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Which Harmonic Balancer?

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Hello again all,

Well, my engine has a weird shutter at about 1000-1800 rpms only when the A/C is running. The A/C still works fine and blows cold but when I come off the gas and the engine hits about 1800 rpm the whole drive shaft seems to shudder. All of the accessories attached shake. This phenomena only occurs while the A/C is on.

The shop is looking at it right now and they think, by the description that it might be the harmonic balancer.

If that's the case, what should the cost from the shop be? $70 for the balancer and then 2 hrs of labor? Or, should I consider doing the work myself and saving the $65/hr labor charge.

If so, what balancer should I look at buying.

Any other ideas?

Thanks everyone and sorry for the short notice.

Russ
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
Your car takes the "fifth design" 8" balancer - it was used on the '66-'68 327/350, and on all '69-'82 350's.

Make sure all your compressor brackets and braces are in place and tight - a similar symptom in the 70's was called "A/C compressor moan", which was cured by adding more braces and bracket attachments. If your car doesn't have the shudder you describe with the A/C off, I doubt if the balancer is at fault.
 
D

Dave L.

Guest
My 81 takes the 63/4" balancer.just replaced it last year.
77 4spd- i can verify yours for sure at work tomorrow and tell you what the GM part# is and what your wholesale cost would be if you like.johnz might be correct,but something tells me that might have used the smaller bal.;)
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Johnz and burglar,

Thanks for the input. I wasn't so sure that it was the harmonic balancer since it only does it when the A/C is on. I would appreciate the info on the balancer though if you get a chance.

Much appreciated,

Russ Fulks
 
D

Dave L.

Guest
Balancer sizes

77 4spd-The L82 all use the 8" balancer.The L48 all used the 63/4" balancers.All 81-82 use the 63/4.The current GM part # for your balancer is 12555879.The current GM wholesale price is $151.32. That sounds like a strange problem you have there.I hope this info may help you. Let us know what you find out on this. Dave
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Thanks Dave,

I should hear from the shop this afternoon and I'll post what they say. If it is the harmonic balancer, I might try to do the work myself. Oreillys will rent the puller, but I don't know about putting the new one on. What do you guys think?

Thanks,

Russ
 
D

Dave L.

Guest
Its an easy job.Make sure you use an installation tool also.Some guys like to pound them on with hammer,not the proper way to do it. Good Luck! Dave
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Burglar,

I was looking at the summitracing web site and they have an OEM balancer for $61. It says it is for the "street rebuild" and that it is 4500 rpm max. It seems strange to me that any harmonic balancer would only be good to 4500 rpm. That's not even redline for any 350 Chevy I've seen.

Russ
 
Joined
Aug 29, 2001
Messages
3,235
Location
Norcross, Georgia, United Stat
Corvette
2017 Arctic White Grand Sport
While I wouldn't describe it is a shudder, I can definitely feel the effect of the a/c compressor running at low RPM's. I just assumed that it was normal as more demands were put on the engine at a low power spot.

Russ, still trying to keep it under $500???:crazy

Bob
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
You got me Bob!

See, the A/C is 2 months outside of the 6mo/6000 mile warranty, but I've only put about 600 miles on the car. And, since this would be the second replacement of the compressor, I think I could talk Napa into paying for the replacement.

If, however, the balancer is the problem, then I will have to take the ~$200 hit, assuming I do the work myself. So I still have ~$300 to go, and only 1-2 yrs! :)

It's just a hobby, it's just a hobby, it's just a....
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
News update!!!

Well, it's neither the balancer or the a/c. It's, drumroll please..............the cruise control servo. WTF!!! The alternator was adjusted all the way out on the bracket, and the bracket was hitting the cruise control servo causing everything to be out of line. Since I have a 4-speed car and the cruise has long since been disconnected and jacked up by Bubba, the shop is going to remove the servo for me at the tune of $40.

So it looks like I'm still under the $500.


They also inspected the balancer and fan assembly and said that the stock balancer rubber looks fine. No deterioration to worry about!

I'll get her back this afternoon. I guess I'll have to put those new belts on this weekend.

Thanks everyone for the input. Looks like JohnZ was right about making sure everything was adjusted correctly. I can't believe I didn't notice the alternator bracket hitting the servo. Doh!

Thanks guys.

