[Jazzbass]

Note: Here is another info post on Porsche electrics I posted on Pelican a couple years ago. Since I'm reposting, might as go all the way since no one gets tired of lengthy, EE geek posts .

One of the things that irritates me about 911s is the fact that the electrical system blows. Circuits that aren't fused, full headlight current run through the turn signal switch, bad grounds, and fuse boxes designed in 1940.

So I'm in Pep Boys today buying some 30W oil for the lawn mower, and I see this for $4:



Not the bandsaw, but the ATO/ATC fuse block. 4 fuses, 80A. The new block actually has mounting tabs which have been cut off in this picture (hence the bandsaw).

My original idea was to use this as a part of the main fuse box rewire. However, at the time I originally did this, my car was undergoing major surgery, and I just couldn't add one more this to the list. So designing, building and installing a new front fuse box gets shelved. But I have this fuse block - what to do? Hmmm... maybe replace the engine compartment fuse block. So I start with this:




I hate those fuses - really. So I pull the whole assembly, remove the relays and the old fuse block. Now, time to install the new one, hopefully in a way that makes it look the way Porsche should have done it around 1979.

Step 1 - Mark the chassis to cut out a hole for the new fuse block.


Step 2- Cut it out with a jig saw and lots of filing. It's aluminum, so it cuts and files nice and easy like.


Step 3 - Fab up some brackets to hold the new block. These are made from 1/8" angle aluminum I got from Home Depot for $5. To make it so the fuse block could slide in and out easily (since the block will be wired directly into the car), I used #6 countersunk machine screws to attach the brackets to the chassis.


Step 4 - Install the block. I used 2 #6 machine screws in the side of the fuse block to bolt it to the chassis.


And its done!


It was really pretty easy - the whole process took about an hour (not counting trips to Pep Boys and Home Depot). The contacts for the fuse box are what they call "open barrel" type, which crimp on with a special crimping tool. These are the type of crimps used in all OEM wiring.

BTW, this rewire only works for Carreras, since they don't have any of the CIS or MFI relays on the top of the fuse box like the earlier cars do.

[Trak Ratt]

I'm very impressed.
Now can you help me with the rewiring of the SC for the '84 3.2/EFI? Only really need the Geek part (wiring diagrams, pin assignments, etc). I don't think I could alford the Per Diem for a trip down here!

[Jazzbass]

Originally Posted by david riley

I'm very impressed.
Now can you help me with the rewiring of the SC for the '84 3.2/EFI? Only really need the Geek part (wiring diagrams, pin assignments, etc). I don't think I could alford the Per Diem for a trip down here!

Yes, I can and no, you couldn't . You're just installing Motronic? That's actually pretty easy. I even have most of the system captured in Visio at the moment.

[Trak Ratt]

Supper! Looks like the Motronic shifts over in mass. Has one "hot" wire to the front fuses. Big hold up is the engine compartment plugs. MarcA did it in his '77 but I think the SC's pin assignments are even different from his. I have the diagrams for both but then you will likely have them after tomorrow's mail

[Jazzbass]

Originally Posted by david riley

Supper! Looks like the Motronic shifts over in mass. Has one "hot" wire to the front fuses. Big hold up is the engine compartment plugs. MarcA did it in his '77 but I think the SC's pin assignments are even different from his. I have the diagrams for both but then you will likely have them after tomorrow's mail

Actually got them today - thanks!

Personally, I'd suggest using new connectors instead of trying to splice into the old ones. This is what they did on TDs cars and it was a clusterf**k. If you order the right connectors, I'll come down when everything is ready and crimp everything up.

Funny thing is, I now know more about the Motronic in a 944 than in a 911 . Oh, the shame. But, assuming they are 95% the same, there will be 2 main power wires to the DME from the front of the car - a large blue/red main power wires straight from the battery and switched +12 from the ignition switch. That switched +12 is what I'm running through the dash mounted kill switch in TD's car. Everything else runs backwards to the engine.

[Trak Ratt]

Originally Posted by Jazzbass

Actually got them today - thanks!

Personally, I'd suggest using new connectors instead of trying to splice into the old ones. This is what they did on TDs cars and it was a clusterf**k. If you order the right connectors, I'll come down when everything is ready and crimp everything up.

I have the complete "donor" car out back. But can buy all new if you think that is the way to go?

[MatP]

Great info guys. I am definitely impressed.

Thanks!

[hoophead]

x2 on hating the bullet fuses. A couple of days ago I touched my fuel pump fuse, and the metal strip on it literally disintegrated on my finger. That would have really blown (heh) if it happened somewhere other than my garage.

If you've ever measured the voltage drop across any of these fuses that are carrying significant power, it's like a volt. That just sucks. You can get your headlights to be noticeably brighter by using a higher rated fuse (and better yet with a relay).

I've noticed that there are two flavors of fuses: the copper metal versions and the version with some other silver/gray metal (tin?). I think what happens is every time you run the car and use that circuit, the fuse heats up and expands. After many, many cycles of heat/cool, the metal gets stretched and builds up even more resistance. I've noticed that probably half of my fuses have metal strips that are bowing out from the plastic part...

The down side of the ATO and mini-ATO fuses (like you've used), is it's difficult to tell when they've blown. Of course they make some nifty ones that have a built-in LED that lights up when the fuse is good (and the circuit is active).

[hoophead]

You forgot my favorite 911 electrical system complaint: running full solenoid power through the ignition switch, with a total voltage drop over 12 feet or so of wire...brilliant. It's ironic that the best fix for the problem involves a part from Ford.

BTW I love the new fuse box. I'm surprised nobody sells a whole-car kit that does the same thing, like they do for some of the older American cars.

[Jase007]

It's ironic that the best fix for the problem involves a part from Ford.

I'll bite ... what chu talkin 'bout Willis?