Melting fuse block
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!
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.
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.
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.
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.
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
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.
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…
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.
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…
Try using an amp clamp and see what you find.
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
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.
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…
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.
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
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!
Did you check why the pids were hi ? If so what were they ?
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.