[Shop-talk] Finding a Blown Fuse (Automotive)

[Pat Horne]

Eric, there is a device that is used mainly in telecom/networking that 
will help you find the problem. Check out the Klein vdv500-820. It's in 
the $65 range, but there are cheaper ones down below $20 from china. The 
Klein unit usually has a stronger signal than the Chinese units but 
since you probably won't have much use for it in the future, so the 
cheaper ones may be fine for you. There are two alligator clips on it. 
Make sure all the cargo lights are off & connect one clip to the wire 
that is dead & the other one to ground. You should be able to follow the 
wire back to the problem. For the tone generator to work well, the 
circuit needs to have no load on the circuit. By this I mean all the 
lights & other loads like the TPMS booster need to be disconnected. The 
cable tester contains a continuity tester mode so you can verify that 
all the loads have been removed.

Do a search for the Klein unit on Amazon. There is a video there that 
shows how the unit is used, but not in depth.

How are you verifying that the fuses are good? If you are just looking 
at them, they may still be bad or making a poor connection. Use a volt 
meter & measure voltage on both ends of the fuses to verify them.

I can walk you through the troubleshooting if you need help.

Peace,

Pat

On 11/10/2021 6:15 PM, Eric Russell wrote:
> Oh wise & beneficent Shop Talkers - I've run into a situation that has 
> me stymied. I need help locating a blown fuse in a motorhome.
>
> We recently bought a new (to us) motorhome. When installing the signal 
> booster for a TPMS (Tire Pressure Monitoring System) I 
> accidentally shorted a 12V wire in the cargo bay lights. There was a 
> small spark (followed by a few swear words). I then went on a hunt for 
> the blown fuse. There are three fuse panels in this motorhome. Two in 
> the Ford chassis (F-53) - one under the hood and the other inside near 
> the parking brake - plus one in the motorhome's 120V - 12V power 
> converter. I could not find a blown fuse anywhere.
>
> I contacted the manufacturer (Thor) who has actually been quite 
> responsive & has tried to help. After asking for my vehicle's VIN they 
> emailed me some schematics (not actual wire diagrams but more a map 
> showing the general location of outlets, switches, etc with 
> indications of which circuit they belong to). There are two schematics 
> that appear to apply to my motorhome - one indicates the cargo lights 
> are connected to the Ford panel by the parking brake, another 
> indicates they are in circuit #6 of the converter's 12V panel. Neither 
> of those fuses are blown. While there I checked all the fuses I could 
> locate - none were blown. I've also signed up to a couple of motorhome 
> forums but have not been able to find an answer there.
>
> Finally - my plea for help - is there any device one can use to trace 
> where a circuit originates? I've seen an electrician use a device to 
> find out what outlet goes to which breaker in our house.
>
> I've followed the hot wire from the cargo bay light back to where it 
> enters a wire harness along the motorhome's frame rail. I'm loath to 
> rip open the wire harness trying to chase the wire further upstream. 
> As far as I can tell there is nothing else in this circuit - 
> everything else is working properly. Of course I can live without 
> cargo bay lights but it bothers my OCD to have it not working (when I 
> know it did before my screw up).
>
> -- 
> Eric Russell
> Mebane, NC
>
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