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Hot start relay tips?


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Hey y'all!


Been having intermittent single click starting issues, it bridge starts with a screwdriver just fine, done all the multimeter testing, etc and the last thing I need to check is if there's a voltage drop on the black & red wire when I try to start it(more than half a volt would be a positive indicator of a bad wire, correct?), but it got dark out and I'll have to do that tomorrow. Plus, it's like every time I try to diagnose it works just fine, but then when I go out and about it's mad at me. Just gotta bring my tools around everywhere and do the diagnostic at Albertsons or something, lol. Regardless, it seems the control wire is the issue because all other ignition parts are new and it is strictly intermittent, either no struggle or single click complete failure.


I have found enough great tips on here to know what goes where, the kind of wire to use for control vs voltage, the pin #'s and what not, but am kind of stuck on some minor details that I couldn't locate info on.


Where should I install the relay? Should it be closer to the starter or in the cab by the ignition(and that is where the other end of the black and red goes, correct?)? I can only find 5-pin Bosch style, with 2 87's or with an 87 & 87a. Should I just not connect an 87 or the 87a? Do I splice in somewhere for the 12v or run a lead all the way to the positive battery terminal(I know this is probably a dumb question but I'm a noob on electrical)? Where's the best spot to splice or connect for that?


And finally, all this is pretty much because it would be major pain in the ass to take apart the harness and replace the control wire, right? Just wanting to understand the why's behind the how's.


Thank y'all!

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The hot start just uses the signal from the ignition switch to activate a relay... the idea is this would give a stronger trigger signal to the starter solenoid...

Easy test when it fails is just get a piece of wire, preferably with a fuse, hook it up to the starter and see if it activates the starter relay....

If it does a hot start relay will help, if not the starter solenoid is going bad... 

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As for the relay...

86 ground

85 positive signal from ignition switch.

30 power from battery 

87 output to starter solenoid....

I put a spade over the center 87a to be safe....


87a will show power when not activated, to avoid that you could do power to the 87 and then use 30 as the output....



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The ideal relay location would be at the shortest path between battery and starter solenoid, and run a dedicated wire, in the heaviest gauge reasonable, from battery to relay to solenoid. 


The wiring picture above it's a little confusing to me as it looks like the switch is not wired correctly...😅

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is this a 510 or 720.?

If 510 make it nice and neat I would but it low and out of the way.

close to the starter selinoid would be best as you have short wire runs. but out of the way in case water runs down on it. If 510 maybe back behind the strut with the light relay?


you will want pin 87 as that is open when NOT energized

pin 85 you can just loop the ground wire to the screw that holds it down if it has the Tab.


720 is crowded Im not familiar with the layout for a spot to put relay and spots to put a relay but close to starter is best with ez access. DRY


I have a FORD relay on my 521 and next to the fuse box and used a bolt as o hold it down and act as a ground as case is ground.

Edited by banzai510(hainz)
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10 minutes ago, NC85ST said:

I did this on my ‘85 and I mounted the relay on the passenger side near the other relay’s. 

Did you lengthen the line going from the control/ignition line to the relay or did you reroute it?



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3 hours ago, thisismatt said:

Why is there a ground to the center tap? Looks like an ON ON SPST switch with a common center that will short out in either position 😅

I believe the switch is switching power , the ground is to light up the rocker switch but I see what you are saying....

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Stoff I also seen the other way where they jumper the main power back to the selinoid also  using that Ford relay which I think you was thinking about/ I never seen this way untill a few years ago  . in this link thers a photo of the jumper



Edited by banzai510(hainz)
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On 8/4/2022 at 3:19 PM, ProJect said:

Did you lengthen the line going from the control/ignition line to the relay or did you reroute it?

I lengthened it. I didn’t cut any wires. I simply used a spade terminal to plug into the solenoid wire and ran it to the relay and ran another wire from the relay back to the solenoid. I do my best never to cut any factory wiring. Also the relay I used has a built in fuse.

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