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Old 09-18-2002, 03:46 PM
sbourg sbourg is offline
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
'The "listings" I referred to were three different service manuals: an aftermarket book on Bosch CIS'

If this is the SAE publication by Bosch, it is by far the best system description for determining qualitative function of the system. I would rely on the Mercedes CD to pull up the numbers and locate the parts, but it is otherwise probably less instructive. The key, though, is regardless of method, all procedures should converge to the same result. E.g., if I found the Mercedes procedure to produce baffling results, I would use the Bosch method - that might show up the problem.

'How many milliamps to the ohm at 8 volts?'

Current is inversely proportional to resistance. 8V across 8 Ohms will produce 1000 mA, and across 80 Ohms you'll get 100 mA; across 1000 Ohms, you'll get 8 mA.

Note that 8V - measured from ground - on one side of the circuit must be compared to the voltage on the other side. That might not be at ground potential. If the ECU uses a current source in its return line, you might measure a fixed current even though the sensor resistance is varying and the high side voltage remains at 8V. This is because the current source allows the voltage on the low side to vary as necessary and this is then the voltage monitored by the ECU to determine sensor condition.

Measuring current is also not trivial. Most meter circuits use a low resistance internally to create a small voltage drop it can measure, due to the current flowing through it when hooked in series with a circuit. This voltage drop must be considered in relation to the circuit you are measuring. If you are measuring a circuit which is sourced by a very small voltage, the meter's voltage drop can affect the operation of the circuit. For this reason, and to avoid having to break a connection in the circuit you are measuring, it is often handier to measure the static resistance of some device already in circuit, and measure the voltage drop across that element. The current can then be calculated from Ohm's Law.

Good luck! I know these systems can try one's patience.

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