the rough idle and misfire. What does this sensor do and should i just replace it?

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Old 05-02-2006, 12:06 PM
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That sensor is one of five inputs to the computer in the pass footwell that determines pulse width to the injectors. Actually there ate only 4 that determine pulse width. The trigger points in the distributer generate time base zero. Normally you can unplug that sensor and not notice much difference in the engine. Yours must be sending a really funky signal. Just change it. I don't think they are even that expensive.
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Old 05-02-2006, 12:40 PM
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Thanks, the part's in the mail....
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Old 05-02-2006, 10:53 PM
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Quote:
Originally Posted by Toddski
My 1973 450SL has suddenly become a rough runner. Rough idle and misses a lot. In the course of trouble shooting, I discovered the air temp sensor on the horn of the air cleaner can. When disconnected, all seems fine. But reconnecting the sensor brings back the rough idle and misfire. What does this sensor do and should i just replace it?
the air sensor will do as you say when disconnected. this engine can do
as you say with all kinds of things going on. check for bad sp plugs,
bad sp plug connectors, vac leaks, etc. might wanna check and see if
you got oil on the trigger points. most trouble shooting will start with
what is the major symptom. is the problem spark or fuel relasted??

george
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Old 05-02-2006, 11:18 PM
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As autozen said, most likely a bad sensor. This sensor will make your engine run VERY lean if there is an internal short. It will run rich with it disconnected, but not rich enough that it will misfire usually - you'll notice more black smoke on full throttle though. Leave it disconnected while you wait for the new one if you must drive it, it's better to burn a bit more fuel than to burn a valve!
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Old 05-03-2006, 12:05 AM
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in over 30 years of mb service i have not replaced one air sensor
that was defective unless it was physically broken.
always a first time but this engine efi system is kinda complicated
and takes good knowledge to diagnose/repair.
you must check for fuel pressure and vac leak issues first.

george
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Old 05-03-2006, 12:18 AM
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If he has a vacuum leak it'd be running rich. Pulling the air temp sensor plug would make that worse, not better. If he never adjusted his mixture or his idle air speed screw, it's got to be a shorted sensor.
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  #8  
Old 05-03-2006, 12:30 AM
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well, anything is possible and i'm always willing to learn. just
stating my experiance.
i would suggest also checking the eng temp sensor. now those
i have replaced.

george
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  #9  
Old 05-03-2006, 11:44 AM
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Thanks for all the input, guys. I'm just starting to dig into the fuel injection system and I've found all kinds or brittle connectors, scraped off wire insulation, exposed conductors, etc. that would be typical of a 33 year old car. Since I don't have to drive it right now my plan is to replace as many connectors as I can and strip back the wires in the harness to a point where the insulation is good. Then I'll check the sensors and switches to see that they're working right and replace as necessary. I've also ordered the pertonix ignitor so the dwell angle (although now exactly right) won't be an issue any more. General tune up things too like cap, rotor, plugs, wires, filters, etc. That should make a difference. One thing I could really use is the Bosch ECU tester. Any ideas on where I can get one?

I"ve had this car for over a year and just found this forum this week. What a find!

Thanks again.

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