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Recon1342
Recon1342 UltraDork
5/7/24 1:51 p.m.

In reply to 4cylndrfury :

8.2" differentials have a smooth lower housing and a funky stamping in the diff cover. 8.5" has two square protrusions on the lower housing and a smooth diff cover. Some of the '70-'72 Monte Carlos also had an 8.5" bolt-in axle, instead of C-clips. You may need to find the axle code and figure it out. 

10-Bolt Chevy Identification Guide. Know What You're Looking At (chevyhardcore.com)

Decoding Rear Axle - First Generation Monte Carlo Club (firstgenmc.com)

 

4cylndrfury
4cylndrfury MegaDork
5/8/24 6:42 p.m.

Nice! Pretty sure I have an 8.5". Build sheet tells me I have 2.73 gear ratio. Now, onto deciphering my axle code. Anyone have an opinion - is 3.23 acceptable for the highway?

Recon1342
Recon1342 UltraDork
5/9/24 9:24 a.m.

In reply to 4cylndrfury :

Curious. 8.5" cover, but no casting lugs on the housing. What's the axle code?

Your next step should be to pull a wheel and drum to see if your axle has bolt-in retainers at the outer bearings. A c-clip type axle will have all the brake hardware and backing plate, plus bolts holding the backing plate in place. Non-C-clip axles will have an additional retainer plate that the axle shaft passes through that is bolted in place to retain the axles and bearings. 

As far as gearing goes, 3.23 should work well for an all-purpose ratio. I had an uncle that had a Mercury Comet with a 289; he ran 3.08s in it and it was a fantastic cruiser.

Seems that '72 was an odd year for  Monte Carlo 10 bolt axles...

4cylndrfury
4cylndrfury MegaDork
6/17/24 3:31 p.m.

Ok, need some help from the hive...

The horn in the Monte doesn't work, so I started to trouble shoot. I jumped the horn to the battery, and it made horn sounds - so the horn works.

Next I tried to jump the ground from the relay to the battery negative, and got nada, so according to the web, that means relay is shot. So, disconnect the main power at the battery and replace the relay.

I bought THIS DURALAST RELAY from advautozoriellys which doesn't look exactly the same, but they swear it is a match for my car.

There's no wiring instructions, just this image which I don't understand because I'm not an electrician:

 

The polarity of the leads is not something you can get wrong, the positive has a big wing on the plug that won't allow you to connect it to the negative post. There's a pair of terminals to add the power to vs the single on the factory relay, but it's fine because I have ring terminals that I can screw right on.

So, connect the power lead to the relay, connect the horn leads, and lastly reconnect the power at the battery and... horn just stays on for about 10 seconds and then dies. Horn is now dead - jumping it to the battery gets nothing. Multimeter says no continuity. Pressing the horn button does make clicky relay sounds, just no horn sounds.

The relay sounds make me happy. But I can't figure why the horn wanted to stay on...you cannot get the polarity wrong, so I didn't have it reverse wired. The thing that has me the most confused is the presence of an additional male terminal on the relay that isn't present on the factory unit -

is it possible to power the other side of the relay?

 

Help

eastsideTim
eastsideTim UltimaDork
6/17/24 5:41 p.m.

If it has an extra connection, I wonder if it has both normally open and normally closed outputs.  If so, it is possible you attached power to the normally closed side, and that ran the horn long enough to break it.

With the relay attached, and horn button not pressed, can you see if the horn connection is getting 12 volts?  If so, for the spare connection, make sure it is at zero volts.  Press the horn button, and see if the outputs switch.

4cylndrfury
4cylndrfury MegaDork
6/17/24 9:52 p.m.

In reply to eastsideTim :

Great idea! That would make sense - wish I had thought of that before I burned out an OEM horn ...

eastsideTim
eastsideTim UltimaDork
6/18/24 6:51 a.m.

If the dead horn has the key on it, let me know what it is.  I have a stash of GM horns in multiple tones.

4cylndrfury
4cylndrfury MegaDork
6/18/24 9:56 a.m.

I bought a cheapo horn from AutoZone just to try and get it working.But I'd love to have a stock type. If I can get it working, I might inquire about purchasing one of yours!

4cylndrfury
4cylndrfury MegaDork
6/18/24 10:59 a.m.

