Fuse protect DME?

[Solamar]

Wrapping up the final details for a 3.2 swap into my '78SC. I believe I have read everything available on the subject (multiple times), but (being a dense Neanderthal) I'll still come up with questions...

All of the write-ups take power directly from the +Battery terminal to pin 30 on the DME relay. Any reason NOT to power the DME off an empty, constant battery power fuse location?


I was thinking of using fuse 19 (headlight washer 25 amp) as it is constant power, bus 30, and unused.

[Solamar]

Well, after exhaustive research (quick Google search and discussion with my Dad, retired EE) I see no material problem with running the dme off a fused connection.

But, all research points to the same conclusion. Run the dme power straight off the battery.

Why? Mojo crazed Gremlins might knock a fuse out or introduce inexplicable random frequency interference to the dme rendering the car a one way suicide machine.

So, in an abundance of caution I will follow the path well trodden and run power directly to the battery.

Was that so tough?

[Quicksilver]

I've always held that when they made the 911 all the German engineers signed up for the exciting engineering tasks: engines, transmissions, suspension, brakes, etc.
... And they ran out of engineers so they went and hired some from Lucas to do the electrical.

The Porsche system of running power from the source, into the car and to the switches, back to the frunk to the circuit protection (fuses), and then finally to the load... Is insane and is an inefficient waste of wire.

The battery is right next to the fuse panel. The obvious and safe way to design the system is to run the power directly to the fuse panel, then to the control switches, and then to the load. That gives you the shortest unprotected run of wire possible and for loads in the rear of the car it also gives you the shortest total wire run.
The stock system is just flawed.
_________________________
In my car I added a circuit for various accessories under the passenger seat: dash cams, radar detector, chargers, power inverter, etc. That circuit runs straight from the battery, to a fuse, and then into the car to a terminal block for distribution.
(I also have a row of relays protected by circuit breakers for lights and whatnot.)

End result? DEFINITELY put a fuse in the power for the DME. There is no valid reason not to and some very expensive reasons why you should.

[86 911 Targa]

I have to agree (as an EE myself), that fusing for our 3.2 is,
to say the least, a bit interesting.

!. The front condenser is not fused.

2. The dash lights are not fused.

3. The DME is not fused.

4. the A/C fuse (#2) is overworked.

5. Headlight relays.

On & On.

I did post "added fuses and relays" a while back.
which resolved all of the above issues.

[Duc Hunter]

I wonder if someone like Kroon makes a fully fused harness for our cars? Or just the “Factory” way....which seems rather “Lucas” of Porsche.

[911pcars]

Recommend fusing the DME. A meltdown is not pretty. Many ways to fuse it. A straight connection to the battery post creates an issue of "spaghetti" wiring and a neat method of clamping numerous wires (audio amp +, etc.) in a confined space.

The nearby fuse box is right there. If you want direct battery voltage, branch a 14 gauge wire from the source (battery) side of the fuse box (e.g. sunroof or other unused accessory circuit) to a dedicated inline fuse holder to the DME wire. Instead of using a typical factory ceramic or glass fuse, use a modern ATO fuse holder and 20A (?) fuse, perhaps the latest type that has a integral light to visually confirm the current path is intact. Then label and mount near the fuse box for easier monitoring. Carry spare fuses, but don't use larger fuses to mask an electrical issue.

Sherwood

[Solamar]

Originally Posted by Quicksilver

I've always held that when they made the 911 all the German engineers signed up for the exciting engineering tasks: engines, transmissions, suspension, brakes, etc.
... And they ran out of engineers so they went and hired some from Lucas to do the electrical.

The Porsche system of running power from the source, into the car and to the switches, back to the frunk to the circuit protection (fuses), and then finally to the load... Is insane and is an inefficient waste of wire.

The battery is right next to the fuse panel. The obvious and safe way to design the system is to run the power directly to the fuse panel, then to the control switches, and then to the load. That gives you the shortest unprotected run of wire possible and for loads in the rear of the car it also gives you the shortest total wire run.
The stock system is just flawed.
_________________________
In my car I added a circuit for various accessories under the passenger seat: dash cams, radar detector, chargers, power inverter, etc. That circuit runs straight from the battery, to a fuse, and then into the car to a terminal block for distribution.
(I also have a row of relays protected by circuit breakers for lights and whatnot.)

End result? DEFINITELY put a fuse in the power for the DME. There is no valid reason not to and some very expensive reasons why you should.

I like your logic!

My first wrenching experience was on my Dad's '52 MG TD. That led me to a very 'educational' experience of owning three Triumphs, just to keep one on the road.

Lucas jokes make the memory slightly less painful

[Solamar]

I'm glad common sense has prevailed!

I like Sherwood's idea of using a modern fuse holder. I can see the exposed ceramic fuses as less the optimum for DME reliability.