What would make this block of fuses go bad?

[heavysixer]

I recently purchased a 911 with some electrical gremlins that need to be sorted out.

If you look at the attached image you'll see that all the fuses inside the red box do not light up when touched with a continuity tester, both with and without the engine running. I know that these things can be notoriously hard to debug sometimes but I am wondering if there is some sort of typical root cause I can check first?

Any help is greatly appreciated!

Best Regards,
Mark

[heavysixer]

I should say that visibly the fuses appear to be good so not really sure what else could be wrong.

[porwolf]

A volt meter and wiring schematic should find the problem.

[Justin@Athens]

I'd look for a ground wire to that subset of fuses in the fuse block. See if there is a singe grounding point for those that has come loose.

[heavysixer]

Ok good idea, will look for schematics and see if I can figure out where the ground wire runs to.

[fred cook]

Originally Posted by Justin@Athens

I'd look for a ground wire to that subset of fuses in the fuse block. See if there is a singe grounding point for those that has come loose.

The fuses on a 911 do not use a gang ground. Each fuse is fed power and then takes the current to an end load. In that group of 10 fuses, only 5 - 6 and 7 - 8 are jumpered together on the top side. 5 - 6 are for the headlight low beams and 7-8 are for the high beams. For starters, check to see if the top white wire on 6 and the top yellow wire on 7 get power when the lights are on. If not, then there is a problem in the headlight switch or the high/low beam stalk switch. The other fuses use will vary depending on the year of the car. You will need a good schematic to figure this one out! Good luck.

[timmy2]

As Fred pointed out, Porsche runs unfused power to the switch or device and then when the device is active the power runs back through a fuse to the branch circuit.
So unless the lights are turned on at the switch, you won't see power at the fuse for the branch circuits to the lights.


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[heavysixer]

Hi Fred,
You were right when i turned on the headlights a subset of the fuses became active. There are several that still don't fire but at least I got part of the mystery solved.

[porsche930dude]

without knowing the year and just looking at that I dont think you have the proper fuse size in many of those. best check before you burn something up

[McLaren-TAG]

I think it would be easier to talk about the electrical gremlins you're troubleshooting and then try to work on those. No sense spending time fixing what isn't broken like you just did.

When working electronics, it is imperative to isolate the issue. If there are multiple issues then you can begin to see if there is commonality in those like a bad general ground point.

[heavysixer]

Agreed,
I was trying to get acclimated to the 911's electrical system first and just make sure i understood the basics of it all before proceeding. But as you say don't fix what's not broken. Specifically the electrical items that I am attempting to diagnose are:
* no high beams
* no trunk light
* no license plate lights
* no working radio
* no interior lights

When I thought I saw a block of fuses not light up I figured I was onto something, but now it appears I am back to square one.

[heavysixer]

porsche930dude,
It didn't even occur to me that these might be the wrong fuses sizes. I found this image on our host and they do have different fuse sizes than mine (also a 1986 911). Do you know if this fuse layout is correct? http://forums.pelicanparts.com/uploads18/+86+911+Fuse+Panel+11290642473.jpg

Where does one find the correct size chart for this car?

[Noah930]

I had this uncommon thing happen to me once:



Could explain why simultaneously everything on that fuse block isn't working. I couldn't tell until I took all the fuses out and took the block out that it had cracked like that and couldn't maintain good springy tension across the terminals.

[fred cook]

This might help a bit. This is a list of the first 10 fuses starting from the front of the car.

1 15A Fog Light Relay
2 5A License Plate Light
3 5A Parking Lights Right
4 5A Parking Lights Left
5 8A Right Low Beam
6 8A Left Low Beam
7 8A Right High Beam
8 8A Left High Beam
9 5A Turn Signal Right Front
10 5A Turn Signal Left Front

This should be common to 84-89 Carrera 911s.

[heavysixer]

Yeah i am seeing that my fuses are most definitely wrong! I really hope it didn't fry anything, our host doesn't seem to see 5a fuses, where does everyone get their replacements from?

[porsche930dude]

most auto parts stores will have the fuses. they are german / european fuses

[jittsl]

I don't know your specific fuse box but it may be you are looking for something that should not exist. In the same way that you don't get power to the headlights until you turn on the headlights a lot of other circuits operate in the same manner. The parking lights ,for example, aren't fused until after the switch - the power goes power source (battery direct or from ignition or from Fuse box) - to switch - to fuse box - to parking lights. So simply checking for power at the fuse box without having the lights on will yield a nil result. If the list sent above is correct then I wouldn't actually expect any power at any of the ten fuses if the items being protected are switched off.

Start by identifying what the specific fuse you are looking for is on the supply line for, then switch that item on and then decide if there is something wrong with the power supply.

Since many of the items (except the radio) are powered through the light switch I would actually suspect an issue with the power supply to the switch or the switch itself. Good luck.

[Noah930]

Originally Posted by heavysixer

Yeah i am seeing that my fuses are most definitely wrong! I really hope it didn't fry anything, our host doesn't seem to see 5a fuses, where does everyone get their replacements from?

None of my FLAPS stocked 5 amp fuses, but the Napa guys were willing to order them for me. That was the easiest way to get a stash of 5 or 10 of them. Of course, since then I don't think I've burnt one of them out.

[McLaren-TAG]

That list seems a lot better to work through now. I agree, I'd work through making sure there are no blown fuses first which look good. I'd also check that the wires exiting the bottom of the fuse are attached tight, there are also fuses that have two wires that one may be lose.

One thing I will tell you is the fuse for the license plate light is a bear to find if you're not familiar with it. It is a glass fuse and it can be buried BEHIND the block where you have to unscrew the block to get to it.

Originally Posted by heavysixer

Agreed,
I was trying to get acclimated to the 911's electrical system first and just make sure i understood the basics of it all before proceeding. But as you say don't fix what's not broken. Specifically the electrical items that I am attempting to diagnose are:
* no high beams
* no trunk light
* no license plate lights
* no working radio
* no interior lights

When I thought I saw a block of fuses not light up I figured I was onto something, but now it appears I am back to square one.

[heavysixer]

Here is an update on where I am now. I ordered three bags of bullet fuses (5,8,16) and they arrived today. I decided to replace the last 10 fuses at the end of the block since they all fit nicely into the amperage range I had.

Before setting the new fuses I gently cleaned the copper terminals with a sanding block to remove the built up corrosion. However, once set my headlights and fog lamps will no longer turn on. I replaced the new fuses with the old ones but no luck. Everything is still off.

Could gently sanding the connectors have messed something up? I would have actually thought I was improving the connection.

Any help is greatly appreciated!