Piggy-back fuseholders - a right way and a wrong way?

A test by voltmeter would give me 12v on both sides, so how can I tell which is +ve?
That shouldn't normally be true if you remove the fuse before testing, then one side should be around 12v and the other 0v.

Measuring resistance is probably a bad idea unless you disconnect the car battery first, you still have to remove the fuse though.
 
Tiny fusebox might be the reason you're not seeing this, but in a "standard" fusebox there will be blank slots, and these will only have contacts on the live side. There also be some without contacts at all, of course.

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Here's a pic of my fusebox - you can see for example, slots 18; 32; 40; 49; 51; 52, all have contacts on the left-hand side, but not on the right-hand side. Some options just not used in my model - e.g. fuse 40 is a tilting/sliding roof, etc. However there are some slots that are not wired at all, e.g. 34 and 35 - these are for rear door windows, something that could never be specified on a two-door coupe ;) - hence no power to these at all. At least that's how Mercedes wires their fuseboxes. Hope that makes sense ...
 
Mose fuse-boxes are made up similarly. The car owner's manual typically covers all models, and it often specifies the fuse capacity for all the slots- even the unused ones- giving you knowledge of where not to tap in because of low capacity.

For electrical efficiency, if you have to tap an active fuse, you should tap into the 'hot' side so that you don't experience any loss from going through unneeded circuitry (such as a fuse). Tapping the 'load' side offers the possible safety advantage of double-fusing, albeit with some possible current loss which could affect either or both 'loads'. If the added load causes the stock fuse to blow, you don't want it to be on a safety-critical system- another reason to tap the 'hot' side.

For practicality either way usually works just fine as long as the total draw doesn't exceed the circuit capacity and the added circuit is smaller than the one tapped into. That's the most important factor here.

Phil
 
For electrical efficiency, if you have to tap an active fuse, you should tap into the 'hot' side so that you don't experience any loss from going through unneeded circuitry (such as a fuse). Tapping the 'load' side offers the possible safety advantage of double-fusing, albeit with some possible current loss which could affect either or both 'loads'. If the added load causes the stock fuse to blow, you don't want it to be on a safety-critical system- another reason to tap the 'hot' side.
The only reason there is to not tap a fuse after another fuse is that if you're going to add a fuse it's totally illogical to put it after another one. ;):)
 
The only reason there is to not tap a fuse after another fuse is that if you're going to add a fuse it's totally illogical to put it after another one. ;):)
No its not. If the hot side is capable of providing 5 amps without burning out the transistor/relay/whatever supplying it and the fuse is a 5 amp fuse and you add another 2 amp fuse. If you tap into the hot side you can now extract 7 amps and blow up your ECU leaving you with a $500 bill before you can start the engine again, if you tap into the cold side then you can still only extract 5 amps and the ECU is still protected.

Normally it is safe to tap the hot side, but unless you have proof that it is safe then you do it at a risk, it is logical not to take that risk and always tap the cold side.
 
Nigel, even tapping the cold side still leaves the total load the same, the 2A added will not blow the 5A drawn from so if it's going to blow the ECU it will still happen either way.

Phil
 
Nigel, even tapping the cold side still leaves the total load the same, the 2A added will not blow the 5A drawn from so if it's going to blow the ECU it will still happen either way.

Phil
No, if you tap the cold side then you can still only take 5 amp max since it all has to go through the original 5amp fuse and so you can't damage the ECU. It is true the load will be the same, but the maximum you can take is 5 amp instead of 7 amp.
 
No its not. If the hot side is capable of providing 5 amps without burning out the transistor/relay/whatever supplying it and the fuse is a 5 amp fuse and you add another 2 amp fuse. If you tap into the hot side you can now extract 7 amps and blow up your ECU leaving you with a $500 bill before you can start the engine again, if you tap into the cold side then you can still only extract 5 amps and the ECU is still protected.

Normally it is safe to tap the hot side, but unless you have proof that it is safe then you do it at a risk, it is logical not to take that risk and always tap the cold side.
Excuse me, but if there's a 5A fuse in a certain circuit it doesn't mean you only have 5 amps in that circuit. Fuses are meant to protect the circuits, not regulate its current. If a fuse with that value is there then the circuit has a lot more amps going through.
And how can an extra load on some random circuit blow the ECU?
 
And how can an extra load on some random circuit blow the ECU?
If the ECU is providing the power to the hot side of the fuse then it can easily be blown if you bypass the fuse protecting it by taking power from the unprotected side of the fuse.

In most cases the power to the hot side of the fuse is provided by the battery/alternator through cable capable of more than 5 amps but unless you know that to be the case then you are taking a risk.
 
No, if you tap the cold side then you can still only take 5 amp max since it all has to go through the original 5amp fuse and so you can't damage the ECU. It is true the load will be the same, but the maximum you can take is 5 amp instead of 7 amp.

Well, thinking this through you are indeed correct. I've been a bit unwell today and not thinking as I should have, my apologies to you and all. Thanks for setting this right!

