DIY Voltage Drop Across Fuses - Quiescent Current or Parasitic Discharge Tests

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Original Document

W220 2003 S500L How To Interpret and Measure the Voltage Drop Across Individual Fuses by BenzWorld Member ricebubbles


Version 1 (Original document): Created 7th Feb 2014 This document: Last Saved: 8 February 2014


This DIY document refers to my 2003 (Update) W220 S500L (Long Wheel Base Sedan) Chassis Number: 220.175, Engine Number: 113.960, Transmission: 9-speed (722.9) (7 forward and 2 reverse gears) 722 901.


W220 S500 Measuring Quiescent Current or Parasitic Battery Drain, Version 01 posted 2012-07-08:


Having published my preferred technique for measuring Total Quiescent Current Battery Draw on BenzWorld, (See Reference) in which I also recommended a simpler technique whereby you can assess the quiescent drain by measuring the voltage drop across individual fuses, I thought I had better check my calculated voltage drops with real fuses.


Fuses Under Test

I tested real M-B 40A (orange or amber) and 10A (red) fuses.

W220 40A (orange or amber) and 10A (red) fuses.JPG

Digital Volt Meter (DVM)

I used my Digital Volt Meter (DVM) (same as the DC Volts scale on a Digital Multi Meter orDMM) on the 200mV Range and measured the voltage drop ACROSS each fuse. When the fuses are installed in the car you can hold the leads on the exposed fuse terminals without withdrawing the fuse from its socket. The important point is to place the DVM ACROSS the fuse. (The DVM has the red and black probes in the next picture and is connected across the fuse terminals. The crocodile clips are supplying the current.

Digital Volt Meter (DVM) on the 200mV Range.JPG

I induced a DC current of 30mA through a load consisting of a short string of LED strip lights. The LEDs were placed in series with the Fuse Under Test.

LED Strip Light Load of 30mA.JPG

Analogue Multi Meter on the 30mA DC Current Scale

I used my Analogue Multi Meter to measure the current. The meter shows just over 30mA on the 30mA DC Current Scale. I know it’s a bit ancient but so am I, but I know it reads very accurately.

Analogue Multi Meter on 30mA DC Current Scale.JPG

10A Fuse Under Test

The next picture shows the setup for the 10A Fuse Under Test.

10A Fuse Under Test.JPG

Result for 10A Fuse

The observed voltage drop across the 10A fuse is 00.3mV ie 300uV. The DVM is on the 200mV Range.

40A Fuse Under Test

The next picture shows the setup for a 40A Fuse Under Test. With a 40A Fuse the fuse resistance is much less and hence the observed voltage is much less. This is pushing the capability of my DVM.

40A Fuse Under Test.JPG

Result for 40A Fuse

The observed voltage drop across the 40A fuse is 00.1mV ie 100µV. The DVM is on the 200mV Range (maximum reading possible of 199.9mV) and cannot display any smaller voltages.


Thus the observed readings agree very well with the calculated readings summarised here from the published document. (See Reference).

I have added two extra columns to the Fuse Resistance Table.

“An easier technique, if you have a sufficiently accurate DVM which can read microvolts, is to measure the voltage drop across each of the fuses. This method does not disturb the circuit requiring a reset or necessitate having to wait the 20 minutes for the car to go to sleep after the first time. The quiescent current can be calculated using the voltage measured, the resistance of each fuse and Ohms Law. Reference: WIS” with added observed voltage drops for a 30mA current measured by a Digital Voltage Meter.

W220 Voltage Drop Across Individual Fuses.JPG

Summary of Technique to Assess Quiescent Current by Measuring Voltage Drop Across Individual Fuses

  • Ensure the car has fully ‘gone to sleep’ by waiting at least 20 minutes after locking.
  • What you need to do is read the voltage drop across each fuse one at a time.
  • Use your Digital MultiMeter (DMM) on the 200mV or higher range.
  • Place one lead on one side of any fuse and the other lead on the other side of the same fuse. It doesn't really matter which way around the leads are.
  • Make a chart and record the voltage readings for each fuse.
  • If you are prepared to wait for the car to go to sleep each time you can activate the circuit being protected by the fuse (remember the door switch activates the interior lamp) and record the before and after car sleeping values.
  • The main idea is to see if you have any fuse showing a voltage drop which is equivalent to more than 30mA. Ie the fuse voltage drop should not exceed the values shown in column 3 of the above Table. (With the car fully asleep.)