Trunk Fuse Box - identifying hot side
Thread Starter
Junior Member
Joined: Nov 2015
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From: London, UK
C63 AMG W204 / CLA45 C117 AMG / ML500 W164 / CLK320 W209
Trunk Fuse Box - identifying hot side
Hi guys i needed to make some connections to the rear Sam for an accessory and i havent seen this specifically covered.
when using a fuse tap the orientation of the fusetap matters. Is there an identifier on the fusebox to know which side of the blade fuse would be “hot”
When you remove a fuse you can see a + and - look. Its a bit hard to get my multimeter probes in to be sure however.
Basically one side is the side getting power and the other side when a fuse is inserted carries the power to the component. Obviously if the fuse is already in outting probes to the top of the fuse will just show both sides with power.
when using a fuse tap the orientation of the fusetap matters. Is there an identifier on the fusebox to know which side of the blade fuse would be “hot”
When you remove a fuse you can see a + and - look. Its a bit hard to get my multimeter probes in to be sure however.
Basically one side is the side getting power and the other side when a fuse is inserted carries the power to the component. Obviously if the fuse is already in outting probes to the top of the fuse will just show both sides with power.
Thread Starter
Junior Member
Joined: Nov 2015
Posts: 58
Likes: 11
From: London, UK
C63 AMG W204 / CLA45 C117 AMG / ML500 W164 / CLK320 W209
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Joined: Jun 2017
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From: Western NC
2009 MB C300 Sport 4matic
If you are really curious as to whether one ( which) side is "hot" when the IGN is ON for instance, put a blown fuse in the socket and check the top as you mentioned. Connecting your accessory to that side will give you an unfused circuit , not sure why you would want that.
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Joined: Jun 2016
Posts: 462
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From: Panama City, FL
2020 C300, 2013 C250, 2000 Boxster S
You are absolutely correct. The orientation is important. My apologies. To verify which side of the fuse socket is the feed side first remove the fuse then turn the IGN to ON. Touch the black meter probe (COM) to a suitable ground point then touch the red meter probe (V) in turn to each side of the fuse socket. The side that shows voltage on the meter is the feed side.
Thread Starter
Junior Member
Joined: Nov 2015
Posts: 58
Likes: 11
From: London, UK
C63 AMG W204 / CLA45 C117 AMG / ML500 W164 / CLK320 W209
If you are really curious as to whether one ( which) side is "hot" when the IGN is ON for instance, put a blown fuse in the socket and check the top as you mentioned. Connecting your accessory to that side will give you an unfused circuit , not sure why you would want that.
I have a full most based sound system which i installed, Hk has been replaced with a BIT dmi, processors and an aftermarket amp. All operated as intended for over a year but with the recent addition of an apple car play unit into the command unit. I am now pulling constant power to my bit dmi which subsequently leaves my amps etc running. Essentially a drain and the culprit is highly likely the carplay module.
the fuse testing is to basically temporarily fit a fuse tap (want to be sure it goes in correctly and safely) so that i can run a test wire out from a few circuits, lock the car and work out powerdowns on the command related fuses e.g. rear cam, tv tuner, sound system amp. This will confirm that all those items in the most loop are getting constant power and not going off.
when carplay module is unplugged these items power down as intended. Its more for a fault find and to confirm suspicions but when using these taps i want to ensure they go in correctly and safely.
You are absolutely correct. The orientation is important. My apologies. To verify which side of the fuse socket is the feed side first remove the fuse then turn the IGN to ON. Touch the black meter probe (COM) to a suitable ground point then touch the red meter probe (V) in turn to each side of the fuse socket. The side that shows voltage on the meter is the feed side.
Thank you both
Senior Member
Joined: Jun 2016
Posts: 462
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From: Panama City, FL
2020 C300, 2013 C250, 2000 Boxster S
thanks again, i did attempt this but unfortunately my probes dont go into the slots as they are slightly thicker. I was hoping due to the design of the fusebox when a fuse is removed you can see one side is like a + and the other a straight - slot. I was hoping that someone may know if that was a simple indicator to tell which side was the powered in source. I will try using an alternative insert or as another posted mentioned a blown fuse so i can get my probes onto the top metal contacts of the fuse to ascertain a reading.
Thank you both
Thank you both
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Thread Starter
Junior Member
Joined: Nov 2015
Posts: 58
Likes: 11
From: London, UK
C63 AMG W204 / CLA45 C117 AMG / ML500 W164 / CLK320 W209
Thanks guys, So i think i managed to figure out the hot side (Cut a fuse in half inserted it on one side and then the other to take a measurement)
For others - It seems the hot side is the side with a (-), The (+) is not powered. When fitting a fuse tap ensure the orientation of the lead out to your accessory falls on the + side. This is based on an assumption of a few i tested in the rear Sam, It would be best to use a similar test to confirm the hot/cold side when using fuse taps
For others - It seems the hot side is the side with a (-), The (+) is not powered. When fitting a fuse tap ensure the orientation of the lead out to your accessory falls on the + side. This is based on an assumption of a few i tested in the rear Sam, It would be best to use a similar test to confirm the hot/cold side when using fuse taps
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