I still think we're "picking nits" here though. First, we're talking about a addition of what, maybe 2+A running 2 cams at the most? And for most of us that would be halved running one cam. Fuse ratings are not 'spot-on' exact and their 'blow' point is usually in excess of their rating (I base that on my experience with known supply's and measured loads; most fuses will carry 15% above their rating quite easily). My automotive experience also indicates that manufacturer's allow for some disparity too, with nothing being run at the limit but always somewhat below that- otherwise one slightly dirty connection would end up as a costly-to-them warranty repair along with the bad reputation this would cause. So unless you have over-fused the tap-in you're not likely to kill any components from overload. But wait, there's more!

What can and does happen is that some of the car's circuitry in the ECU or sensors use input voltages to regulate it's response, so drawing voltage from one of those areas could well cause a malfunction. But once again, manufacturers rarely overlook limiting responses so that an out-of-spec input should not cause any component failure. Another variable here is 'load ramping' or the rate at which a load comes online. A simple tiny spark can induce a massive momentary drop in current and a spike in apparent voltage when the arc connects which can damage sensitive electronics. So if a cam were turned on or drew any current when connected you could have a problem. Today's cars are certainly far more sensitive to these things than an old 'breaker points and condenser with carburetor' engine management system.

As with anything know fully what you are doing and if you don't, then hand the job to someone who does. If your car is still under warranty, contact the dealer for advice before altering or adding anything- they are there to help you avoid problems.

Phil
 
I think you're overthinking things. Car's electricity is as basic as it gets. Unless you're talking about cameras that run directly on 12V, there's a very small chance that the camera can affect the car's circuitry and vice-versa.
 
To avoid potential problem discussed above, I normally tap into power socket fuse (usually 20A for the front one and 30A for the rear); which themselves sit behind larger fuses of 50A each.
Did just that recently for my newly acquired car, adding 7.5A fuse tap on a front power socket, for the custom loom I put together, feeding two dashcams, GPS tracker and Garmin satnav. Rear dashcam (0801) and satnav use 5V, and each have their own voltage converters (CPT for dashcam and Garmin own for satnav); front dashcam (DR400-II) and GPS tracker take 12V direct.
 
Another bump for an old thread, thanks Google!

Just installed my mobius dashcam, using a piggy back fuse. It worked great for a couple of weeks and today blew the fuse.

I've used an extra cigarette lighter, which I've installed in that glove box, piggy backing off the BMW cigarette light9 fuse. The BMW one is 20amp and I installed a 5amp fuse next to it, which came with the piggy back connection.

Have I put the original 20amp fuse in the correct slot? The original cigarette lighter still works.

aac883612eb6c772b90acb13b7d5e3ef.jpg


If it's correct, what amp fuse should I use for the 2nd one?

Thanks
 
yes, you are correct. the yellow fuse is in the original place. in the upper slot (in-line with the red power cable) you should place a small fuse like 3A. if you are operating only the mobius on that line, 1.5A is enought.
 
yes, you are correct. the yellow fuse is in the original place. in the upper slot (in-line with the red power cable) you should place a small fuse like 3A. if you are operating only the mobius on that line, 1.5A is enought.
OK thanks, just concerned as the 5amp fuse blew, but I was messing with the USB cable at the time, so maybe the had something to do with it
 
"i was messing with the usb cable"=?
cutting the cable?
attaching more than one electronic consumer to usb splitter?
describe what you actually did.
 
"i was messing with the usb cable"=?
cutting the cable?
attaching more than one electronic consumer to usb splitter?
describe what you actually did.
Swapped 1 USB cable for another, then I pulled a bit more length of cable through the glove box (the fuse box is behind the glove box) , it's at this point, I noticed the light on the cigarette lighter adaptor had gone out, I checked the fuse and it had blown
 
I've been installing an extra 12v socket today with a piggy back fuse. I know which side is positive & know which fuse is to be in which place.
However I'm in a bit of confusion as when the ignition is turned off, the socket stays live, yet the original 12v outlet that I'm tapping from turns off. I'm not sure how this is possible, but hope someone on here can shed some light.
 
I've been installing an extra 12v socket today with a piggy back fuse. I know which side is positive & know which fuse is to be in which place.
However I'm in a bit of confusion as when the ignition is turned off, the socket stays live, yet the original 12v outlet that I'm tapping from turns off. I'm not sure how this is possible, but hope someone on here can shed some light.
There is a lot of magic going on behind the wiring in modern cars. I am a bit confused about your question though.. You have identified a source that is not switched (ie 12v on all the time).. you put your piggy back fuse in that slot. The original output now is still switched as it was before (ie turns off with key). And.. your new pigtail outlet is ...? (on all the time?) If that is the case then there is some relay some where that is turning off the accessory (radio) with the key.. or.. your have miss identified your original location.
 
There is a lot of magic going on behind the wiring in modern cars. I am a bit confused about your question though.. You have identified a source that is not switched (ie 12v on all the time).. you put your piggy back fuse in that slot. The original output now is still switched as it was before (ie turns off with key). And.. your new pigtail outlet is ...? (on all the time?) If that is the case then there is some relay some where that is turning off the accessory (radio) with the key.. or.. your have miss identified your original location.

Thanks for the reply.
That's right. The original 12v socket turns off with the ignition, but the new one doesn't. I've tried it in 3 or 4 (non critical) different locations & it's the same with all of them. I've got 2 piggy back adapters and it's the same with both.
Should have been a 30 minute job, but instead it's been a couple of hours & I still don't have it sorted.
 
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