What would make this block of fuses go bad?
 

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,
Markhttp://forums.pelicanparts.com/uploa...1412969865.jpg

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

A volt meter and wiring schematic should find the problem.

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.

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

Porsche fuse panel
 

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.

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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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.

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

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.

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.

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?

I had this uncommon thing happen to me once:

http://i5.photobucket.com/albums/y18...0/DSC_0773.jpg

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.

1986 Carrera Fuses
 

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.

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?

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

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.

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.

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.

A series of unfortunate events
 

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!

http://forums.pelicanparts.com/uploa...1413549155.jpg

Heavysixer, I am guessing the fuses you just bought are plastic in the core. Those are OK for short term use but they are junk. The fuse gets hot and the plastic gets soft and will squish.

Try a local VW dealer. They often have the ceramic core proper fuses. They are silly how expensive they are but they are worth using.

If you stay with the plastic fuses, keep a LOT of spares in the car.

@86911Targa thank you for the wiring diagram I will double check that I have the right fuses in place.

@GH85Carrera, actually I bought all ceramic fuses here is an example:
Amazon.com: Install Bay GBC8-25 8 AMP CERAMIC Fuse 25 PACK: Industrial & Scientific

They seem to be as good as the Bussman fuses that you can buy at the auto supply store.

I tested several of the fuses for continuity and they pass, and even swapping the old fuses back still doesn't make the headlights turn on.

I have zero experience testing electrical systems so I am wondering if:
1. Lightly sanding the contacts caused an error
2. I managed to blow something else like a relay?
3. My car hates me

I noticed that with the headlights in the on position the top of the fuses block is not hot according to the test light. I would assume it would be. This seems to be true for all of the fuses that control the high and low beam circuits. I do see the interior gauges light up though so I think that means the knob itself is functioning.

Thanks everyone for the help in debugging this!

The issue you have is a lack of source voltage, NOT existing fuse capacity, even if the wrong fuses are installed.

Fuses are merely current sensing elements of a circuit. If there is continuity through the fuse, it should either work to conduct circuit current or "blow" (create an open circuit) if circuit current exceeds design specs.

Use the factory electrical schematic to search and identify the source of battery voltage common to all the affected fuses. Many are related to the headlight switch and ignition switch.

Sherwood

fuse panel.
 

http://forums.pelicanparts.com/uploa...1413557961.jpg
^^^^^
For our '86.

pm me as needed.

Good luck,

Gerry

this is the first thing that popped into my mind .... a cracked fuse block will be a random pain in the ass, sometimes everything works , breath on it wrong or poke at it and then half of the stuff in the car stops working ...if more than one circuit in the car is giving you random issues check the back of the fuse blocks for cracks....and like others have said after you fix the main issue swap out those plastic fuses

Are those fuses using aluminum for the conductor? If so you likely will find the aluminum will oxidize and cause problems in the long run.

It is a pain to find ceramic core copper or brass conductor fuses. Anything else will cause problems eventually.

I tend to keep my cars for a long time. I have had my 911 for 19 years and before that a 914 for 21 years. Trust me, if you uses anything but the ceramic and copper good fuses you will be cussing at yourself down the road. BTDT.

Electrical problems never seem to happen at a convenient time.

Sherwood I am starting to agree with you. Considering the top of the fuse block is not hot I do think there is a power problem.

I tried swapping the horn relay for the fog lamp relay and the horn continued to work, so I don't think it is the relay itself.

The headlight knob turns on the interior gauge lights so I think the headlight knob also works.

I am looking for a wiring diagram now to see where the source of the voltage originates from.

Just to clarify for others reading this thread, the fuses I bough are ceramic and brass, even though the amazon listing appears to show aluminum. I even scratched the surface of one of the fuses to confirm it was not just a light coating of brass on the surface of the metal. It appears to be solid all the way through.

Additionally, I did a visual inspection of the block itself i unscrewed it from the body of the car and felt around behind it and did not feel any breaks. I could disconnect the entire thing but I don't want to break something else before I've tried to track down the lack of voltage. If I can't figure anything else out I will resort to unwiring everything and inspecting the block.

Thanks again everyone for the great images and schematics you have posted it is really helpful!

Ok so I'm an idiot (or at least mostly an idiot). As it turns out either the high beams or the low beams don't work and when the stalk is set to that setting inside the car you get no headlights AND no fog lights. I am not sure if that is normal or not though, and could be symptomatic of a larger problem.

Should you be able to run your fog lamps in either high or low beam settings (seems like you should)?

I haven't yet experienced a burned out Lo-Hi beam column switch, but I believe those are the symptoms. Do you have aftermarket headlamps and/or high wattage fog lamps (e.g. H1, H4)?

Sherwood

May be the perfect time to replace the orig fuseblocks with the updated fuseblock panel kit...... which I and many 911 owners have done....which uses the modern two-prong push in fuses found in all current cars.

You can search this forum and see Fred Cook's panel. You can also get the headlight relays add on package with the new panel.

It's not that difficult to do.....if you are careful and take your time doing it

Regards,
JT

Respectfully not agree. Faulty fuse block? No problem. However, it's not wise to install a new fuse panel just to end up with the same symptoms. If you have to sublet repairs to a pro, they will either decline the job if an unknown aftermarket part was installed on top of the malfunction or charge you additional time to 1) confirm the upgrade was equivalent to factory spec and 2) to find and repair the original symptom.

Repair the malfunction first, then upgrade if desired.
Got a copy of the factory schematic yet? If so, grab a pencil and trace the path from the battery to the fuse block or headlight switch (I don't recall the exact current path). The headlight circuit should work with the ignition ON or OFF. Have you tried to simply tighten the circuit wire hold-down screws at each fuse position?

In my early 911, fuse no. 1 provides source voltage to the ignition switch which energizes all ignition switch-related circuits. If an open circuit occurs here, the entire ignition, fuel pump and cranking circuits are dead.

Sherwood

Unless changed from the factory design, the fogs will only work on low beam. There should be a blue/white wire going from the bottom of fuse 6 that activates the fog light relay when the low beams are on and the fog light switch is pulled on. The ignition switch also has to be in the run position.

Fred thank you for the info! That explains why it works only with the high beam switch disengaged. I am going to try and take off the steering wheel and see if the high beam light is shorted now!

First things first............
 

I agree with Sherwood. Fix the problems before changing anything you don't have to!
Unless there is a fault in the original fuse blocks, you should be able to get everything working. Good luck!