Let's all learn how to debug a GAUGE fuse blown due to a short circuit

[vegas]

The proper fix of course is to replace the capacitor with a new one with the same capacitance and voltage tolerances. Here's a closeup of the markings on the capacitor:

So this is a 47 uF 35V tolerant electrolytic capacitor. It's likely an aluminum one but the inner material doesn't really matter in this case for reasons I don't want to explain. Here's a good replacement: https://www.mouser.com/ProductDetail/Vishay-BC-Components/MAL203870479E3?qs=G99oyVYclWVNTUIdkadjAg==

And look, it's only $0.43.

 

[vegas]

If any of you guys are ever in Vegas, you should stop by the Las Vegas hacker space called The Syn Shop. I learned this electronics theory from playing around with circuits on nights where everybody solders up some electronics kits.

 

[South Texas 97]

That is an amazing and inspiring piece of diagnostics. Thank You for sharing your work.

 

[jht3]

It is pin #9. This is a female connector and there are only 5 pins on the the row with the catch:

View attachment 3013570

Here's how to count them:

View attachment 3013571

So here it is numbered:

View attachment 3013573

It doesn't matter though because you'll notice each pin in the picture is labeled as "B" which means in the wiring diagrams every J4 just has a B wire going in and a B wire going out. So there is no way to know which circuit goes to which pin in J4. I worked it out manually last night and I posted it above but I think there is no guarantee that your truck matches mine.

you are still counting incorrectly on the bottom row. numbering should go from low->high as you read left->right on BOTH rows.

but if you've chased the wires with your DMM and it DOES indeed terminate in the A/C amplifier....then i don't know what to say. hopefully when you've re-capped that board, the short goes away.

i have plenty of experience replacing components on vintage amps, speakers and vehicle ECU's from the 80's that I can see you have the experience to work on that board in quick order. i have enjoyed following your deep dive into the truck.

 

[vegas]

you are still counting incorrectly on the bottom row. numbering should go from low->high as you read left->right on BOTH rows.

Oh, you're right, it is pin 10. I just took a closer look at the note on page 5 of the EWD. I was looking at the arrows and not the numbers in the note

So it is pin 10. It really doesn't matter though because the junctions label all pins as "A" or "B" and so knowing the pin number doesn't really help because the wiring diagrams for each circuit just show "A" or "B" so there is no way of knowing which wire in a junction goes with which circuit. The only thing you do know is the color of the wires correspond to the letters. For J4 for instance, all "B" wires are yellow and all "A" wires are green. I still had to Ohm out each circuit to map pins to circuits. Let me correct my chart then and make edits to previous posts identifying the error.

And them my list of which pin is for which circuit becomes:

J4, 11 - IF1, 2
J4, 9 - IH1, 6
J4, 12 - IG1, 1
J4, 13 - R4, 5 rear heater switch
J4, 4 - A10, 7 AC system amplifier
J4, 10 - C16, 4 cooling fan

I apologize for my thick skull slowing down the message on this one. You're right about the numbering.

 

[vegas]

But it turns out there is a hidden and undocumented splice that changes this:

J4, 10 - C16, 4 cooling fan, also A/C Air Amplifier and two other places I haven't followed the wires to yet.

 

[jht3]

But it turns out there is a hidden and undocumented splice that changes this:

J4, 10 - C16, 4 cooling fan, also A/C Air Amplifier and two other places I haven't followed the wires to yet.

!@#$

keep up the progress. i'm still trying to work out why my headlight auto-off feature isn't working right even though the wiring tests pass AND I swapped in a different relay. there are a couple similar threads on the subject....with no resolution by the thread starter. i haven't found the energy to tear apart the lower dash and post my fix (hopefully!!!)

 

[vegas]

!@#$

keep up the progress. i'm still trying to work out why my headlight auto-off feature isn't working right even though the wiring tests pass AND I swapped in a different relay. there are a couple similar threads on the subject....with no resolution by the thread starter. i haven't found the energy to tear apart the lower dash and post my fix (hopefully!!!)

That sound super frustrating. Dang.

 

[vegas]

Well, s***. Now that the sun is up and nap time is over, I decided to dive in deeper. One thing I forgot to mention last night is that there is no guarantee that the first filtering cap is the short. I got ahead of myself late last night. In theory, anything on this board could be shorting, its just that the capacitors are the most likely culprit. You don't really know until you desolder the capacitors and test them and keep checking the board.

