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I retrofitted my 74 with a fuel supply pump Inertia Switch today. If you don't know, inertia switches cut power to the fuel supply pump in the event of a collision. Early 70s Alfa's did not have this safety device. Obviously is it very undesirable to be in a collision, have a fuel line breached, then have the fuel supply pump continuing to pump lots of gasoline all over the ground.

Also, for those of you who have inertia switches factory installed, I thought you might like to see what's inside of one. The used unit I bought off ebay was a little corroded and was a little erratic in allowing current to pass through the switch . . . so I disassembed it and cleaned the contacts. The contacts and magnetic ball were somewhat corroded. As hopefully you know by now, it is very imporant the the SPICA fuel supply pump receive full current from the battery . . . meaning connections and switches should be clean and tight.

The inertia switch that I got was very easy to disassemble with only a few parts. Basically it consists of a large magnetic ball bearing that is held in its seat by another magnet, then unseated by unusually high G-forces, such as in an accident. When it is unseated it breaks the contacts it the upper part of the switch. To reset, the operator must push down on the button on the top of the unit. Like I said, the unit I bought was intermittent in allowing current to pass, but after cleaning, was like new. Tests at the pump electrical leads showed full current to the pump. In wiring the inertial switch, I just spliced in the switch between the plastic single connector in the trunk. That way, if the switch ever does become inop, then it can be bypassed by just reconnecting the orginal leads in a few seconds. Here's some pictures. First is the normal operating position, then the "tripped" position, then an exploded diagram of the parts. Finally is a picture of my retrofit.

Hope this proves informative.
 

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My former '81 Spider already had a factory inertia switch installed on the firewall by the washer bottle on the passenger side... it looked rather like yours - a grey cylinder with a button on top to reset it. Later cars have a switch that looks like a black box with a rubber button on top to reset it. I can't speak for 1974, but Alfas by the mid 1970's had them standard, although when they got old and corroded, a lot ot POs bypassed them.
 

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Bill77 said:
Those are great photos showing the inside of the inertia switch. Thanks!
I second that. I have an eBay inertia switch too, and I plan to retrofit it into my car. Sounds like I should clean it first.
 

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Thanks John for another concise explanation of our Alfa's. My 76 spider has the inertia switch mounted on the passenger side behind the washer bottle. I will clean it now!
 

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I am resurrecting this ancient thread because it should probably be a sticky. I just cleaned up my inertia switch and reconnected it because of this excellent thread.

And someone might like to see one work, so I shot a little video:

 

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The S3 (& I think S4) Spiders also have a Bosch relay that cuts the fuel pumps if the engine isn't running. This is important because the inertia switch alone won't protect in low-G accidents. For example, Alfaparticle spun off into a ditch, which severed a fuel line under the car but didn't trigger the inertia switch. It can happen to you.

HOWEVER, the Bosch relay is NLA and the available replacements don't provide the pump-cutoff function. My Spider had that kind of relay now, so I added an aftermarket tach sensor that cuts the pumps when the engine isn't running. It's inexpensive and easy. See the Bosch Fuel Relay vs Inertia Crash Switch thread from 1 month ago.

Don't give away safety.

David OD
Laguna CA
 
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