Opened VW Fuse

Matthew Technician Nova Scotia Posted   Latest  
Resolved
Electrical
2015 Volkswagen Jetta 2.0L (CBPA) 5-spd
No Crank / No Start

Hi all, I had this TDI Jetta brought in on the hook yesterday after a cold snap (-20C). Vehicle was idling in the owners driveway warming up for morning commute and instantly lost all power inside. Interviewing the customer I was able to find out the blower was on full, heated seats on, rear defrost and heated mirrors on and customer was trying to pair their phone. 

Inspection found the 80A fuse #4 in the underhood panel open. I was able to make a temporary fuse to pull the car inside. Trouble codes were a lot of low voltage codes, except for two SCR reductant heater circuit low/open faults. Faults cleared and they have not returned. 

I checked current draw through the fuse #4 circuit via amp clamp while loading the electrical system. Heated seats draw approx. 10amp, rear defrost 20A, blower motor 20A, etc. With all creature comforts on the circuit was drawing 90A at one point (not spike amperage but holding amperage) but would not pop a new 80A fuse. Commanding the SCR heater 1 & 2 showed approx. 5-10A draw each, I couldn't verify these were being turned on at the time. I attempted to run active tests for each component on that power feed circuit as well and found none to have high draw. 

The EM box main fuse is fairly expensive for what it is. The car hasn't blown a “macgyvered" fuse since in my hands, I just hate to ship it without thinking every angle. If the 80A fuse #4 feeds other fuses in the interior panel and none of those have blown am I safe to assume that maybe there were just too many loads on at once for this circuit and not just one component?

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Interesting
Erik Diagnostician
Florida
Erik
 

I looked for any updated Tech Info or TSB and there was one for the 200A fuse for a no charge condition. Apparently a voltage drop could find an issue with the threaded area being faulty, corroded or having high resistance. I’d check that before you let that guy go but even with all those loads on it should be good. Something is pulling more than it’s share of amperage. Got a thermal imager by…

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Matthew Technician
Nova Scotia
Matthew
 

Unfortunately I have not jumped the thermal imager band wagon yet, a few in “cart” reading reviews. This car had me ready to buy on the spot though.

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Obie Technician
Washington
Obie
 

Was the fuse blown ( melted) or open (cracked)? Load thru a fuse causes heat, the metal expands. As the load reduces or is removed the metal contracts. That can cause fuses to fail. It is a common failure especially with flat metal fuses. At full voltage that could be 1000 watts. If the voltage drops it may go higher. That is heat. Is the battery in good condition and the charging system up to…

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Matthew Technician
Nova Scotia
Matthew
   

The fuse in question unfortunately is “blacked” out. Made of non-transparent black plastic. Battery and charging systems check out great.

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Richard Instructor
Florida
Richard
 

I might be looking at something wrong, but this fuse layout doesn't make much sense. According to the aftermarket and OE wiring diagrams that I have access to fuse #4 shows that it feeds 22 other fuses. The thing that doesn't make sense to me is, those smaller fuses total well over twice the 80a fuse rating of fuse #4. Looking at what they power, it seems unlikely that they would all be on at…

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Matthew Technician
Nova Scotia
Matthew
 

Hi Rich, thanks for the input. I don’t think you’re looking at the diagrams wrong. Fuse #4 powers a lot inside, some of the diagrams have modules for hybrid vehicles or vehicles with stop/start engines which this does not though. Maybe this question did turn into a thermal imager discussion lol

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Dan Owner/Technician
Texas
Dan
   

… You can get a FLIR ONE for LESS than 200-250 bucks and pair it with your smart ass phone… a TG165 That I got was about 400 …a month after I got mine for 500 (figures)… I actually like the FLIR ONE better because of the LARGE screen on my android phone…and a few features it has that are better than the handheld TG165 has… not much of a reason to NOT get on the thermal imaging bandwagon… Dan H…

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Matthew Technician
Nova Scotia
Matthew
 

Noted, the Flir has been in my sights for some time I've just been hesitant to commit. Thanks for the input.

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Obie Technician
Washington
Obie
 

I don’t believe the amount of current that is possible for the circuit to be loaded with has any bearing on the size of the fuse in this case. The fuse he describes I believe is a slow blow fuse. It functions slightly over the capacity. As almost every component is turned on by a controller the internal logics would prevent all circuits from powering up together. There are many vehicles that…

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Matthew Technician
Nova Scotia
Matthew
 

I had assumed the same thing, we've seen lots of failing electrical components due to charging system issues. I mainly wanted to cross off all my options as the fuse in question is on the expensive side. I'd hate to have to face the customer if I didn't at least do my part to rule things out. 90 amp accumulated loads draw made me wonder how much further the car would push that circuit but I…

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Obie Technician
Washington
Obie
 

You do have a fun one. I will watch for the resolution.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Mathew, I have to ask if you investigated each component for appropriate Amp draw using the service information/data. Specifically, the blower motor. When you say it is drawing a total of 90 Amps, was that a cumulative figure adding numbers together or was that measure using a Fuse bubby (fused test jumper) and an Amp clamp? A common issue is for this problem is power quality phone…

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Matthew Technician
Nova Scotia
Matthew
 

Thanks for the advice Glenn, I hadn't found much in the way of amperage draw specification for what I was testing, just shooting from the hip hoping I'd see one component much higher than one would assume it would be. Unfortunately the fuse in question is black, non-transparent so I can't see what the link looks like. As for your "pay as you go" diagnostics, I couldn't agree more. The…

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Glenn Owner/Technician
Texas
Glenn
 

Hi Mathew, If this model has a fuse box mounted on top of the battery, pay special attention to the alternator power lead coming into the box. I have run into quite a few issues with them and had to replace the alternator cable assembly. A FLIR would pick that up easy once you get a chance to work with it. It takes some time to get use to them. A circuit under load will show some mild heat…

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Matthew Technician
Nova Scotia
Matthew Resolution
   

Well a couple of weeks later and the owner of this vehicle hasn’t had anymore issues this far. I will update in the future if problems come back around. Thanks everyone for the help.

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