12v source only when engine is on

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Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=113856
Printed Date: May 09, 2025 at 8:46 PM

Topic: 12v source only when engine is on

Posted By: livehho
Subject: 12v source only when engine is on
Date Posted: May 17, 2009 at 1:52 PM

22R-E 1995

I'm installing a water injection system and need the water pump to be on ONLY when the engine is running. I don't want it to turn on when the ignition is on, but when the engine is actually running.

I'm gonna do this through a relay so I would need a 12V source ONLY when the engine is running.

In most cars you can just tap into the fuel pump or fuel pressure sending unit cable to achieve this. However, in the 22R-E '95 they get 'hot' (12V) when the ignition is on. So, no good.

any hints?

Replies:

Posted By: icearrow6
Date Posted: May 17, 2009 at 2:01 PM

..so there are time when you have the key in the "RUN" position without the engine running?

I would suggest maybe the VSS wire (tachometer).
READ THIS

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Posted By: i am an idiot
Date Posted: May 17, 2009 at 2:18 PM

I would not connect a relay to either the vehicle speed sense wire or the tachometer wire.

Posted By: livehho
Date Posted: May 17, 2009 at 2:34 PM

i am an idiot wrote:

I would not connect a relay to either the vehicle speed sense wire or the tachometer wire.

because it sends pulsed signals instead of a steady 12V signal?

Posted By: i am an idiot
Date Posted: May 17, 2009 at 2:43 PM

The pulses are not the problem, the 90 Ohms resistance of the relay's coil will probably be a bit much of a load for the circuits to drive. You can connect it briefly to the tach wire and see if it makes the car idle rough. If it does not affect the running of the car, then yes the diagram I posted on the other thread may work for you. If it does make the engine idle rough, we may be able to rectify that problem with a transistor.

Posted By: ckeeler
Date Posted: May 18, 2009 at 11:02 AM

just use a relay in conjunction with the oil pressure switch on the engine. if you only have a gauge sender and not a switch, you can pick up a simple 1/8" pipe N/O oil pressure switch at any decent parts store and any fitting you need to plumb it in. engine is running=oil pressure=switch will turn on relay. simple.

Posted By: livehho
Date Posted: May 18, 2009 at 11:32 AM

ckeeler wrote:

just use a relay in conjunction with the oil pressure switch on the engine. if you only have a gauge sender and not a switch, you can pick up a simple 1/8" pipe N/O oil pressure switch at any decent parts store and any fitting you need to plumb it in. engine is running=oil pressure=switch will turn on relay. simple.

cool. thanks.

Posted By: KPierson
Date Posted: May 18, 2009 at 1:09 PM

You can pick up a frequency switch pretty cheap if you do a search on the net. You can then hook the frequency switch up to a fuel injector - have it activate a relay if the fuel injector is cycling. That is probably the best way to actually tell if the car is running (that or a cam or crank angle signal feeding the frequency switch).

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Kevin Pierson