Spent about 45 minutes on this. Now I'm totally second guessing that this relay really is what this circuit needs. With everything connected, it's sending 12v constant, and when you press the horn button, no change. When you RELEASE the horn button, it jumps to about 15v and then drops back down to 12v constant.

Also, it sends 12v constant even with the black ground wire disconnected. If I try to use the other male terminal (blue arrow above) for the green power wire, it sends no voltage, even with the horn button pressed. When you release the button with that configuration, it jumps to 1.5v then drops back to 0.

The only connection I haven't changed is the main power source to the relay. I could attempt to power the blue arrow terminal, but that seems super sketchy and out of my league electronics-wise...I'm not sure how to verify I won't be sending power to a no-no place that releases the magic smoke.

Unless anyone has suggestions, I'm probably going to just keep searching for an original style relay.

 

Also, EastsideTim, I haven't pulled the horn out, but this is what I can see as far as a part number:

 

 

 

4cylndrfury
4cylndrfury MegaDork
6/18/24 12:02 p.m.

I also suppose the horn switch in the wheel could be stuck closed. But the relay clicking when the button is pressed makes me think maybe not?

eastsideTim
eastsideTim UltimaDork
6/18/24 1:19 p.m.

Yeah, that sounds odd.  The clicking makes me think the horn button is fine.  I'm afraid I'm not familiar with the 1-5 numbering sequence on that relay, so hopefully someone else can interpret that.  I'll dig through my horns and see if I have anything that matches that part number.

4cylndrfury
4cylndrfury MegaDork
6/18/24 1:59 p.m.

Thanks EST... No rush.

I searched a bit through YouTube and found a video of this exact relay misbehaving in the same way, being stuck in the switch closed position because of shoddy mfg.

That was enough to convince me to just find an actual replacement part vs a cross-reference mutt. Classic industries had it in stock for not much more than I got it at Autozone

Ordered under the original GM part number 3996283

Should be here in a few days. Cost me more with shipping, but I probably should've just forked over the  money the first time and saved my factory horn...

4cylndrfury
4cylndrfury MegaDork
6/19/24 11:16 a.m.

Just for fun I opened up the non-functioning OEM relay, and found an incredible amount of corrosion inside. I didn't take pics, but I spent some time cleaning it up as well as I could and reassembled it. When I get a few moments I'm going to try to see if it works now. Still have the new one on its way, but would be cool to get this old one back on the road...

4cylndrfury
4cylndrfury MegaDork
6/20/24 12:25 p.m.

Welp...that OEM relay appears to be well and truly borked. No clicks, no horn sounds.

 

But she does look pretty

4cylndrfury
4cylndrfury MegaDork
6/20/24 9:18 p.m.

Starting to get pretty serious about ordering my new engine. Anyone have a good resource for tips&tricks or how tos for a SBC remove and replace?

Luckily, blueprint motors arrive ready to drive...just add oil, gas and coolant, and go. But I've never done this before, and while I know enough to be dangerous,I'd never turn down free advice. The internet is a big place and there's lots to digest. Anyone have a link or source they'd recommend?

preach
preach UltraDork
6/21/24 2:11 p.m.

Sweet car.

This just rolled through my algorithm:

 

eastsideTim
eastsideTim UltimaDork
6/21/24 2:30 p.m.

In reply to 4cylndrfury :

I don't know of any specific link, but I'd suggest finding some generic instructions for SBC V8 removal, and checking to see if there are any issues specific to your chassis.  Also, may want to put out the call for help from local forum members.  Depending on timing, I'd be happy to help out, and I suspect others are the same.

Racingsnake
Racingsnake Reader
6/21/24 3:28 p.m.

Should be pretty straight forward to remove and replace a small block in one of those. You might need an engine hoist with extra long reach though because the engine is a long way back from the front of the car

eastsideTim
eastsideTim UltimaDork
6/21/24 4:28 p.m.

In reply to Racingsnake :

I think at least once, I had to pull or install an engine from the side because the reach wasn't long enough on the engine hoist I was using, probably on a 3rd gen Firebird.  Would have been less sketchy to have a longer reach.

 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
6/21/24 6:26 p.m.

The horn relay is weird because it's also the main electrical junction block of the car.

Definitely don't screw around with cheaping out.  Last summer I had to do some rewiring on an elderly Chevy that blew all the insulation off of the alternator cable after the relay decided to short that positive to ground. 

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