TAP fuse holder is questioning my understanding of electricity

[roblkc3]

Hello everybody!

I am facing a problem that is starting to question my understanding of electricity. I wanted to add a TAP holder the connect the dashcam in my girlfriend's car. "No problem", I thought, "I did it multiple times and it's very basic stuff". I have, therefore, bought the appropriate holder for the car (an Open Mokka -- see image below for the item), found the fuse corresponding to the cigarette lighter, and plugged it in. The access to the fuse box is not very comfortable, therefore I couldn't find out which side is the hot one, but no problem -- I thought that at worst I would have to rotate it 180 degrees.

Looking at the picture, the original (cigarette lighter) fuse has to be between C and D. If I put it in, the two legs are short circuited (which is what I expected). If I put the new fuse between A and B and remove the one between C and D, the legs are not short circuited but I see continuity between the first leg and the blue end of the cable (which is what I expected). Note that, due to the way the item is cabled, if I put both fuses both legs are short-circuited with respect to the blue end.

Now on the weird part:
- I've put back the original fuse in the fuse box: the cigarette lighter is ~12V wrt to a bolt on the car (from now on "GND") -- good.
- I remove the original fuse and put the TAP holder with both fuses in: the cigarette lighter is still ~12V wrt GND (good), but no voltage whatsoever is present on the blue end (what?!??!?!).
- I've tried rotating the holder 180 degrees but with no change at all. - I've tried buying another TAP holder, same results.
- I've tried plugging a cable inside the A hole and checking the voltage: 0V wrt GND (while the cigarette lighter was still at 12V).

Desperate, I thought that I was missing something so I've done a small circuit where I've connected my power source to one leg and used the two circuits (one from the other leg, the other from the blue end) to power two separate LEDs: both were on.

I am start to go insane... and my girlfriend is laughing at me (all my lab full of Star Trek stuff, and in two months I haven't been able to connect a fuse holder...... :S).
Can anyone help me, please?
Thanks a lot in advance 
Rob

[rstofer]

With no fuses installed, there should be no continuity between the two blades of the adapter.  Installing a fuse at C-D should establish continuity across the blades and the original cigarette lighter should work.  But you don't know which leg should be the HOT leg.

Without a fuse in A-B, there should be no continuity between any adapter blade and the blue connector.  Installing a fuse should establish continuity between one blade and the blue connector (remove the C-D fuse).  This blade is the HOT blade and is in common with both fuses.  You could also find this by just checking the internal connections with no fuses installed.

You still need to know which blade is HOT on the fuse block.  Can't be too hard to figure out...

As the device is shown in the picture, I'm betting the left blade is HOT, the right leg goes to the cigarette lighter and the blue connector is the fused tap.

You can 'ring' all this out on the bench...

[ArthurDent]

There is a possibility that you removed the wrong fuse from the fuse block. If you remove the fuse you previously did and check the cigarette lighter outlet and still have 12 volts, you removed the wrong fuse.

Insert 2 wires in C and D and check to make sure you have continuity between the corresponding pins and the inserted wires. Plug the tap into the fuse block and check from each wire to ground to see if you have 12 volts on either one. You already know C has no voltage from previous checks.

After you find the problem check to see if the tap is inserted backwards because if it is the tap circuit current and the cigarette lighter current will both go through the fuse between C and D so it is important to get the tap inserted in the correct orientation.

[Brumby]

With both fuses fitted, you should get 12V at the blue connector with the TAP plugged in - irrespective of which way the TAP is inserted.

One orientation will have the blue connector powered through both fuses and the other will have it powered only though its own fuse.  While both arrangements will work and be safe, it is preferable that the two circuits are independently fused - which means there is a "right" way.

The way to check for the right orientation is to only fit the AB fuse and leave the CD one out.  Plug the TAP into the fuse block and check the voltage at the blue connector, then remove, rotate 180º, refit the TAP and check for voltage at the blue connector.  One way should give you 0V and the other way should give you 12V.  The orientation that gives you the 12V is the one you want.


But - back to your problem...  There is not really much that can go wrong here - so it's going to be something else .... but what?

I can only think we need to look at everything involved in your testing - such as:
 - Have the fuses been individually checked?
 - Have you tried other fuses?
 - How reliable is your "earth" point?  I am very wary about using bolts - especially if there is any paint around.
 - When the TAP is fitted, is it twisted or under mechanical stress of any kind?
 - Are your meter and leads in good condition? (No internal intermittents?)
...... and the list goes on, getting weirder and weirder.

What is important is that you need to be very critical of every aspect.  Even the most experienced of us can get caught out by something that we have taken for granted - because it's never raised its ugly head at us before.  Assumptions need to be examined when you get to this level of confusion.

