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My engine builder wants me to be able to turn my car over without it starting to pre-oil the motor due to a remote oil filter. The car needs to be wired so that the coil will not have juice until I push a momentary switch which will activate the latching relay/coil until the ignition is turned off. It will also act as an anti theft device.

I bought a latching relay (Hella latching/bistable relay, 30 A, 12 V part # 74994) and a momentary switch to accomplish this, but the latching relay didn't come with instructions as to which terminal does what. Can somebody help me with a schematic so I can wire this up?

I plan to use the battery/hot wire to the HEI ignition coil as the entry point for the system. Also included would be the ignition from the key and the momentary switch.

ocrdu
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Bigbaldy33
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    Read the datasheet. If it doesn't have a datasheet, don't buy it. – Hearth Apr 19 '19 at 20:23
  • Unfortunately, it's already been purchased. It's a Hella product so I'm sure it's quality. I'll go to the website and see if I can find the data sheet for it. Thanks for your response! – Bigbaldy33 Apr 19 '19 at 20:29
  • Is there no marking on it? – Tyler Apr 19 '19 at 20:33
  • There are markings on it but I don't know their meaning or what does what. Unfortunately, I'm electrically ignorant LOL! I'll try to post a photo. – Bigbaldy33 Apr 19 '19 at 20:42
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    just wite a oil pressure sensor in parallel with the points – Jasen Слава Україні Apr 19 '19 at 23:41
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    Crank only switches are very problematic on any engine. We used to have these on Diesel generators, but they were interlocked to a fuel injector bypass. The same would have to apply to gasoline engines, you CANNOT JUST CRANK an engine since it will suck air/fuel through a carb, or inject fuel on more modern engines. It would be really easy to flood an engine or cause a fire if you do crank only. If you really need to preload an oil system, use an electric oil pump on a bypass line. – Jack Creasey Apr 19 '19 at 23:48
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    You don't want to turn the car over. You want to turn the *engine* over in an upright car. It took me a while to figure out what this was about. – Transistor Jul 03 '20 at 12:16

1 Answers1

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Relays have different "forms" with standard pin numbers. While your Hella P/N does not seem to match up with their catalog.

enter image description here

The upper relay uses Set + Reset inputs using dual SPST contacts to Gnd with 12V to pin 6.

The lower relay uses bipolar DPDT contacts to reverse the current in a coil at both ends then open with 3rd SPST contact or the DPDT switch has a centre off position.

My guess is you have the 1st one. Latching relays require much more current. (e.g. 120mA to drive 20A)

Alternative.

  • Normally one only needs a momentary SPST relay to drive the solenoid.

  • electric oil pump in ACC position

  • All coils need a snubber reverse diode to clamp the arc current on release to V+, rated for the same current.

  • do you really want the starter motor to keep turning on its own (latched) in a theft scenario? ( not a great idea)

On my old '63 MGB the solenoid had a manual pushbutton as part of the solenoid to do what the mechanic wants. But that was under the hood.

Tony Stewart EE75
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  • You are correct in that I have the top or first relay. I already have the momentary SPST switch. It's actually the second anti-theft device. I already have a master battery disconnect solenoid in place. This is more to pre-oil the engine. Thank you for the info. – Bigbaldy33 Apr 19 '19 at 21:22
  • OK so 2 switches are needed then, Set and Reset – Tony Stewart EE75 Apr 19 '19 at 21:24
  • Won't turning the key off reset the relay? – Bigbaldy33 Apr 19 '19 at 22:39
  • Yes but in a theft scenario , it would burn out the starter and battery and in normal use Start enables Solenoid and does not RESET after start, since it is momentary. – Tony Stewart EE75 Apr 19 '19 at 22:45
  • What you described is exactly what I want. I'm not concerned about the anti theft capabilities (I already have something else in place). I just want something to prevent the car from firing until I press the momentary button (once the motor has turned over for a few seconds and oil is run through the motor, I hit the button and the ignition fires normally). Then the car starts and I drive it. Once I turn the key off, the latching relay resets and the next time I go to start the motor, I go through the same procedure. This oils the motor prior to start-up since I have a remote oil filter. THX! – Bigbaldy33 Apr 19 '19 at 23:30
  • Sorry but it doesn't work this way. You only want a momentary Solenoid relay not a latched one to operate the starter motor. Also turning off ignition does not intiate a Reset. So although I answered your question, it is not a complete solution and I see more problems than solutions with a latching relay. – Tony Stewart EE75 Apr 19 '19 at 23:59
  • I guess I'm not being clear or not understanding your suggestion. The ONLY item connected to the latching relay will be the ignition to the coil. All other items will stay wired as is stock. I'm just looking to temporarily kill the juice to the coil on start up so that the car will not fire when initially turning over. Once the car has turned over a few times, I'll hit the momentary switch, the relay will latch and the coil will become "hot" and spark will be produced to fire the car. Then once I turn the car off, the switch will unlatch. Am I missing something? I apologize if I'm being slow. – Bigbaldy33 Apr 20 '19 at 00:15
  • So, the latching relay stays latched once electricity is turned off? – Bigbaldy33 Apr 20 '19 at 00:17
  • It does not unlatch by removing power. The Reset coil also needs ground current to reset it momentarily .... yes as the name implies. – Tony Stewart EE75 Apr 20 '19 at 00:17