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

Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
I was thinking this morning that the short could be between J4 and the gauges/switches or between J4 and J7, IH1, and EA1. That’s easy enough to figure out while J4 is pulled apart. As I pull the gauge connectors and the other connectors apart, all I have to do is test for continuity between the downstream connector and ground. I do that by probing the correct pin in the downstream connector. The EWD has the pins listed in the circuit diagrams.

I just wanted to point out the possibility so it doesn’t get overlooked.
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
So it's not J1. Let's go find J4. I really had to rip the dash apart to get to this one. It is almost impossible to reach.

View attachment 3011669

Hello there, J4. I see yellow wires. This is straight back behind the right side of the instrument cluster, behind the air vent. It's really really hard to get to, let alone pull it apart. With both J1 and J4 pulled apart, the short circuit went away. I reassembled J1 and it was still gone, so I know it is a downstream circuit of J4.
What bugs me is that the EWD doesn’t identify which pins in J4 feed which downstream circuit. If it did, I could prove each one at J4 and know right away which circuit has a problem and make this search go quicker.
 
Joined
Jan 20, 2013
Messages
1,692
Location
Bay Area, Ca.
Maybe I missed it but did you try unplugging IH1? That one feeds the main engine harness, so also low hanging fruit.
it's the big square connector above the glove box, iirc.
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
Maybe I missed it but did you try unplugging IH1? That one feeds the main engine harness, so also low hanging fruit.
it's the big square connector above the glove box, iirc.
I haven’t yet. That’s tonight. In an earlier post I listed J7, IH1 and EA1 as the key junctions to figure out which circuit it is.
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
I pulled the dash apart. Discovered that the short goes away when I pull apart J7, but not when I pull apart the other two. So it’s got to be one of the circuits that goes through J7.

Just for s***s and giggles, I probed all of the Pins in J4 to see if I can find one that is grounded. There’s only one:
7C5730FA-9853-4D9B-ADE2-47EF9CE8B90E.jpeg


Now I’m trying to decide if that is how it is supposed to be or not. It’s a green wire and not connected to any of the yellow wires coming from the gauge fuse. I *think* this may be a red herring.

I think my best bet is to see which J7 pins are grounded. There should only be one: the wire that goes to the ground point.
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
Oops, I had J7 apart when I was probing the yellow wires. After putting J7 back together I found a yellow wire that is shorted:
AF7BE264-350D-4E50-86E8-5D8EB0F83FBD.jpeg
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
EDIT: This message is INCORRECT on the numbering of the pins. I was counting wrong. The correction is later in this thread here: Let's all learn how to debug a GAUGE fuse blown due to a short circuit - https://forum.ih8mud.com/threads/lets-all-learn-how-to-debug-a-gauge-fuse-blown-due-to-a-short-circuit.1283459/post-14451189

Looking at the pin numbers, I *think* I'm looking for a yellow wire that hits J4 as pin 9 because the retainer tab is on the bottom of the connector in this photo. Given the way I have the numbers in the photo, the yellow wires in J4 are numbers 4 - 10.
J4_grounded_pin_numbered.jpeg
 
Last edited:
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
The only problem is that the EWD labels all of the pins in J4 as "B" and doesn't make any distinctions. So now I need to go hit all of the connectors downstream of J4 to see which one corresponds with pin 9 in J4. :bang:
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
EDIT: The numbering of the J4 pins in this message is INCORRECT. The correction is later in this thread, here: Let's all learn how to debug a GAUGE fuse blown due to a short circuit - https://forum.ih8mud.com/threads/lets-all-learn-how-to-debug-a-gauge-fuse-blown-due-to-a-short-circuit.1283459/post-14451189

I was mapped out all of the pins on J4 to where they go:

J4, 8 - IF1, 2
J4, 10 - IH1, 6
J4, 7 - IG1, 1
J4, 6 - R4, 5 rear heater switch
J4, 4 - A10, 7 AC system amplifier
J4, 9 - C16, 4 cooling fan

BOOM!! That’s it. The stupid cooling fan hot lead. Testing it with ground also has conductivity. So now I just need to pull the bundle apart and find how/where it is grounded.

F41DAC27-8837-4F9E-802A-7A7DE1F56331.jpeg


All zeros on my meter means the two points are connected. :rofl:
 
Last edited:
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
Well, I traced the wire back to behind the steering column and then I couldn't reach it anymore. I'm about 50% sure that it goes into a bundle of wires going to the...hrm...cruise control ecu? what is the "Denso" white-ish box bolted up under the dash, to the right of the steering column above the driver's right foot? I think it goes there. I'll unbolt that and unplug it and look for the wire there and test for continuity. If that's the case then I think the cruise control ecu is bad and has a short inside. I think I'm getting really close to having this electrical problem resolved.
 
Joined
Jan 5, 2017
Messages
3,638
Location
Charlotte, NC & Alexandria, VA
What bugs me is that the EWD doesn’t identify which pins in J4 feed which downstream circuit. If it did, I could prove each one at J4 and know right away which circuit has a problem and make this search go quicker.
EDIT:
just saw your post #52. I feel your pain.

...and what's worse, there are further undocumented splices within the cabling between the connectors. I'm going through the engine harness right now and have found six splices between the ECU and firewall bundle so far, and I'm not even in to the "engine" connectors yet!
 
Joined
Jan 5, 2017
Messages
3,638
Location
Charlotte, NC & Alexandria, VA
Has anybody heard of the cruise control ecu shorting out?
Yes, it does happen. Unfortunately, I haven't seen any documentation on the ECU itself; since it's a Denso "problem", the school solution is to replace the unit and not try to fix it (or even identify it internaly to/within the ECU) - Toyota considered that a supplier problem to solve, and never allowed for Denso to actually solve it.

