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Old 01-22-2007, 02:59 PM
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Join Date: Dec 2006
Location: Hawaii
Posts: 7
1992 500sl engine dies

When driving steady and letting off the acelerator pedal, the RPM indication (tach) goes to 0, and if the car is brought to a complete stop, the engine is dead. Same condition: While driving steady and letting off the acelerator pedal the engine tach still goes to O RPM, but then if the pedal is pushed further down, the RPM indication reads normal & the engine continues to run. I checked the fault code and it had 7 blinks. The MBZ chart indicates ( TN-signal (rpm signal incorrect or open/short circuit). This is intermitent and I was told it happened mostly when making a turn. I drove it one day and nothing happened. The next time I drove it, it happened 4X. Any ideas where to look? What is "TN-signal" & "DI" control module? The MBZ chart 12.0 test looks like I check for voltage (5-7.5V) from pin 76 - 5. Any help would be appreciated.

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Old 02-06-2007, 03:25 PM
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Join Date: Dec 2006
Location: Hawaii
Posts: 7
I have not been able to get the same problem with the loss of RPM signal. I continue to have the problem with the accelerator pedal though. Intermitently, when the accelerator pedal is actuated, there is no response until the AP position sensor mechanical linkage makes contact with the EA/CC/ISC actuator linkage, then the rpm increases. The car can be driven but there is no accelerator pedal response at the low end. When this happened last night, I turned the ignition off and re-started the car and everything was fine again. There are no "check engine" lights when this happens. Is this the "limp home" mode? I checked the AP position sensor (R25) at the plug on the rh fender disconnect (X1) and with a meter found that between pin 6 & 7 there is fluctuatting resistance. It reads anywhere from 4 ohms to .8 ohms. If I wiggle the AP positio sensor shafe, the resistance flucuates also. I know that on aircraft, if it is over .5 ohms, that will cause trouble. All the other resistance readings appear to be normal and smoothly increase or decrease in resistance. I compared The AP position sensor readings to the EA/CC/ISC actuator readings and all are close to being the same except for the AP ohms between pins 6 & 7. Am I looking in the right place? Any help would be appreciated.
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Old 02-15-2007, 01:04 PM
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Join Date: Dec 2006
Location: Hawaii
Posts: 7
I am slowly getting more familiar with this problem the more I read about the other ASR problems posted. It looks like I am definetly going in the limp mode. I found out the ASR & ABS lights do not work when in the limp mode or when the ignition is first turned on. I do get the ASR function lamp in the center of the spedo on when the ignition is first turned on, then it goes out after starting the car. I also have the "Snow Chains" switch lamp working.

Is there any way to check the ASR & ABS lamps without removing the instrument panel?
If a faulty diode in the A7k1 relay can cause the ABS not to come on, can it also cause the ASR lamp to not come on? (the wiring diagrams show 2 diodes in one drawing and none in another drawing)
What is the purpose of te R25 "safety switch". ( I have the high resistance across the contacts)

I built the LED test lamp and it works great. It looks like I may have a TN problem. I cleared the codes and will drive again and recheck the codes tonight.
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Old 02-19-2007, 01:12 PM
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Join Date: Jul 2003
Posts: 536
Where are you pulling codes from? My 92 500 sl has only a round cover on driver side with a 9 pin connector under it. Is yours a 9 pin also?

Thanks, Juan

Last edited by deportes; 02-24-2007 at 09:57 AM.
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