Really weird fuse circuit

[Josh G]

I had something really weird happen the other day that made me a lot less careless when checking fuses.

I was having some electrical issues in one of my cars. I tested a fuse with a logic probe and saw that it lit red on both sides. However, I suspected that fuse so after checking all fuses I went back with a multimeter and found that it was actually blown and reading 12V on one leg and 5V on the other leg (which was enough to light the probe). What a pain if I hadn’t caught that (new fuse fixed the problem the 5V was normal). Multimeters do work good for fuses but when checking continuity if you are on a ground side fuse you will probably have continuity even if the fuse is blown. So I would say DVM is the way to go and just pull ground side fuses and check them out of box. I know checking fuses is simple, but has anyone else run into this kind of thing…if so, it can really through off your trouble shooting.

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[Bill]

I have been made a fool of before myself. I think when you press the test leads onto the fuse it pushes on the broken trace inside and it makes slight contact.

Now I remove the suspect fuse for visual inspection.

[Kevin Criswell]

When you say “ground side” are you meaning the “load”side?

Not trying to be a smarty, but there really should not be a “ground side” to a fuse in that the fuse should be inline with the hot conductor.

[Josh G]

[quote=”Raistian77″ post=77677]When you say “ground side” are you meaning the “load”side?

Not trying to be a smarty, but there really should not be a “ground side” to a fuse in that the fuse should be inline with the hot conductor.[/quote]

I mean that the fuse is on a ground not a hot…I know it’s weird but I have a car were the headlight fuses read green on both sides when probed and show 12V from fuse to batt + with a DVM. I’m going to examine the wiring diagram to see why that is…but I sure hope I’m not seeing things :whistle:

[Kevin Criswell]

[quote=”JayMann07″ post=77711][quote=”Raistian77″ post=77677]When you say “ground side” are you meaning the “load”side?

Not trying to be a smarty, but there really should not be a “ground side” to a fuse in that the fuse should be inline with the hot conductor.[/quote]

I mean that the fuse is on a ground not a hot…I know it’s weird but I have a car were the headlight fuses read green on both sides when probed and show 12V from fuse to batt + with a DVM. I’m going to examine the wiring diagram to see why that is…but I sure hope I’m not seeing things :whistle:[/quote]

Oh, okay I get it.

That is because you are seeing the ground potential through the light filament and to the fuse, that is normal. On most cars when the lights are turned off the fuse WILL show ground and no voltage till you activate the lights.

You will see this on motors also, if you pull the main fuse for a cooling fan motor, on one side you will see ground, that is because the motor is grounded on the other side of it’s windings. Now if you do a resistance test on that ground side you will see the resistance from the windings or the filament on a lighting system.

[Josh G]

That thought had crossed my mind, but I’m still confused about the behavior of this circuit…here’s a diagram

http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=64820
PS the fuses are the two 15A at top center

The fuse is reading green at all times (key on & off, fuse in & out, HL on & off)except it turns red when brights are on…pulling the fuse does nothing except it causes the headlight to go out when the brights are activated (i.e. disables brights)

What are your thoughts on that & when a car has fuses like this should I be using a DVM and just forget the probe?

[Kevin Criswell]

[quote=”JayMann07″ post=77724]That thought had crossed my mind, but I’m still confused about the behavior of this circuit…here’s a diagram

http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=64820
PS the fuses are the two 15A at top center

The fuse is reading green at all times (key on & off, fuse in & out, HL on & off)except it turns red when brights are on…pulling the fuse does nothing except it causes the headlight to go out when the brights are activated (i.e. disables brights)

What are your thoughts on that & when a car has fuses like this should I be using a DVM and just forget the probe?[/quote]

Okay, the way yours works is that the headlights receive 12 volts at all times the relay it turned on.

The light switch turns the relay on by grounding the control circuit for it. Than 12 volts are sent to the headlight fuses and then on to the headlights. The headlights are grounded through the headlight switch, depending on which wire is grounded determines if the high or low filament is energized.

Okay if you turn the headlights on and you get 12 volts to both 15 amp fuses than you know the relay is working fine.

the c-12 combination tells you which terminals are bridged together in which positions.

in park the taillight wire (not shown on your diagram) is grounded to the EL terminal which is the ground wire (white with black).

in headlight the taillight wire and headlight wire are grounded turning on the headlight relay, if the headlights are on low beam the HL terminal is grounded, energizing the low filament.

if the headlight switch is on high the HU terminal is grounded energizing the high beam filament.

You are going to need to unplug the switch and check for continuity with a DVOM to test it.

[Josh G]

Oh man I was pulling my hair out trying to understand that wiring diagram :S …the diagram works exactly like you said, but it wasn’t matching up with what I was seeing when looking at my vehicle and the result of pulling certain fuses.

Then I looked more closely at my service manual and noticed that there was a diagram for headlights for Canadian models and for some reason that diagram matched my vehicle and showed certain relays I found that weren’t in the first diagram.

Now I can see that with the highbeams off the 15A fuses are dead and do in fact run through a resistor and then to ground, with the highbeams on the 15A fuses are energized and a relay bypasses the resistor and sends the current straight to ground.

Shew the right diagram sure does make a difference hahaha 😆

[Kevin Criswell]

Yeah, if you have the Canadian version, you have daytime running lights, which completely changes the way it is wired.

[Josh G]

[quote=”Raistian77″ post=77776]Yeah, if you have the Canadian version, you have daytime running lights, which completely changes the way it is wired.[/quote]

I have no idea how you knew that, but that’s exactly right 🙂 …it does have daytime running lights and that does seem to change the wiring a lot

[Kevin Criswell]

[quote=”JayMann07″ post=77826][quote=”Raistian77″ post=77776]Yeah, if you have the Canadian version, you have daytime running lights, which completely changes the way it is wired.[/quote]

I have no idea how you knew that, but that’s exactly right 🙂 …it does have daytime running lights and that does seem to change the wiring a lot[/quote]

Canadians adopted DRLs long before we did, so I got used to dealing with 2 separate headlight wiring diagrams.

[EricTheCarGuy]

I think DRL’s were required on all vehicles sold in Canada somewhere around the turn of the century. Don’t quote me on that though. Since that time you’ll see more than one diagram for headlight circuits depending on where the vehicle was sold.

[Author]

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