"The speedo's packed up on Curtis' merc, apparently there's some burnt bits, can you have a look?"
Yeah ... why not?
It's from a 1986 Mercedes 500SL
It's an electro-mechanical speedo. The speedometer itself is basically a big voltmeter. The odometer (mileage) is driven by a small solenoid.
The electronics is driven by an IC, an ITT UAF2115, and the datasheet for it is available on the web.Pulses come in from the speed sensor (presumably on the gearbox), and this IC interprets them and creates the appropriate voltage for the speedo, and pulses to increment the odometer. There's some timing components to set the range.
The circuit board is removed by removing the 4 rubber mounts, and desoldering the solenoid and speedo
Tracing the circuit through, 12V comes into the circuit, and is filtered by a C-R-C circuit. The R bit of this is the bit that has burnt up. It is (was) quite a substantial component. Poking around with a meter, and sure enough, something after the resistor is dead short (in the picture above, you can see I've already swapped it out)
How's that second capacitor looking? Dead short! Bingo!
It's a Frako brand 22uF 16V fellow. I've had these particular caps fails dead short before. This one looks like it's popped it's seal too...It's replaced, along with the resistor (33 Ohms) and we're in business. Feeding some pulses in from my signal generator, and the speedo is reading.
I change the other capacitor as well, as it's probably not far behind the first ....
Curtis reports that the speedo is reading perfectly. Great.
Another saved from landfill...
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