Leon Speed

The aftermarket alarm system (which I want to keep) triggers when opening the hood by contact to ground. This requires a normally-closed hood switch, which the alarm guys installed.



I want to put the original hood switch back in, but this is a normally-open switch. Is there a way to add the normally-closed functionality to the original hood switch? Maybe using a relay, but which one and how? Thanks for any tips, my electric knowledge is very basic.

Podguy

Sure use a relay. Most of the 928 relays have an NO and a NC position. Hook the relay coil to hot and then to the NO switch. This will cause the relay to open when the hood is closed and close when the hood is open. Then simply pick the pair of contacts you want to reverse the signal.

This can probably be done electronically with solid state stuff but I am too long out of that business to know what is on the market today. If you are confused just draw a little diagram and check your logic before hooking things up.

depami

I assume you are saying the switch is closed when the hood is closed. If so, I personally wouldn’t use a relay as the coil will be energized most always. Of course if your car, like mine, spends 90% of its time with the hood open, this may not be a problem.

I would also be cautious about connecting anything else to the alarm system wires. It could cause unwanted behavior.

Your best solution would be a DPDT (double pole, double throw) switch to have N/C and N/O connections and keep the two circuits isolated. The chances of finding a DPDT that will fit are slim but probably better than finding a 928 with no electrical issues.

A mercury switch would work but it would have to be attached to the hood. IIRC they were used in some cars as hood light switches years ago (at least in the US). They can also be found in wall thermostats but with much smaller wires. You should be able to have N/C or N/O just by how you mount it and it can easily switch the current directly.

Hope this helps,

Denny

Leon Speed

Thanks for the excellent ideas. I will do some more research to find the best solution.

Alan

The stock hood switch is normally closed.

This terminology describes the switch - not the application. when the switch is at rest dissconected from the car it is closed circuit - only when operated (plunger pushed) is it open circuit.

In the context of the car the hood is normally closed - so the switch plunger will be pressed & will be open circuit and when the hood is operated (open) the switch will be closed circuit. Again however this is not a description of the switch.

However the switch does exactly what you want it to do. On the later models it even drectly switches the ground side of the hood light. On your year it connects only to the fan controller & alarm - but you can connect your alarm directly to the Brown/Green wire. In fact this is already a connection at the Stock alarm controller connector - so you don't even need to run the wire...

BTW why is the switch out - did they remove it? - that was a mistake - it also controls the safety interlock for the fan operation.

Don't ever let the morons butcher your car - they usually don't care - but even if they do - they have ABSOLUTELY no idea what they are doing... Just ask here first (also only remove the stock alarm as a last resort - it is better than most that ever get put in to replace it).

Alan

dr bob

I'm regulary re-amazed at what aftermarket installers come up with for solutions to relatively simple challenges. That picture of the switch reminds me of the time a wild African feces flinger got loose in the alarm installation bay...

Podguy

The way I was suggesting the relay would only be engaged when the hood is up. It is simply a matter of selecting the right wiring set up. Isolation can be done with a diode.

Leon Speed

So it is!

How did I ever come up with the idea it is normally open??? I have been asking for a solution to a problem that doesn't exist....The stock hood switch will do fine without any modding.

They replaced it with the one in the picture. Why I don't know, at the time I was just getting to learn about my car. They did attach the brown/green wire for the fan safety feature.

After a few mishaps with mechanics I decided a long time ago to do everything myself. And thanks to Rennlist I can Luckily the stock alarm system is still untouched - as far as I can tell it still functions.

Yes, the quality does show, doesn't it.


It is a good idea but I diodn't realize I won't need it... If the fan controller signal interferes with the aftermarket alarm signal it might be a good idea to separate the two circuits though.

Thanks everybody, another "problem" solved (sofar).

depami

Alan is correct but what I have found in my 30+ years of working in the electronics field is that the most installers, many mechanics and even some electricians don’t know this. They apply N/C and N/O to the application not the switch itself.

The aftermarket installer is likely one who refers to the application and not the switch itself. It could be that the alarm wants a normally open switch (closes when hood closed) so that cutting the wire to it would set off the alarm.

All I’m saying is, verify what is actually there before you change anything.

Good luck and let us know what you end up doing.

depami

I just got off a call with an Otis Elevator mechanic in TX on this very topic. He had "Dead Mode - Both Open and Close Limits Active" due to confusion over "switch N/O" and "application N/O". I get about ten calls per month on this.

Alan

You need not worry about connecting both to the same switch in this case - it will be fine.

Alan