My suggestion is to:
 1. Check everything (yes again) fit the fuses and fit the TAP.
 2. Set up your meter with solid connections to earth and the blue connector - so that you do not need to hold anything in place.
 3. Push, prod, twist, wiggle, etc. around the area of the TAP and fuse block and see if anything causes meter fluctuations.  If so, investigate.

When you do find out the problem - your first reaction will be relief and a sense of victory.  This will soon be followed by either a "WTF!" moment if it was some obscure problem ... or a giant facepalm for something really simple you overlooked.


Good luck!

[roblkc3]

Thanks for all the answers!

With no fuses installed, there should be no continuity between the two blades of the adapter.  Installing a fuse at C-D should establish continuity across the blades and the original cigarette lighter should work. 

Yes it does, and to answer ArthurDent:

There is a possibility that you removed the wrong fuse from the fuse block. If you remove the fuse you previously did and check the cigarette lighter outlet and still have 12 volts, you removed the wrong fuse.

If I remove the fuse, the cigarette lighter does not work anymore, so I am confident it is the right one.

Without a fuse in A-B, there should be no continuity between any adapter blade and the blue connector.  Installing a fuse should establish continuity between one blade and the blue connector (remove the C-D fuse).  This blade is the HOT blade and is in common with both fuses.  You could also find this by just checking the internal connections with no fuses installed.

Yes, I had played a bit with cables and the multimeter to check all the possible connections (with and without fuses installed) on my bench.

As the device is shown in the picture, I'm betting the left blade is HOT, the right leg goes to the cigarette lighter and the blue connector is the fused tap.

Indeed, I was unable to determine the hot side on the fuse box though (it is very uncomfortable to access it -- and doing that outside with a -10C temperature does not help either...).

The way to check for the right orientation is to only fit the AB fuse and leave the CD one out.  Plug the TAP into the fuse block and check the voltage at the blue connector, then remove, rotate 180º, refit the TAP and check for voltage at the blue connector.  One way should give you 0V and the other way should give you 12V.  The orientation that gives you the 12V is the one you want.

This is the weird part -- I've already tried that, and although it works well (as expected) on the bench (both by applying 12V and by simple continuity test), once I plug it in the fuse box, I see no voltage at all irrespective of the orientation.

- Have the fuses been individually checked?

I did a continuity test with the multimeter and they work fine.

- Have you tried other fuses?

Yes, I've bought an entire new TAP holder with its corresponding fuse.

- How reliable is your "earth" point?  I am very wary about using bolts - especially if there is any paint around.

Huge bolt with no paint --- I've tested it thoroughly using the cigarette lighter as reference and it seems to be good.

- When the TAP is fitted, is it twisted or under mechanical stress of any kind?

I had thought about it, therefore I have tested continuity when applying all the strain types I could think of --- seems to work well...

- Are your meter and leads in good condition? (No internal intermittents?)

Perfect conditions, and I periodically check it against a reference voltage.

My suggestion is to:
 1. Check everything (yes again) fit the fuses and fit the TAP.
 2. Set up your meter with solid connections to earth and the blue connector - so that you do not need to hold anything in place.
 3. Push, prod, twist, wiggle, etc. around the area of the TAP and fuse block and see if anything causes meter fluctuations.  If so, investigate.

On my todo list for the weekend!

When you do find out the problem - your first reaction will be relief and a sense of victory.  This will soon be followed by either a "WTF!" moment if it was some obscure problem ... or a giant facepalm for something really simple you overlooked.

100% sure it is something very very dumb.... but still cannot figure it out!  Guess I'll wait for the weekend again to play a bit more with the car

Thanks again for your help -- at least now I am confident that I am not overlooking something totally trivial. I'll wait for some spare time and some sunlight, I'll try again, and I'll let you know the outcome!
Cheers,
Rob

[Brumby]

Now on the weird part:
- I've put back the original fuse in the fuse box: the cigarette lighter is ~12V wrt to a bolt on the car (from now on "GND") -- good.
- I remove the original fuse and put the TAP holder with both fuses in: the cigarette lighter is still ~12V wrt GND (good), but no voltage whatsoever is present on the blue end (what?!??!?!).

Hmmm.......   There is one test you did NOT do (or if you did, you didn't say).....

Here's a hint:

There is a possibility that you removed the wrong fuse from the fuse block. If you remove the fuse you previously did and check the cigarette lighter outlet and still have 12 volts, you removed the wrong fuse.

The question is: Are you sure you've picked the right slot in the fuse block?

All your head banging could be answered if the fuse position you have been using is not powered......


Edit: The more I think about this, the answer must lie in the fuse block.  You've been over every other permutation and possibility a dozen times.

If the position in the fuse block IS powered, then the last option would be that the TAP blades do not make contact with the connectors in the fuse block.  If the TAP blades are the same thickness and length as the normal fuse blades then one would not expect that - but if it were the case, it would be a really weird.