Deck lid ground strap- why?

[JAE]

Curious, there are two very high quality copper braided ground straps running from the hinge engine bay attachment point to the deck lid.

I have seen lesser ground straps on lightning rods. Why? There seems to be plenty of metal to metal attachment points to the lid to dissipate static charge.

Joe
'80 SC Targa

[campbellcj]

I had assumed it was for a proper ground to the license lights. But you're right - they sure are robust ground straps just for that!

[Early_S_Man]

I am quite sure that it is for RFI shielding, and that is as much black magic as science! Kind of like the old inside joke at NASA that Von Braun couldn't build a rocket without fins, rf ground straps are always braided and oversized ... just to be sure! There is logic behind the use of braid ... it increases surface area, and rf energy always travels on the outside of conductors, and 16, 14, or even 12 gauge copper wires don't have much surface area! But, a nice 10 mm wide piece of braid does!

So, just like the fins on a Saturn V booster, Porsche lids get grounding braids, even if it might be overkill, from an engineering standpoint!

------------------
Warren Hall
1973 911S Targa

[Nickshu]

If your SC has a engine bay light like mine does...its for that.

The switch for this light (lower left corner) is just a ground switch which turns the light on when it closes (becomes grounded).

Nick.

------------------
_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

[rstoll]

Hey Warren, can a 911 makes it's own lightening??

------------------
Robert Stoll
83 SC
83 944

[Leland Pate]

I have always wondered that as well...my car has no lights in the lid. And I can see that the strap goes body to lid...nothing else. Warren, what do you mean RF energy. Did Porsche have cases where mischevious engine lids were killing radio reception???

[JAE]

Nick,
The light is definitely not turned on by grounding- not only would this be really bad engineering, but the pin at the far end of the light assembly assures constand ground. The hot wire is interupted by closing the light.

The only thing I can figure is that the extremely high rpm's of the fan creates tons of static electricity that transferrs to the lid, creating a possible shock hazard.

Joe

[Leland Pate]

JAE...that makes perfect sense. I almost laughed when I read your post.

How obvious.
Very good work~!

[cds72911]

Would it be fair to assume if you have a decklid that s 100% fiberglass that you wouldn't need these ground straps?

Charlie

[wckrause]

You won't need the ground straps if you have a glass lid. They wouldn't do much good.

RF (radio frequency) energy is generated by the ignition system. Along with shielding the ignition wires, the deck lid is additional shielding. This is to prevent interference with your radios, or other people's radios. I wonder if cars have to pass some FCC EMI tests.

My neighbor had a pacemaker installed. Now he can't work on his cars, or even open the front hood and stand near a running engine because the RF energy from even a lawn mower engine can mess up his pacemaker. Maybe he should get a diesel.

------------------
Bill Krause
'79 911SC Euro

[Doug Zielke]

You're on the money, Bill. My friend's 911 has unshielded ignition wires. When his car is running (with lid open) in my shop, the radio on the workbench is full of static.

------------------
Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm

[Stinky 911T]

JAE - I disagree with your statement that grounding a light to turn it on would be "bad engineering". It is very common for all auto makers to use the body of a car for the return path for most of the circuits. The way the door switches work on my 911T (and probably on all other 911s, too) is that when the door is opened, the spring loaded switch connects the brown (ground) wire to the body to complete the cabin light circuit. When the door is closed, the switch opens the connection to ground and thereby turns off the lights.

The deck lid ground straps are to ground metal lids and reduce the amount of radio frequency electrical noise radiating from the high voltage ignition wires. They do not do any good if you have a fiberglass lid.

------------------
Bob Thayer
1971 911T
rthayer@hdrinc.com

[Roland Kunz]

Hello

The reason is called "Funkfernentstörung".

Since the 50´s all cars must pass the german DIN 59 879/VDE 0879.

I think comparable to the MIL-STD 461/462.

One thing on the 73 RS was that the cars failed the test and Porsche had many trouble geting the car street legal in germany.
To pass that the cars where officialy sold with the standard lid and then the costumer ordered the Bürzel seperatly as "Zubehör". Some Bürzel had a Steelframe and a fine mesh in the laminat but had only seen one of those at a company done the race ignition development for Bosch. In 1974 the Bürzel get a shielding sheet inside.

Radio supression is very important in europa as many bands whern´t usable for the puplic ( Well germans had to use FM for the Radio by the lack of frequencys ) and well its a small country.

The HKZ can generate very high radiation output if the system is negletet.

The groundstraps must have very low resistance and are made from OFC.

BTW Porsche Weissach is one of the leading companys in EMV

Grüsse

[wckrause]

Roland, How does the shielding work when you've got that big grill opening. Are the grills conductive?

------------------
Bill Krause
'79 911SC Euro

[wckrause]

oops

[This message has been edited by wckrause (edited 06-07-2001).]

[Early_S_Man]

I suspected that it was the VDE (equivalent to the US FCC) requirements, and I expect most countries now have similar regs! On GM cars with metal bodies and plastic hoods (Camaro, for example, aluminum foil is applied to the underside of the hood, and that foil is grounded by a couple of braided copper straps! The RS 2.7 could have used such a fix, too!

No, the grill is not a problem on most models of 911, because underneath the bars of the grille lies a nice mesh of expanded aluminum, quite effective at frequencies under 10 GHz or so!

And, Robert, if you doubt that there is lightning under the hood of a 911, just look at the warning sticker on the coil. And, talk to anyone who has grabbed a Beru connector and wiggled it to find out where a mysterious miss was coming from, only to find out what 50KV feels like traveling through the body from hand down to kneecap or so while becoming the return path for said ignition impulses!

------------------
Warren Hall
1973 911S Targa

[wckrause]

I was thinking of the plastic grills on the newer cars. Are they conductive?

------------------
Bill Krause
'79 911SC Euro

[rstoll]

Yeah Warren, 50K is better than Ex-Lax! Just thinking about that beautiful Saturn 5 making it's own lightening.

------------------
Robert Stoll
83 SC
83 944

[Nickshu]

Just went out and looked at mine. You're right...its not for the light. The light has a wired ground from the car body itself. Interestingly, the hood light operated as a ground as I described before...but not the rear decklid light

Sorry for the stupid assumption.

Nick.

------------------
_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

[pbs911]

I thought those were straps to hold the deck lid close to the car in a crash. LOL Don't all race cars have parts secured by wire?
----------------
Paul
78SC Targa