So...I did just that. I desoldered that first cap and there's still a short from hot to ground. I tested the capacitor after removing it and it works just fine. I could--and I will--desolder all of the caps on this board and check each one but I don't have time for that right now. My deadline is approaching and I need this truck back on the road. I decided to shell out for a known good, used unit from cruiserparts. The saga continues. I'm also now worried that my sleuthing on tracing the short was inadequate as well. Today I'm going to put my truck back together enough to convince myself that without the A/C Air Amplifier plugged in that the short is indeed gone. Wish me luck. Be sure to sacrifice and consume a beer or two in honor of the truck gods for me.

 

[vegas]

aaaaand, of course just after a I order the new part and spend the hefty amount of money, I followed the hot traces to the second board and found a hefty diode that was next in the circuit. I desoldered it and sure enough it was actually the short:

It's hard to take pictures and test parts at the same time. But there you go. All zeros shows an internal short. Normally diodes will read a moderate amount of resistance, say a few hundred kilo-Ohms with the leads in one direction and mega-Ohms with the leads in the other direction. A reading of very low resistance or no resistance is a sign that the part has failed and that's what we see here.

Now to check the circuit board to see if the short is gone:

And there you go, the short circuit is gone. No more continuity from the hot lead to the ground. Now to order a replacement part and resolder.

 

[vegas]

Here's the photo from before desoldering the capacitor and the diode.I'm testing the same points on the board as I did in the "after" shot above.

I just wanted to show that there was a short before desoldering and then there isn't one after desoldering the capacitor and diode.

 

[vegas]

So to be clear, the failed part is a diode which regulates power and is also a common point of failure in electronics. If you ever have a spinning hard disk die because the platters fail to spin up, a quick trick is to find the power diode and desolder it to remove the short circuit. The drive will spin up and work long enough for you to get your data off.

 

[vegas]

Based on the markings on the circuit board and the fact that the cathode is connected to hot, I'm 99% sure that this is a zener diode used to provide a reference voltage for this part of the circuit.

The next question is, based on the markings on the diode, what are its specifications so that I can find a replacement? The markings say "27 620 C". I was searching the internet for all of the secret decoder rings and nothing is coming up. Here are the markings:

My best guess at this point is this is a 27V max (reverse?) voltage, 6.2V zener voltage diode. But maybe not. Does anybody else have any ideas on finding a replacement part? Does anybody know how to decode these markings?

 

[vegas]

It can't be 6.2V zener voltage. The input to the cathode on this part is straight from the GAUGE fuse, so 12-14V. If the zener voltage is 27V then it wouldn't ever kick in as a voltage regulator. I have no clue what the specs are on this part. The only way to really know is for somebody with a perfectly good working one to desolder it and test it with a zener tester to figure out what its zener voltage is.

 

[george_tlc]

It quite likely is a MOV/TVS to protect from spikes etc. Especially since they show it as a Zener on the silkscreen which is the same as what would be used by the PCB layout guy for a MOV/TVS. Unidirectional based on the silkscreen and the fact the original has a band on it. It's big to handle fairly large avalanche events without dying - though it did die in this case

cheers,
george.

 

[vegas]

It quite likely is a MOV/TVS to protect from spikes etc. Especially since they show it as a Zener on the silkscreen which is the same as what would be used by the PCB layout guy for a MOV/TVS. Unidirectional based on the silkscreen and the fact the original has a band on it. It's big to handle fairly large avalanche events without dying - though it did die in this case

cheers,
george.

I hope you're right. Given the markings, will you help me find a replacement part? It sounds like the zener diode symbol on the board sent me in the wrong direction.

 

[george_tlc]

TVS/MOV are common for auto electronic to protect the front end. I use them in my designs for the exact same reason. Just find a MOV/TVS that avalanches at around 25V or higher and you should be fine. They aren't "precision" devices

cheers,
george.

 

[vegas]

I just looked up MOV varistor and those look like ceramic capacitors. TVS diodes look like this part.

 

[george_tlc]

Yeah, TVS diode is what that device is highly likely to be. Just find something in the 25V-27V range and similar in size, they usually are rated at several hundred watts (short duration spike energy).

cheers,
george.

 

[vegas]

Yeah, TVS diode is what that device is highly likely to be. Just find something in the 25V-27V range and similar in size, they usually are rated at several hundred watts (short duration spike energy).

cheers,
george.

So something like this? Sorry, I guess we can't post amazon links in here. Search amazon for: "NTE Electronics NTE4934 Transient Overvoltage Suppressor Unidirectional Diode, Surge Clamping, Axial Lead, 1500W, 27.05V Breakdown Voltage"