1992 500sl engine dies

[tmike]

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.