Relay Wiring Question

6

65 Mustang

Well-known member
Hello Everyone. I’m installing the Autolite 1100 Holley Sniper EFI on my 1965 mustang. For those familiar with Holley’s wiring harness, there is a pink wire that powers the device. It requires a 12V ignition source in both the run and cranking position. They say not to connect it to the ignition coil for this purpose. After speaking with Holley’s tech support, they said I could run a relay to bring power to the wire. My question has to do with the ignition trigger for the relay. The relay connections will be as follows: Pin 87 (pink wire connected to Sniper unit), Pin 30 (positive side of battery), Pin 85 (ground). My question concerns Pin 86. It needs to be connected to some sort of key switch. The 1965 mustang has a started solenoid near the battery. The solenoid has an “S” and “I” terminal. If I attach Pin 86 to the “I” terminal, would that be able to trigger the relay to power the device? Any info would be helpful. Thanks very much.
 
Thanks for super fast response. Just watched video. Very informative. Ok. I’ll look to connect to S terminal. Does the I terminal not provide power any more once the engine is done cranking?
 
65 Mustang said:
Does the I terminal not provide power any more once the engine is done cranking?
Click to expand...
YES. But not full 12V from the solenoid (fender relay). If you look at the wiring diagrams for your '65, you'll see the RED/GRN ignition wire splits at the firewall to the coil(-) and the "I" terminal (BRN wire). They are joined. So, while cranking, un-resisted full-power feeds from the "I" terminal to the coil(-) for increased starting spark power.

When the key moves to RUN, the solenoid disengages, stopping full 12V feed. However, the wires are still joined at the firewall, so coil(-) power still flows to the "I" terminal, which is now a dead-end. Fortunately, it is still hot with power from the PINK resistance wire, and makes for a point we can tap for the relay 86 terminal. See the opportunity? This is what many refer-to.

Having said all that, I do not suggest the "I" terminal to energize the relay, unless you test the relay for voltage required for energizing, and drop-out (relay release) voltage. The issue is that the power on that BRN wire (except when cranking) is not full 12V, more like 6 to 7.5V, and standard 12V Bosch-style relay operation at low battery voltage (terminal voltage down to 5V or less) may not be reliable to hold the relay ON when the key moves to RUN.** My preference is to avoid the resistance and just pull power directly from the key switch "C" terminal, or the RED/GRN wire from it, before the PINK resistor wire. This ensures full battery voltage to the relay 86 terminal any time the key is in RUN or START. :cool:

** Here is an example of a "typical" generic Bosch-style 30/40A relay and specifications from a reputable supplier. Better or more-suitable ones are available with careful shopping, but that's typical with kits and such. Note the "Must Operate" voltage is 8.4V. That's higher than the PINK wire resisted voltage is on the coil(-) wire when the key is in RUN. While it will probably work, you can see how we are on the edge of unreliability, especially with a low battery when you really want it to work.
 
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Thank you so much for that very detailed explanation! It certainly illuminated a lot for me. This makes sense now why the “I” terminal still has power after cranking has stopped. I always meant to bypass the resistor wire once I installed my pertronix electronic ignition and Flamethrower ignition coil. They recommend it to supply the coil with full battery voltage since I am not using points anymore. Never got around to it, though.

Back to the relay, I completely see your point about connecting the 86 terminal to the RED/GRN ignition wire prior to the pink resistor wire. However, if I am going to make a connection to the RED/GRN ignition wire, why not just connect the pink wire from the Holley Sniper directly there and not use a relay at all? It seems as though that would give the EFI 12V in both the Run and Cranking positions. Anything wrong with that option?
 
Nothing "wrong" with that scheme either. A couple considerations are that your switch is both old, and limited in its current-carrying capacity, as are the minimal-gauge wires to and from it. The Holley ECM does not pull much power, but is additional direct load, so not a biggie unless you have yet more stuff on it... and it's old.

Conversely, the relay pulls almost nothing to energize, and your ignition coil can also be powered via relay (bypassing the resistor wire), removing switch and wire loads instead of adding them, for extended switch life. 😉 Those are just options that may interest you if looking of optimal reliability, but should not be required if your system, wires, connections and loads are good.
 
Ok. Thanks again for all the great info. Definitely gives me a lot of options to think about and consider. I see the advantage of the relay vs going straight to the RED/GRN ignition wire. Hopefully, this weekend I’ll have time to start working on this. Just want to put one more thing out there. When speaking to Holley’s tech support, they suggested the use of a toggle switch as another option. They said I could connect the toggle switch to the battery positive and the other end (pink wire) directly to the EFI. Any thoughts on this approach? Thanks again.
 
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Not an option for me. Something to forget ON when you're late for work, and has other odd implications. Little things such as adding prime in unusually (or un-tuned) cold weather, you just key on-off-on to get another squirt of fuel. Stuff like that becomes unnecessarily awkward with a toggle. K.I.S.S., but reliable. That's me, but you have lots of options.
 
Ok. Yeah, I was not leaning towards that option as well. Just thought I’d get input on it. Appreciate your feedback. Alright, I’m going to process all of this and come up with a plan. Thanks again so much! Great info. 👍
 
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