August 2001
911 HOOD LATCH ALARM
By Steve Grosekemper
The
other morning I got a call from one of my customers (Carl Scragg) who had just
purchased a 1984 911 Carrera. Naturally, I assumed he was calling to make an
appointment to bring this car in for track set-up. Unfortunately he was calling
for body shop referrals. You see, instead of telling me stories of how incredible
the new car was, he was telling the story of what it is like to have the hood
fly open while traveling at a high rate of speed. For details, see Carl's re-enactment
here.
At
this point I pondered whether or not I should tell him about the front hood
latch alarms that I have installed on a few cars. This is the one thing that
could have avoided the crumpling of his precious Porsche sheet metal. Well,
I took my chances and let him in on the secret. After a rundown of how and why
this calamity occurs Carl assured me that the first stop after the body shop
would be to me for a front hood latch alarm.
THE
HOW AND WHY-
A
911 trunk can become unlatched in many ways. The first and most common reason
is that the trunk is not properly closed before driving. The second most common
reason is that the release cable and/or latch will get out of adjustment and
release the hood while driving on rough roads, or under heavy cornering. If
this does happen there is a safety latch which is intended to keep the hood
from flying open. Unfortunately this safety latch does not always work as intended.
They are often so bent and abused from years of service that one good bump in
the road can pop the hood right open. If you think you can stop the car before
the hood becomes part of your sunroof, think again. If the safety latch fails
and the wind catches the hood at freeway speed, it will take about 2.3 nanoseconds
for the summer landscape out your windshield to turn into a wall of painted
sheet metal, usually followed by an excess of high pitched screaming. I have
seen this result in damage to the hood, cowl, windshield and roof panel, so
this cannot only be inconvenient and frightening, but expensive as well.
HOW
TO AVOID DEPLOYMENT IN THE FIRST PLACE-
The
key to avoiding this catastrophe is to know exactly when the hood becomes unlatched.
A longtime customer of mine and a pretty bright guy, Jack Cooper come up with
the idea of installing a push-button grounding switch near the fuse box of his
'74 911 which activated the oil light to warn of impending hood deployment.
While this is a very good and simple idea, I have made a few refinements to
it, resulting in a safe, professional and reliable warning system.
So
what are our goals for this warning system? The alarm must provide both visual
and audio warnings so there would be no chance of missing the signal, even with
the radio on and the sun's glare in your eyes. The warning system must also
be single purpose so there is no confusion as to what the emergency is. The
system must come on only while the car is running, so the key can be on with
the trunk open and not activate the system. (For instance when cleaning the
car with the radio on and trunk open).
HOW
IT WORKS-
The system I have come up with is a conglomeration of systems used by Porsche in various models. To power up the system I use a relay, which is activated by the voltage output from the alternator. This makes the system operational only after the engine has been started and the charging system light goes out. After the engine is running and the relay is powered up voltage is present at the warning light and buzzer. If the trunk then opens the contact switch provides a ground and completes the electrical circuit. The driver is then bombarded with visual as well as audible warning to his situation. Believe me, this is definitely preferable to the hood in your face and screaming alternative. At this point the driver can safely stop the car and close the trunk lid.
(See
electrical wiring diagram figure #1) .
SYSTEM INSTALLATION-
PARTS LIST:
Bosch relay kit, part # WR-1
Porsche indicator light, part # 477.919.209B
Buzzer (Napa/Echlin) part # BZ6562
Ground switch (Napa/Echlin) part # DJ6300
Assorted
electrical wire and connectors.
FIRST
DISCONNECT THE BATTERY
1-
Pull all the gauges out of their holes with the exception of the tachometer.
I
use a Bosch starter relay kit (part # WR-1) because it is the cheapest way to
get a relay along with the necessary mounting plug, connectors and wiring harness.
2-
Leave the #87 wire full length. Cut the remaining 3 wires to 6 inch lengths
and remove the provided fuse holder. Attach piggyback spade connectors to the
shortened #86 and #30 wires.
3-
Attach the #86 piggyback connector to the blue wire on the charging system light
(located in the oil gauge). Attach the #30 piggyback connector to the red/black
wire on the other side of the charging system light. If space is limited this
wire can also be connected at the back of either oil gauge, at the (+) terminal,
also a red/black wire.
4-
Attach the #85 connector to an open ground tab at the back of the gauges.
5-
Run the long #87 wire behind the tachometer to where the speedometer lives and
replace the oil and fuel gauges.
The
power side of our project is now complete and our next step is to install the
buzzer, warning light and ground switch.
6- Open the hood and drill a 1/4" pilot hole 6 inches from the centerline of the front trunk latch, towards the drivers side. You will go through one layer of plastic and then one layer of sheet metal. Now remove the long black plastic cover concealing the hood latch. It is held in place with four spring clips accessed from the inside of the trunk. Re-drill the hole in the sheet metal to fit the grounding switch
(See figure #2).
Washers may be needed to space the switch up for proper activation (this may vary from car to car). Pre-1974 cars may require a slightly different installation process. Increase the 1/4" hole in the plastic cover to 1/2" and reinstall. The 1/2" hole will just fit around the hex nut of the switch making a perfectly flush mount
(See figure #3).
Install
a bullet type electrical connector to a wire, connect this wire to the switch
and run it up next to the #87 wire behind the speedometer.
7-
For the warning light we use a factory oxygen sensor light. Because we don't
want to confuse the issue, we will remove the "OXS" white letter stamping
from this light. To do this we put a small amount of acetone on one shop towel
and water on another shop towel. With the first towel lying on a flat surface
rub the face of the light on the towel for a few seconds. The acetone will melt
away the top layer of the red plastic switch and the lettering in SECONDS! Now
rub the light on the water soaked towel to neutralize the process. Be quick
and careful during this process or you may end up with a piece of sticky melted
plastic!
8-
On 1974 and later 911's there is a hole in the dash that this light will fit
into. It is located between the tachometer and the speedometer. Push on the
vinyl covering to find the hole. Slice a small cross in the vinyl with a razor
blade and then use a rat tail file to trim the edges of the vinyl. This will
make a perfectly matched hole in the vinyl. '75-83 cars will have an emission
control light in this place. You can either replace it with the warning light
or drill a matching hole on the other side of the tachometer. On pre-'74 cars
you can just drill a hole in this same location.
9- Attach two 12-inch wires to the warning light and install it in the new dash location. Attach one of these wires to the #87 wire coming from the relay using a flat screw connector. Attach the remaining wire to the trunk switch wire using the same type connector. Now attach the flat screw connectors to the clamping screws of the buzzer. In order to gain the greatest effect from the buzzer we want to give it a solid mount. The best place for this is behind the speedometer on the wiper motor mounting plate
(See figure #4).
10-
Reinstall all of the gauges. Reconnect the battery, and start the car. When
you pull the trunk release you should get a loud alarm accompanied by a bright
red light. This should be the case before the safety latch is released!
Don't forget to check the trunk release and safety latch for proper operation
and adjustment. Now that your new safety system is installed, lets hope that
you never see or hear it operate while you are driving your newly improved Porsche.
Good Luck
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