Alternator and horn relay wiring - ClassicOldsmobile.com


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Old December 2nd, 2017, 09:11 AM   #1
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Alternator and horn relay wiring

Ď70 Cutlass, 350, no a/c... Every wire under the hood has been cut and attached incorrectly somewhere, and Iím trying to fix it.

I have two red wires coming from the firewall towards the battery - a heavy gauge (10-12?) red w/black stripe, and a lighter gauge (14-16). I cannot find the red/black on a schematic. For sure, this is not the pink/black that goes to the relay. P.O. had the heavier going to the (incorrect) horn relay, and the lighter one going to the alternator. I think both should go to the horn relay stud and a new wire should run from the stud to the alternator. I also think that the heavier gauge should have a fusible link wire. Can anyone help me straighten this out?
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Old December 2nd, 2017, 04:31 PM   #2
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There should be 4 red wires to the horn relay power stud. 1 from the bulkhead connector (has the fusible link at the stud), 1 from the battery, 1 from the alt, and 1 from the voltage reg. Trace the red/blk wire back to see where its coming from. You may have an aftermarket harness in yours.
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Old December 2nd, 2017, 04:38 PM   #3
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Originally Posted by oldcutlass View Post
There should be 4 red wires to the horn relay power stud. 1 from the bulkhead connector (has the fusible link at the stud), 1 from the battery, 1 from the alt, and 1 from the voltage reg. Trace the red/blk wire back to see where its coming from. You may have an aftermarket harness in yours.
The red/black goes straight to the upper right connector on the firewall. These wires are old - no sign that they are aftermarket. But from what youíve said, it should go to the power stud. The second red in the pic is coming from the regulator. Iíll make one to go from the stud to the alternator, and the one from the battery makes four. Iíll post a pic of the red/black connection when I get back to the car later this evening. Thank you!
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Old December 2nd, 2017, 04:54 PM   #4
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There is a pink/blk that runs from the horn relay (key buzzer) to the bulkhead.
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Old December 2nd, 2017, 11:05 PM   #5
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Here is a pic of the red/black wire coming from the firewall. Why it is different, I donít know - but I ran it to the power stud on the horn relay.


I think I have everything in the right place along the fender and across to the alternator now. Firewall and starter is tomorrow. Iíve never seen such a wreck of a wiring job - who knows what this car has been through since 1970....

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Old December 5th, 2017, 08:31 PM   #6
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that alternator output wire looks seriously undersized.

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Old December 6th, 2017, 08:45 PM   #7
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Thanks, Bill - after unwrapping the rear harness to the regulator and such, I found such a mess of cut wires that I just ordered a full new wire set for under the hood. If you think the output wire here looks small, you should have seen what was there. I thought I was doing much better by increasing it to this size... It's all coming out and the proper is going in. Thanks for the heads up on this - it sent me in the right direction.
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Old December 10th, 2017, 07:23 PM   #8
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Wire to alternator size

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It's all coming out and the proper is going in.
I'm not sure what size your alternator is but this table will help you decide what wire size to use. http://www.usawire-cable.com/pdfs/nec%20ampacities.pdf. In the event my link won't post, just look up "amperage wire size table" and you'll find plenty of references. You're gonna be shocked at how big the wire needs to be for those moments when your alternator is maxed out. I'm going to run multiple smaller conductor wires to achieve the same effect as one big one (again, tables available for figuring this out). Since I'm going with all kinds of extra electronic goodies (FI, pump, electric fans, rear defrost, alarm, windows, locks, blah blah blah), I'm going with a much more stout than factory alternator (125 or more amps). Are you adding any electronic extra goodies now or at any time in the future?

I'd immediately either "add to" or replace the factory or replacement harness alternator wire based on calculated requirements...

I'm now concerned that a standard v-belt won't do well under heavy load and my understanding is that serpentines on the 350 olds platform are not practical or worse, non-existent. hmm, maybe it's time for an LS motor... anyone want to buy an FI unit? ahh, there I go dreaming again.

btw, I re-wired my brother's 72 Cutlass that happened to have a partial 71 harness.. which of course is different enough to cause lots of issues at the firewall connection point. I layed under that dash a lot more than I want to admit. The diagrams from the Olds manuals were priceless (both years)! My own "interpolation" diagrams became my gospel because we ended up with a "Frankenstein".

Last edited by GammaFlat; December 10th, 2017 at 07:29 PM.
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Old December 10th, 2017, 07:41 PM   #9
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I'm only running a 55A alternator - I have very few accessories. I may add an amp for the stereo at a later date. But I see what you mean about beefing up that cable from the alternator to the relay. Is this table telling me to use an 8 gauge wire for the 55? The wire that was on the car when I got it was 18 gauge And I think I sorted out the under dash wiring the other night - but I won't know for sure until the new harnesses get here and they get installed. What an insane mess this car is. There just isn't any single wire that someone hasn't hacked or moved or removed or tapped....
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Old December 12th, 2017, 06:29 PM   #10
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Quote:
Originally Posted by BSiegPaint View Post
I'm only running a 55A alternator - I have very few accessories. I may add an amp for the stereo at a later date. But I see what you mean about beefing up that cable from the alternator to the relay. Is this table telling me to use an 8 gauge wire for the 55? The wire that was on the car when I got it was 18 gauge And I think I sorted out the under dash wiring the other night - but I won't know for sure until the new harnesses get here and they get installed. What an insane mess this car is. There just isn't any single wire that someone hasn't hacked or moved or removed or tapped....
Those more experienced can help out but here goes... If you assume your alternator is running at maximum output (55 amps), you'll need to get that level of output to (at least) the distribution point if your load reaches 55 amps (add up your likely simultaneously running components). If you get to a spot where your battery (dead-ish) is charging a lot and you have lots of stuff running.... oh boy.... turn some stuff off for a little while . You don't want to outrun your alternator with load or you'll steadily drain your battery. The good news is that your wire won't need to cover any more than 55 amps with that alternator. I'm looking at ones like this (150 amp max): http://www.qualitypowerauto.com/item....00&unt=15&sd=
..but again, I'll be running a lot of high draw stuff and I don't want my daughter calling me from the side of the road .

So, yes, I think 8 gauge would be ample for your application. I'm not sure how yours is wired exactly but if your alternator goes to the horn relay terminal, you'll also need an 8 gauge from your battery to the terminal to ensure enough capacity for instances when you have a lot of electrical load with no alternator support (car not running, belt broke or slipping or alternator dead, and lots of accessories on). I understand this isn't likely but it is what I'd do. We have to assume there may be failures and you don't want one failure to cause others (cascading event - under hood fire).
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