Melting fuse block

Chad Mobile Technician Eustis, Florida Posted   Latest  
Resolved
Electrical
2001 Volkswagen Beetle GLS 1.8L (APH) 4-spd (01M) — 3VWCD21C11M465937
Stall

I’ve changed many of these fuse blocks on top of the battery. Never investigated anything unfortunately. This customer had a melted block. It was the second one this year and she does not drive far when she does drive. First time I’ve seen this car so I replaced it To start diagnosis. With my autel I noticed the generator pid is often 100% at idle. The battery is good. The alternator cable has about 60 millivolts(.060)of drop upon start up. At operating temperature with hood up I notice 170 millivolt(.170) drop. And the cable was 150+ degrees. Hot to the touch and the terminal end at fuse black ”will” burn you if you touch for 5+ seconds. AllData does not have good info for this system and this is the only service info I have. Nor do I have a charging system tester so I’m using my labscope. Shopstream and I are not getting along at the moment. The fist capture is alternator full load at idle. 500ms screen, looked ok to me. The next capture is full load at idle and I believe 20ms screen. Looks good to me. Third capture is full load at 1500 rpm. Good IMO. Something odd I noticed is when I was key on engine off using scan tool there was some noise on alternator cable. This is the last capture. For those of you who are not absolutely lost due to my rambling. Do you think I made the right call saying the alternator is good but that the cable needs to be replaced and is the underlying issue burning up the fuse block. Thanks guys! 

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Steve Instructor
Irvine, California
Steve Default
 

Most of those fuse blocks melt because of high current and inho low quality plastic Check the in rush current, fix what ever is high.

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Stephen Owner/Technician
Orange, Texas
Stephen Default
 

We used to see this problem all the time. Replacement of the alternator cable always fixed the problem for me. Also use a good quality battery fuse box replacement part.

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Helpful
Jason Technician
Arlington Heights, Illinois
Jason Default
 

New cable and Quality new fuse box And your good to go. I say quality because I’ve seen some really crap ones sold on the market.

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Steven Instructor
LaGrange, Georgia
Steven Default
 

We had one of our teachers VW doing the same thing. We replaced with a larger cable and the temp dropped. Notice the temp of the wire for alternator.

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Helpful
Agree
George Owner/Technician
Mobile, Alabama
George Default
   

Is the Idle speed at specs at Idle ? The PID seems high unless you had a heavy load on....regulator voltage should be 14.5v

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Chad Mobile Technician
Eustis, Florida
Chad Default
 

Agreed. PID is what had my attention. Only at idle tho. % would decrease to 70% at 1500rpm. I had no specs to reference. Trying to fix the cause, I was concerned if it was being told to full charge incorrectly. That being said I didn’t see 14.5v. I was curious if the cable was causing alternator to work harder... I will be replacing cable and updating.

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George Owner/Technician
Mobile, Alabama
George Default
   

At 70% that seems still high if the battery was fully charged....Also short trips even with a good alternator and good wiring eventually runs the battery down & heats up the wiring, It keeps the alternator maxed out if the trips are not long enough to recharge the battery This is just something to know.......... Some mfg at one time put a spec on the alternator (50/120A 13.5V ) the 50…

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Matt Technician
Gastonia, North Carolina
Matt Default
 

Chad If you have a battery load tester. Voltage drop the wire from the Alternator to the Fuse. Use the load tester to max out the alternator load. I’m willing to bet you’ll see some high numbers. It seems as VW did not crimp that lug very well. The good news is VW can get the alternator harness.

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

You did a voltage drop of the cables but what is the actual output of the alternator?Sound as if the alternator might be full fielding itself and overcharging​.​Since full fielding alternators used to be part of charging system diagnosing tears ago when vehicles had external voltage regulators,alternators will also do it by itself if the voltage regulator is shorting itself…

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Victor Owner/Technician
Doral, Florida
Victor Default
 

Try using an amp clamp and see what you find.

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Chad Mobile Technician
Eustis, Florida
Chad Default
 

I was using a clamp. With the snap on clamp, 100mv/1amp. So these captures are everything on, full load. It has a remanufactured Bosch alternator. I wasn’t able to cross reference but i think it’s rated at 120 amps

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Michael Owner/Technician
Montague, New Jersey
Michael Default
 

Replace the alternator to fuse box harness. Replace the fuse box. If it has an aftermarket alternator replace it unless it's a Bosch or Valeo unit.

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Mike Educator
Raleigh, North Carolina
Mike Default
 

VW does seem to have some issues with the fuse blocks. As a general rule of thumb a localized hot spot in an electrical circuit is always the result of excessive resistance. I am willing to be that if you loaded the battery down and did a voltage drop across the alternator cable to the fuse block that you will find a decent drop. I would think that the alternator was commanded to 100% because…

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Agree
Tim Owner/Technician
Wilmington, North Carolina
Tim Default
 

I have done 2 of these in the last month. Both cars required the same parts to fix it. We replaced the melted fuse box and upgraded the alternator cable. I fabricate my own cables and solder the lugs on. P.S. Had to also replace alternator on one of the vehicles due to the voltage drop taking it out.

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Chad Mobile Technician
Eustis, Florida
Chad Update
 

I am going to be fabricating a cable for the alternator. Factory size is 4 gauge and I’m going to use 2 gauge. I will be doing this Thursday or Friday and will post my results. Thank you everyone for your input

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Chad Mobile Technician
Eustis, Florida
Chad Resolution
 

Today I replaced the cable. Yes I did use 2 gauge instead of 4. This being said, 100% for sure a fix. The temperature of the cable and box were 110 Fahrenheit. That’s after idling to operating temperature and then a test drive. Prior to repair temps were above 150* just idling with hood open outside in a driveway which is well ventilated. Thanks everyone for your input. Great community!

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George Owner/Technician
Mobile, Alabama
George Default
 

Did you check why the pids were hi ? If so what were they ?

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Chad Mobile Technician
Eustis, Florida
Chad Default
 

George I was unable to check the pids yesterday. Snap on did not offer pid and autel was dead. Also my amp clap would not fit around the 2 gauge wire lol so I was unable to capture any wave form. However I watched voltage and it stayed about 14.1-14.3.

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