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#1
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Fused ACC as Relay Trigger
Title pretty much sums this one up. Is it safe to use my fused ACC pin off the box to trigger a relay?
I need to add a couple interior accessories and I'm already pulling power from ACC for my tach as well as the led halo's in my headlights. I've not had any trouble, but the electronic speedo drive I'm adding, along with a sender will need more power. Is this a safe and effective solution here, or should I be looking at doing something different?
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-Jason 1969 Pontiac Firebird |
#2
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A relay coil typically requires pretty low current to operate, probably in the range of 200 milliamps or less. As long as your total load on the fused accessory pin, including the other things you're operating from it, is less than the fuse capacity you should be okay.
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#3
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Yeah I would only be using the fused accessory pin as the relay trigger. The relay itself would be powered off the battery and take the actual load.
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-Jason 1969 Pontiac Firebird |
#4
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You should fuse the power lead, the closer to the battery the better.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic. |
#5
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This might be overkill, but depends on future plans. Because of add-ons and desire for cleaner relays (using Holley Sniper) I recently added a MSD 75643 stand alone solid state relay. Mounted very close to the battery with an easy to make bracket for a slick finish.
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#6
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I might actually pick one of those up. I've got the FiTech and ignition on a relay and I'm about to add a relay for both an electric fan as well as this electronic speedo drive. It looks like I could run everything off this one deal.
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-Jason 1969 Pontiac Firebird |
#7
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You probably noticed, it is separately fused for each relay.
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#8
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Yup, and I noticed it can be triggered by ground, or power, which will work with the ground relay needed for the fan.
I was going to create a fused power distribution node off the battery, then trigger relays for various systems, but this is a much more elegant solution.
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-Jason 1969 Pontiac Firebird |
#9
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MSD 75643 on 76 F Body
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#10
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I'm far from competent with wiring. I can "do" most of the stuff, it's the conceptual side that I lack a good bit of understanding.
As is right now, I have the following add-ons that I need to account for (some installed, some not) Fitech (Installed) headlight halos (installed) Tach (Installed) Electric Fan (Not installed) Electronic Speedo Drive (Not installed) Speedometer sending unit (Not installed) As of the current state, I have both my HEI and FiTech being powerd off of a 30 amp relay that is triggered by the coil wire. The halos and tach I have combined off of the ACC terminal. This setup has been rock solid for years, without issue, but I realize it's probably not "correct" and there's a bit of a rats nest on the firewall because of where the relay is mounted. I'll be yanking out the HEI in favor of a billet distributor and traditional coil. I'm wondering if I should put everything on this relay, or if I should keep a few things separate. The FiTech and ignition need power under cranking. Nothing else does here. I'm also a bit worried about the amp handling. I'm using a Flex-A-Lite Blackmagic and it wants 18 amps continuous draw, but I'm not sure how much it once at startup. the FiTech will control that and it's not PWM, so there may be significantly more amp load when the fan turns on. Thoughts?
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-Jason 1969 Pontiac Firebird |
#11
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I'd take a look at the comments on Summits site regarding this product and ecu's. Some had problems with back feeding and components running when they should've been off. If nothing else maybe a conversation with MSD tech folks about how they're handling flyback and stuff of that nature before you use it with anything triggered by your ecu. Agree it looks like a handy thing to have, but not if it's gonna jack with your ecu.
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'65 Tempest 467 3650# 11.30@120.31 |
#12
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I'm not certain that would be an issue with my setup. The FiTech's wiring is pretty simple. It has a constant power direct from the battery and a trigger to turn the thing on or off.
Since this isn't bypassing the FiTech's harness which turns everything in the system on (ECU, Pump, Fans etc.) the relay can really just be an on/off for the trigger. It's how it's functioning now, just with a standard 30amp relay triggered by the coil wire. The only thing that is known is the run-on issue you get with firing an HEI via a relay triggered like this. Unless you diode protect the excite wire from the alternator, it'll feed back. My car has been setup with a 1 wire alternator since I got it however so that's not an issue unless I try to run the internal regulator pigtail. I can deal without having a functioning idiot light though. Should functionally work the same here, just with consolidated relays. I could still keep the current setup and just use this for accessories only.
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-Jason 1969 Pontiac Firebird |
#13
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If you have the Fitech controlling accessories (fan) that's where it'll show up probably. Hopefully you have no issues. Just wanted you to be aware others have.
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'65 Tempest 467 3650# 11.30@120.31 |
#14
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I'd have to look at how the fan circuit is setup in the FiTech harness. The fuel pump driver is internal to the ECU, so if the ecu is off, the fuel pump can't run.
The fan control for FiTech is a ground signal only. The ECU can't directly control the fan. It's on above a set temp and off below a set temp. But I suppose that could still cause an issue.
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-Jason 1969 Pontiac Firebird |
#15
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Best way to assure no unwanted interference is to have relays that are only powered when the ecu is powered. Coil side, at least. Not sure if that's possible with these. And I really don't wanna speak out of turn on this, just bringing up the potential for issues.
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'65 Tempest 467 3650# 11.30@120.31 |
#16
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Perhaps it's still best that I look at doing just a relay off the acc terminal for a couple of these. I've already purchased the ground triggered relay, for the fan. It's nothing to go down to the local auto store and pick up another relay. Definitely the cheaper option, it's just a bit more wiring that I have to find places for.
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-Jason 1969 Pontiac Firebird |
#17
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I understand on the wiring, completely. Adding modern equipment to our old vehicles that was never intended to have it, is a challenge for sure.
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'65 Tempest 467 3650# 11.30@120.31 |
#18
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Only for the sake of information. I am running the Sniper EFI, MSD Pro Billet distributor (which is phased and timing is controlled by the Sniper ECU) with MSD Blaster Coil, and the electric fan for transmission cooler through the MSD Solid State Relay. The left-over relay is sent to the passenger fuse box area where I am running some basic needs and taking advantage of the cleaner circuit. I am adding an electric fan shortly and will swap out the lead to the passenger compartment to use with the fan which will be managed by the Sniper ECU. This system works 100 percent, and it didn’t come easy. The reason for going to the MSD solid state relay is that my journey to a RFI free system for a well running EFI was not an easy one, amongst other smaller gremlins.
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#19
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How are you triggering the relay? I will have a pretty similar setup in my bird. Though, I have not dealt with RFI related issues.
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-Jason 1969 Pontiac Firebird |
#20
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In my case, the MSD relays are triggered by the ignition. The MSD relay has constant power and ground. Each is triggered by either power or ground. As example, the fans I am about to install will be triggered by ground output from the Sniper ECU. My initial RFI problems were largely a function of 40 plus year old grounds and corroded ground wires, and positioning of my MSD 6AL box and Blaster Coil. As Scott65 mentioned, adding of some modernization performance items can be a challenge, but not always.
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