Author Topic: Need help sorting out 74 engine wiring harness  (Read 547 times)

twov8sandat4

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Need help sorting out 74 engine wiring harness
« on: May 28, 2021, 07:39:50 AM »
Good morning all,

In the process of replacing the brake lines and master cylinder/booster, I had to remove the bulkhead connector to get access to one of the booster bolts.  While I had it out, I decided to remove the entire engine/front light harness to give it a close inspection to make sure it was still in good shape after 47 years of use.  Shame on me that I did not label everything when I removed it.   I figured I had the factory service manual, and I am fairly confident with my wiring abilities, what could go wrong?

Fast forward a couple of months, and I had some time last week to work on the harness.  Turns out it is not as virgin as I thought it would be.

Below is a screenshot of the engine harness from the FSM.  I am having issues figuring out some of these components/wires.




For example, the alternator has five wires coming out of it.  The two prong plug with the two 18 gauge red and brown wires, and three wires out of the post.  One red 10 gauge as shown in the diagram, and two additional 14 gauge wires that are fused at 20 amp each, and are not in the diagram.  These two wires are crimped together and connected to one 14 gauge wire that runs along the same harness in the valley between the intake and valve cover and goes to a two prong connector in the back of the engine.  the other prong in that connector is connected to an 18 gauge wire that goes to nowhere.  There is nothing in the back of the engine for that two prong plug to connect to.  Also, the fuse holders and connector both look factory, or at least vintage correct.  I have owned the car 31 years, so those are not recent additions.



The fused wires coming out of alternator


Connector at the other end of the two alternator wires above

The car was originally a unitized distributor car.  Only options per PHS are the AM/FM radio with 8-track, center console, and the Unitized Distributor.  It is a 400/TH400.  No A/C.  At some point early in its life the distributor was ditched for an aftermarket HEI.  The factory shows yellow and pink wire for the BAT and TACH signals, but mine has Pink and light blue, with the light blue having the resistor.  The resistor was bypassed, and the power lead goes to a 20 amp fused brown wire that goes through a firewall grommet into the pass compartment.  This one looks factory too, but not sure.

The car no longer has the factory Quadrajet.  It has a 750DP Holley, no choke.

From the FSM, I was able to determine that I do not need the Idle Stop Solenoid (one yellow wire plug).  However, I don't know what the Cold Feed Switch is for, and I don't know where the Dist Vac Solenoid is, if it has any.  The TCS GRD and Downshift switch plug has the two wires (Black/org and black), but the switch in the back of the transmission where this plugs into (I think) has only one prong. 


TCS downshift switch on transmission

If anyone has any insight on how to sort this out, please let me know.  I have done some complex wiring on much newer cars  before, but I had decent wiring diagrams to go by.

I also contacted Ames Performance to order a replacement engine harness, but those are 12-14 week lead times...

Thanks for any help.






Little Miss G Racing

firebirdparts

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Re: Need help sorting out 74 engine wiring harness
« Reply #1 on: May 28, 2021, 09:06:43 AM »
One thing is for sure:  All that stuff hanging off the back of the alternator is just raw power.  If you have something on the car that's unpowered, you'll figure it out eventually, but it may be that was all dangling for 31 years.  Who knows.  I cannot enlarge your shop manual pic, and we need that.  I have a 74 FSM but anyway it'll help the discussion if you post a big version.

Being a 74, it should have originally had seat belt non-start interlock under the hood.  I haven't looked at one in a long time, but my brother has a 74 GP that is pretty original.  The TCS, I think, is something you want to remove.  It shut some of the vacuum advance off some of the time.  The transmission comes into it so that the spark advance is allowed in third gear.  I am thinking as it sits you may not have all the vacuum advance you are entitled to.

On my brother's car, we wore the transmission out, and of course out it came.  So at least there's that perspective; He has the same situation of a 2 wire plug to the transmission but only one wire is actually there, and that's the kickdown wire.  I was there when it happened, so I'm not in doubt at all where it went.  GP's are all TH400.
« Last Edit: May 28, 2021, 09:10:03 AM by firebirdparts »
I want to be like Paul Kenny when I grow up
Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
1988 T-bird Turbo Coupe

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #2 on: May 28, 2021, 09:24:14 AM »
There is nothing that is unpowered that I can think of, maybe the cigarette lighter, but that does not need 40 amps  :D  I am all for removing any wires that have no place in the car, so I will leave the extras coming off the alternator out.

The seat belt interlock is part of the lighting harness.  That part of the wiring is untouched and is still in the factory wire wrap.

I will start the reinstall this afternoon and see how far I can go.

Thanks for the quick reply.

I will try a larger picture.  It is my first time using Flickr.  What a pain...
Little Miss G Racing

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #3 on: May 28, 2021, 10:15:59 AM »


Hopefully this is better
Little Miss G Racing

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #4 on: June 08, 2021, 04:39:26 PM »
So I eliminated the extra two fused leads off the alternator, plus the cold feed and vac adv solenoid plugs and wires. I also rerouted the wires from the valve cover area with the forward light harness, and rerouted both between the inner fender and the fender for a cleaner look.  I also eliminated the starter as a junction point for the +12v for the car.  The +12v for the car comes from the battery directly now rather than the starter, all wired and protected the same way as factory.  I had to extend a couple of wires, but nothing major.