Russ
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Well,

I knew it couldn't be that easy. The mechanics didn't fix the problem I wanted fixed, they fixed another problem I had. So, here's the deal:

I have a flex fan. I also have A/C. The problem is an audible souund and physical shudder when coming off throtle with the A/C running. The compressor is fine. The shudder/sound only happens from about 1400-900 rpm. When the fan is taken off of the car, the shudder/sound goes away. Now before you say it is the fan, the belts to the A/C shake violently at the 1400-900 rpm range with or without the fan on the vehicle. None of the other belts shake as bad as the A/C and none shake with the A/C turned off. I don't think the belts should shake at all, but the mechanic says that's usual since the A/C belt is the longest in the chain.

So, anybody want to venture a guess? I am stumped.

Thanks,

Russ
 

Jack

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Joined
Dec 27, 2000
Messages
1,825
Location
Florence, SC (Timmonsville SC)
Corvette
71 War Bonnet Yellow VERT 71 BH Blue CPE (SOLD)
check stamped pulleys, belts, comp

/RUSS ... See, the A/C is 2 months outside of the 6mo/6000 mile warranty, but I've only put about 600 miles on the car. And, since this would be the second replacement of the compressor, I think I could talk Napa into paying for the replacement...The problem is an audible souund and physical shudder when coming off throtle with the A/C running. The compressor is fine.

Russ:
First, after I'd verified the pulleys are round and they have no spots where bent that could effectively squeeze the belt into an eccentric (check closely any stamped pulley by measuring the groove width at many points, the cast/forged ones aren't so prone to bending) as the belt(s) may be riding up & down within the groove yielding a natural frequency for vibration at 9-1.4K rpm ... If pulleys OK, I'd consider changing the belts to another name brand (GM, Dayco, Gates). Also ... Maybe the compressor is "bad" ... even if it's pumping gas OK ... maybe you could get napa to exchange. Longer belts do tend to vibrate ... as does inadequate bracing ... seem to recall the short fat R4 ac comps moaned & shuddered more than the older A6.
JACK:gap
 

77-4speed

Well-known member
Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
I have checked the pulleys and they are indeed round. And I do have the short type of compressor. I'll make sure they test the compressor and if it's okay I guess I'll just have to try a new set of belts. If that doesn't work, then a march compressor bracket. If that doesn't work, dual spal fans.

Thanks for the info Jack.

Russ
 
V

vmrod

Guest
I had a similiar problem. My compressor was starting to lock-up on me. Of course, when the A/C is off, the internal A/C clutch allows the pulley to spin freely, however when the A/C is activated the compressor spins with the pulley. If the compressor is locking up, then the pulley will tense up too.

I actually saw the pulley siezing up, causing the belt to squeal and smoke and cause the whole engine to 'freak out' a bit. As a quick fix, I removed the A/C belt. Problem temporarily solved!

I'm betting that it's your A/C compressor. The internal A/C clutch however seems to be fine.
 

77-4speed

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Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
I suppose I should expound.

The shop "tested" the complete compressor assembly and said everything is ok. They removed the flex fan and the belt still shook when the A/C was on, but no noise. I installed the stock clutch fan (temporarily) and got belt shaking but no noise. So I think I can logically deduce that the load placed on the engine while running the A/C puts the flex fan in a mode of vibration at 900-1400 rpms. If I want to get rid of the noise I'll have to reinstall the clutch fan permanently or try to figure out how to balance the flex fan, or go with dual spals :)

Russ
 
V

vmrod

Guest
I would think that the clutch-fan vibrates less, because the clutch system can absorb the braking action of the A/C compressor (if the compressor is the problem). The flex fan on the other hand is forced to spin at the same rate as the pulley which is driving it. Therefore the flex fan would be prone to more vibrations due to centrifigal force.

I'm still suspicious of the A/C compressor. Is the shop that tested it 'reputable'? It is possible that the compressor still 'checks out' as far as compressing the freon, however maybe they did not check to see if it was binding up excessively.

Think about it. Everything works fine until the A/C compressor kicks on. The compressor seems to be putting a larger than normal strain on the motor. Maybe you can look real close at the A/C pulley while the car is running and notice it binding up, but maybe it has not reached that point, or it could be something else.

I just had the exact same problem before. That is why I suspect the compressor.
 

77-4speed

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Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
vmrod,

I understand your rational and I have the same thoughts, but all I can do is trust that the shop checked the compressor correctly. The shop seems good and everyone that I talk to has good things to say, but they're not a corvette shop. Unfortunately, there aren't any reputable corvette shops in the area.

I don't notice the compressor pulley binding up but then I don't know exactly what to look for. By the way, my rpms drop from about 900 to about 500-600 when the A/C kicks on, how far do yours drop? Do you know of any way to test the compressor?

I really wish that I could figure this thing out for sure but I'm stuck right now with the differential off of the car for a short while.

Thanks,

Russ
 

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