I do have a spare, if you want to try swapping parts. Not the correct, scientific method, obviously, but hey....
 
Joined
Jul 24, 2006
Messages
195
Location
Fairfax, VA
Looking at the pin numbers, I *think* I'm looking for a yellow wire that hits J4 as pin 9 because the retainer tab is on the bottom of the connector in this photo. Given the way I have the numbers in the photo, the yellow wires in J4 are numbers 4 - 10.
View attachment 3012592
you sure that isn't pin 10 you are pointing at? the way i read the EWD, pin 6 should be under pin 1.

which if i'm reading the EWD properly again, sends you through the park/neutral position switch. it just so happens to interact w/ the trailer socket, connector BQ1, etc when in R.
 
Last edited:
  • Like
Reactions: ppc

ppc

M Go Blue
Joined
Aug 18, 2003
Messages
3,780
Location
Nashville, TN
you sure that isn't pin 10 you are pointing at? the way i read the EWD, pin 6 should be under pin 1.

which if i'm reading the EWD properly again, sends you through the park/neutral position switch. it just so happens to interact w/ the trailer socket, connector BQ1, etc when in R.

Yes, as @jht3 has indicated, in post 48 the prob is pointing to pin 10 in the 14 pin connector. Even if a connector does not have a terminal in it it still counts as a pin position.
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
EDIT: This information is INCORRECT. I was misreading the note in the EWD (look at the numbers, not the arrows) and smugly correcting everybody with incorrect information. :bang: My apologies. The correction is later in this thread here: Let's all learn how to debug a GAUGE fuse blown due to a short circuit - https://forum.ih8mud.com/threads/lets-all-learn-how-to-debug-a-gauge-fuse-blown-due-to-a-short-circuit.1283459/post-14451189

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

j4.png


Here's how to count them:

pin_numbers.png


So here it is numbered:

j4_numbered.png


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.
 
Last edited:
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
Alrighty. So I found where the wire went. It goes to the Air Amplifier unit:

air_amplifier.jpeg


And I know the wire isn't shorted because probing from here to ground shows no continuity. Probing here to the wire up top does. That tells me that the problem is in the air amplifier unit itself. Given the age of the truck, I'm guessing that a diode or an electrolytic capacitor has failed. Let's take a look....
 
Joined
May 7, 2017
Messages
308
Location
Las Vegas, NV
Ok, cracking this open and getting it up in the clamp. First let's identify the input pin. We know that the yellow wire's job was to bring 12V power to this computer.

air_amp_hot.jpeg
air_amp_hot_bottom.jpeg


So we now know that that pad is hot. That makes sense because look at the thick trace that goes from there up and round to lots of other areas on the board. *Usually* the ground is very close to the hot because in every circuit board I've ever seen, there's some form of capacitor connecting the hot to the ground to filter out noise and to smooth out the power to the circuit. For those of you that don't know, capacitors act like little batteries that get charged and then release the charge when voltage between the hot and ground drops. That ultimately smooths out the voltage over time and is why they are sometimes called "filtering caps" because they filter out noise. One way to double check is to look for marking on the circuit board that identify the ground or hot side of a filtering cap. Sure enough there is one here. (NOTE: This mark on the board is really to document that the capacitor has polarity and that the ground pin for the capacitor must connect to that trace. If there isn't a mark on the board, you can tell which pin is ground because the capacitor will have a white stripe nearest the ground pin of the capacitor. If there is no stripe then the capacitor doesn't have polarity and you'll have to do other tests to figure out which trace is ground on the circuit board.)

air_amp_ground_mark.jpeg


So, there is an electrolytic capacitor immediately adjacent to our hot lead. Electrolytic capacitors typically fail at around 20-30 years of age. If you ever see a dead video game console at a thrift store, buy it, replace the one electrolytic capacitor near the battery leads and 9 times out of 10 the machine comes right back to life. I'm willing to bet that is exactly what's going on here. Capacitors fail in one of two ways, either they "ground" or "gap". When they "ground" the hot and ground connect inside and it becomes a direct connection between the two causing a short circuit. When they "gap", the opposite occurs and the capacitor stops working but doesn't cause a short. We know that the hot lead has a direct path to ground so double or nothing, this capacitor has grounded inside. Here's the ground lead on the capacitor on the other side of the board, we know that is ground.

air_amp_ground_bottom.jpeg


Now let's check to see if the capacitor has grounded inside:

air_amp_power_cap_uhoh.jpeg


Uh oh. All zeros means that the capacitor has grounded inside and THAT my friends is where the short circuit is. (EDIT: not necessarily, keep reading, the next morning I realized I got ahead of myself with that statement.) After all of that work, it was just a failed electrolytic capacitor. Looking online, this air amplifier unit is a pricey piece of kit. $500-$600. This electrolytic capacitor is $0.25. It's important to note that in *theory* I could just desolder this capacitor and everything would work. If you're stuck in the outback grab a poisonous snake and use its teeth to just nip this bugger right off. That will get you home with A/C and working windows and gauges. This cap just smooths out voltage and it's likely that removing this cap won't add so much noise that the digital parts of this circuit cannot function. I'm not sure though, I've seen computers that are super sensitive to noise and if any one capacitor is missing, it just won't work. I'm willing to bet the boys at 'yota added a little redundancy.
 
Last edited:

Users who are viewing this thread

Top Bottom