One side “feature” from this harness work, is the seat belt interlock now works. I have to figure whether I should keep it or disable it. What a pain in the neck that feature is  :lol:

The car did start right up and idled great. Have not had a chance to drive it yet, as the brakes are not bled yet.

Little Miss G Racing

Re: Need help sorting out 74 engine wiring harness
« Reply #4 on: June 08, 2021, 04:39:26 PM »

firebirdparts

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Re: Need help sorting out 74 engine wiring harness
« Reply #5 on: June 08, 2021, 07:35:53 PM »
Wow!  You're the first person I've ever heard that had that interlock working.
I want to be like Paul Kenny when I grow up
Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
1988 T-bird Turbo Coupe

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #6 on: June 09, 2021, 04:26:55 AM »
Wow!  You're the first person I've ever heard that had that interlock working.

No kidding.  Imagine my surprise when I got in, turned the key and nothing happened, no crank, nothing... I figured what are the odds of that thing being the culprit, and sure enough, pressed the button, buckled up  :lol: and the car fired right up  :-D
Little Miss G Racing

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #7 on: September 25, 2021, 02:08:08 PM »
Bringing this back up.

I went to get a new inspection sticker, and got rejected because the horn wasn’t working. When I got home I did the following:
1. Ran a piece of wire from battery directly to the horn and it worked.
2. I checked continuity between the relay (12g black wire) and horn and it was good.
3. I checked for 12v at the relay (12g red wire) and I had it.
4. I replaced the relay, but still no horn.
5. Fuse is good.

I am trying to do the test described in the manual, but there are no markings on either the original or new relay pins. I need to know which wire corresponds to which pin so I can do the test. The manual lists them as pin 1, 2, 3. Everything online that I could find has the regular relay pin designation of 85, 87, 30.

Red wire —— pin #
Black wire —— pin #
Green wire —— pin#

I appreciate any information you have.

Thanks.
Little Miss G Racing

firebirdparts

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Re: Need help sorting out 74 engine wiring harness
« Reply #8 on: September 26, 2021, 06:36:14 PM »
I'm at work, so I'm useless, but I will say this.  The horn button is ground switching.  It may not be working.  So you could run a wire from the appropriate pin to ground and check that.

You could decipher the relay itself by ohming it out.  There will be some resistance between the power wire and the ground wire.  There will be infinite resistance between the horn wire and the other two wires.  When you put 12 volts across the coil, there will be continuity between the power wire and the horn wire (that is to say, the coil will work either way, but once you turn it on, it has to be the "right" way for the horn to work).
I want to be like Paul Kenny when I grow up
Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
1988 T-bird Turbo Coupe

twov8sandat4

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Re: Need help sorting out 74 engine wiring harness
« Reply #9 on: September 27, 2021, 07:56:19 AM »
I'm at work, so I'm useless, but I will say this.  The horn button is ground switching.  It may not be working.  So you could run a wire from the appropriate pin to ground and check that.

You could decipher the relay itself by ohming it out.  There will be some resistance between the power wire and the ground wire.  There will be infinite resistance between the horn wire and the other two wires.  When you put 12 volts across the coil, there will be continuity between the power wire and the horn wire (that is to say, the coil will work either way, but once you turn it on, it has to be the "right" way for the horn to work).

Thanks Joe.  I was able to figure out the relay based on your description.

I spent a couple of hours last night and finally got the horns working.

Tested the relay.  It was good
Tested the horn button.  It was good.
Tested continuity between relay and horn, and it was good.
Tested the horns.  One was good. Two were bad...

Upon more investigation, looks like at some point in the past, before I got the car, the driver's side horn stopped working.  So rather than replace it, the PO got another horn and mounted it in the radiator support center brace, and put the connector on it, and left the original driver's side horn disconnected in place because it is such a pain to get to.  Then at some point in the 3 year downtime while I was addressing other issues on the car, the passenger side and center one stopped working (it would make popping noises when hitting the horn button.)  I tested both by running a wire directly from the battery pos. while they were still mounted in the car.  The passenger side made the popping noise, while the center one worked fine.  When I reconnected the factory connectors, no horn, just the popping.  And when I disconnected the passenger side horn, and left just the working center horn on the factory harness, the center one would not work  :?

Finally decided to remove all three and test them on the bench.  The factory originals did not work.  One was dead.  The other one was making the popping sound.  The center one worked.  The only thing I noticed was the poor contact area of the bracket with the chassis.  So I cleaned both sides, reinstalled the working horn, connected the factory harness, and voila.... I got horn.

Now on to the washer motor...

Little Miss G Racing

Re: Need help sorting out 74 engine wiring harness
« Reply #9 on: September 27, 2021, 07:56:19 AM »
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