Fuse question (what is this for? Empty on a schematic)

[ChuckN]

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On my fuse box, the empty slot next to “D” in the schematic has a 30A fuse in it. What would that be for? There’s something up with my temp sensor wiring and when I tried to ground the wire out to check for “hot” the 30A fuse blew. I’m thinking that whoever swapped in the engine before me has a wrong wire hooked to the temp sensor. Here is a picture of my fuse box.

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[fast 99]

Can't answer what that fuse is for but get that 30a out of there. It's a fire waiting to start. Put in a lower amperage until you figure it out.

Just went out a looked at a 81 K20 it's got a different configuration.

 

[ChuckN]

Thanks! Will do.

 

[CalSgt]

My ‘80 K10 doesn’t even have contacts in that slot

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[Bextreme04]

Thanks! Will do.

Those open spots are places to pick off power. Some of them are constant power. Some are ignition switched, and some of them are pick-off points of those fused circuits. Grounding anything on a fuse panel is a terrible idea. It blew that fuse because you effectively shorted that circuit to ground.

D is for the radio circuit and should have a 10A fuse.

The temp sensor gets power from the cluster wiring through the headlight switch. If any of your other gauges are working, it could only be a power problem from the printed circuit on the back of the cluster. After power goes to the gauge it comes out of it and runs in a single wire all the way to the temp sensor in the drivers side cylinder head. The sensor provides a variable resistance to ground based on the temperature of the sensor. That variance in resistance is what makes the sensor move. If it pegs all the way hot, it likely has the wire from the gauge to the sensor shorted to ground somewhere. If the gauge pegs all the way cold, it is likely unplugged or you have no power to the gauge.

 

[ChuckN]

Those open spots are places to pick off power. Some of them are constant power. Some are ignition switched, and some of them are pick-off points of those fused circuits. Grounding anything on a fuse panel is a terrible idea. It blew that fuse because you effectively shorted that circuit to ground.

D is for the radio circuit and should have a 10A fuse.

The temp sensor gets power from the cluster wiring through the headlight switch. If any of your other gauges are working, it could only be a power problem from the printed circuit on the back of the cluster. After power goes to the gauge it comes out of it and runs in a single wire all the way to the temp sensor in the drivers side cylinder head. The sensor provides a variable resistance to ground based on the temperature of the sensor. That variance in resistance is what makes the sensor move. If it pegs all the way hot, it likely has the wire from the gauge to the sensor shorted to ground somewhere. If the gauge pegs all the way cold, it is likely unplugged or you have no power to the gauge.

Thanks for your reply! So, something is definitely up. And I'm suspecting it's not good at all.

I pulled that 30A out just to leave it a blank, and guess what- I have no gauges at all. So, I'm guessing that there could be something seriously wrong with the wiring and somebody bodged it by using that circuit to route power through as a major artery, but that's obviously just a guess. Or is there any other sensible explanation that you would have?

Sheesh, instead of that suspension kit, maybe I should be shopping for a wiring harness this morning

 

[Rickf]

Why not replace the 30 amp fuse with a 10 amp or less circuit breaker? If the gauges work without tripping the breaker then you're good to go, plus it's easier and cheaper than a harness.

 

[Bextreme04]

before we start messing with too much... what is the actual problem you are trying to solve? It seems odd that someone would do a "bodge job" by putting in a whole new fuse circuit and wiring it into the fuse panel. Like was suggested above, replace the 30A fuse with a resettable CB in a slightly smaller rating and see what happens. There were a lot of different fuse panel layouts, and the one you are referencing might not be the right one.

 

[ChuckN]

Why not replace the 30 amp fuse with a 10 amp or less circuit breaker? If the gauges work without tripping the breaker then you're good to go, plus it's easier and cheaper than a harness.

I hear what you're saying and I will do that as a temporary measure- I guess the bigger question that bothers me is why would the gauges (and who knows what else) rely on an accessory slot in the fuse box that isn't normally used?

Soon I'll be putting in the tach kit from LMC, so it seems like it would be the obvious time to do a harness anyway, since I'll already be in there, and the wiring is now 40+ years old.

 

[ChuckN]

There were a lot of different fuse panel layouts, and the one you are referencing might not be the right one.

Well you might be on to something there. I was under the impression that these trucks were mostly wired in the same way. Maybe there are other variations out there.

I did find it interesting that the 30a fuse blew instead of the 5a instrument panel fuse when I tested the temp sensor wire to ground.

 

[Bextreme04]

If you have the time and money, it isn't a bad idea. I've thought about doing it too, but I'm replacing the entire engine harness anyways when I do my swap.. hopefully next month. My 67 Riviera is a basketcase of wiring and has the old glass fuses, so I decided to just get a whole new painless kit. It's a nice quality kit and was about $500 for the entire car harness kit.

 

[Bextreme04]

Well you might be on to something there. I was under the impression that these trucks were mostly wired in the same way. Maybe there are other variations out there.

I did find it interesting that the 30a fuse blew instead of the 5a instrument panel fuse when I tested the temp sensor wire to ground.

It wouldn't be the first time someone just found a circuit with power and spliced it into another circuit that didn't have power. You might want to pull the cluster and see what the wiring looks like back there, it isn't a big job to pull the dash and cluster out to see what it looks like back there.

 

[Bextreme04]

Shorting the signal wire SHOULD NOT blow power to the cluster BTW... unless the gauge itself is shorted internally.

You still haven't actually described the issue you were trying to solve that started this...

 

[ChuckN]

Shorting the signal wire SHOULD NOT blow power to the cluster BTW... unless the gauge itself is shorted internally.

You still haven't actually described the issue you were trying to solve that started this...

Ok, so here it goes, I had another thread on it, but it started with the temp gauge always being pinned on Hot.

After that, long story short, I removed the "temp sensor" wire to the intake manifold to test the gauge and see if it would go to "cold" and then grounded it to the intake manifold to see if it would go to "Hot".

It blew that 30a fuse, instead of the instrument panel fuse and then I lost all of the gauges entirely.

So I pulled apart the wire loom since the color wasn't correct for the temp sensor. I found the green wire for the actual sensor buried in the loom, unused, with no power to it (checked both with test light and voltmeter).

The wire that is hooked up to the temp sensor has to be the incorrect one.

I bought the truck with a half-done SBC swap, the truck was originally a straight six. And more or less, that's where I'm at.

 

[ChuckN]

And yes, I do have the 30a removed, just have to go to the parts store today to get a 10a because of course....I have a 5, 15, 20 and a 25 in my drawer. Lol.