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Old 06-18-2007, 08:01 PM
dieseldiehard's Avatar
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,346
Actually, there was a discussion about the circuit that Graminal95 posted.
It was decided that the 470K resistor from pin 14 goes to vcc and the diode in the upper right from pin 8 should be a 6.2 Volt zener (at first someone thought it was a 9.6 V Zener but they then measured about 6 volts on pin 8. I do not understand why there would be a resistor across the diode on pin 8 and believe there is still a possible error in the schematic.

My experience has been that the highest failure item in German clocks and electronic modules are electrolytic capacitors. I suspect the 100 uF electrolytis are prone to failure because the Germans did not use much conservatism when they selected 15 volt rated caps. All the failed resistors I have found are burned because the cap they feed has shorted or partially shorted.
We know that the electrical systems charge up to something like 13.8 to 14 Volts or higher and there is some ripple on the charging system that causes electrolytics to heat up and dries them out. I suggest using 50 Volt caps if at all possible even if it means using a lower cap value of capacitance, if employed as a bypass cap it won't cause any problems to use say a 50 uF 50V cap provided it fits the available space. A tantalum cap would also be a useable substitute. If someone went into the repair business on these VDO tach modules I could have sent them a few dozen modules over the years. They are a sad thing to speak for a supposed high quality outfit like VDO.

VDO is introducing a new fluidless braking system, it will pulse a solenoid at each wheel to grip the pad, no hyydraulics at all! This is for the Greens who want to get hydraulic fluid out of the world by some future date. Pity the poor driver who steps on the brake pedal and it doesn't work because of some 2-bit cap that went poof like these darned tach modules

'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!
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