78SC - Fuse Amperage on Rear Box ???

[acapella8]

Can anyone confirm the fuse amperage ratings on the rear fuse panel/box on a 78SC?

The thing shipped with all 3 being 25A, but I'm just trying to confirm that is correct. Been having some strange things happen when my brake lights and turn signals are both active with headlights on.

Thanks.

-Tom

[Jim Sims]

These fuses likely have nothing to do with your lighting problem; for a car of the vintage of yours the fuses are typically for the rear engine heater blower motor and the rear window defroster circuits. Check the owners manual for the proper fuse ratings.

Your light problem could be due to someone installing single contact base bulbs in the turn signals sockets instead of the correct double contact base bulbs. Check the bulbs. Jim

[acapella8]

Thanks Jim -- the bulbs are proper type.

The original owners manual has 30+ pages stuck together and unreadable -- I'm ready to anteup the $40 that everyone seems to want for same.

The topmost fuse definitley controls the brake lights, as it was busted on the previous piece and now works fine since replacing.

[PorscheGuy79]

Tom,

According to the Bently manual Souk loaned to me that engine compartment fuse box that you have a picture of there contains only

1. 25amp fuse for Rear Window defogger and wiper
2. 25amp fuse for Heater Blower
3. 5amp Fuse for fan relay.

So according to this your last fuse should be a 5amp and those fuses should have nothing to do with your lights, those should be found in the front trunk. We can take a look tonight.

[acapella8]

Sounds like a plan, am looking forward to it!

2 Pelicaners meeting at my house around 6pm-8:30pm.

Any others interested? I might have to get more beer.

[Mysterytrain]

Ok, I'm seeing this differently in my Bentley. On the 78/79SC the brake light fuse is fuse number 11 in the front trunk and it is rated at 16 amps. The three fuses on the rear engine relay panel are as follows. Number 1, the top fuse is 5 amps and is for the heater fan relay [relay coil]. Number 2 [the fuse in the middle] is 25 amps and is for the heater fan [the heater fan in the rear of the car. This is the 12v 'hot at all times' power source from the front of the car, goes thru the fuse and then to the switched contact of the heater fan relay.] Number 3, the last fuse, the one on the bottom is also rated at 25 amps and is for the Defogger/ Rear window wiper and like the fuse above it is connected to the 12v hot at all times source. The Bentley also has an error. On page 970-20 the schematic looks like its labeled incorrectly but really what they are doing is extending the front fuse position numbering to the rear. Since the front panel holds 21 fuses..they are calling the top fuse in the rear panel 22 and then others, 23 and 24. So, if you are looking at page 970-20 fuse 22 is the top fuse in the rear panel and bentley has it rated incorrectly at 25 amps [ should be 5 amps, 25 is bit beefy for a relay coil.] Both of these 'hot at all times' connections make a nice point to grab power when you are using your timing light.
Sounds like someone got into your harness and has mucked things up. You need to go thru the wiring and correct it. Bad wiring, melts and burns..not good!

[acapella8]

Ron - thanks for the advice. This is looking more complicated than I thought....

[acapella8]

So -- according to the Bentley for my year, in order from top to bottom (looking from above) would be:

5 A
25 A
25 A

That is -- IF it is wired to spec with the OEM pathways in the manual.

[sagacis]

You might verify grounding throughout the car. If the electrical ground has to take a route through some odd connections to complete the circuit, wierd things can happen.

[Mysterytrain]

acapella..that is correct. I just went out to the parking lot to verify this on my 78SC.

[Mysterytrain]

Also, after looking at your picture again..those black wires look suspect. I'll see if I can take some photos of my fuse box and post them.

[acapella8]

Many thanks Ron, this is much appreciated....

I do not know what is more difficult than tracing a bad ground through many yards of old wires in an older vehicle.

Add to it -- my wiring might have been "customized" due to the adding of an after market alarm etc.

-Tom