pulsing vibrtion at about 2500 rpm

[Paul P.]

Hi.

My 62 vette has recently started to produce a pulsing vibration at approx 2500rpm (tacho still to be repaired).

This is not a wheel balance issue, as it occurs when reving the engine when standing still.

Today I discovered that the top of the generator belt can be seen vibrating from side to side when this phenomenon occurs.

So, is it the belt, is it the generator, or is it the top pully on th engine that is causing this?

I don't beleive it is the harmonic damper, as this is at the bottom of the belt system, and there are the generator and top engine pully's that I think would dampen any vibration in the belt caused by this.

Has anyone had any similar experiences, and what was your "cure" if you did?

Paul

[John F.]

Re: pulsing vibrtion at about 2500 rpm

I would say look at the damper first, then motor mounts (both front mounts and trans mount.

[Patrick B.]

Re: pulsing vibrtion at about 2500 rpm

I once that a similar vibration with my 70 LT-1 when it was less than 10 years old. It turned out to be the engine fan. I took the fan off and just held the pulley on with washers and nuts. The vibration stopped. I replaced the fan with a new one from the chevy dealer (probably not up to NCRS standards). There was no visual indication that there was anything wrong with the original fan. I still have it but I am not tempted to put it back on.

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

thanks Patrick, at least that's an easy test.

Let me see what I can find out.

PP

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

John,
are there any specific tests, or tell-tale signs, when checking these things?

PP

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

OK, here are the results:
1. I took off the fan and it's assembly (see attached pics) and ran the engine - no vibration!
2. take a look at the fan (attached pics). look at the side of the fan I have highlighted. The whole fan is asymmetric! This is not because of it being bent, it's manufactured like this. Is this normal?
3. The assembly if very strange, The fan is bolted to a cast aluminium (or aluminum for you north americans) that has some sort of heat actuated clutch on it, and this is bolted to the pulley on the engine. Is this normal?

2012-07-23 14.50.07.jpg 2012-07-23 14.50.31.jpg

PP

[Patrick B.]

Re: pulsing vibrtion at about 2500 rpm

The asymetric 5 blade fan and the fan clutch are normal. The same type fan was original equipment on my LT-1 and it was the cause of the vibration. The factory replacement may have been a 6 blade fan possibly for A/C cars. I will check when I go to my garage later this week. In my case the problem was only the fan. I suppose it is possible that the fan clutch could be out of wack. Can you try a fan of the same diameter off a friends car to see whether the problem is the fan or the fan clutch?

[Terry D.]

Re: pulsing vibrtion at about 2500 rpm

I agree with John, pull the dampner and look at the backside fpr missing rubber. These things do not last forever

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

Hi Terry,
see my response to Patrick Boyd re the latest outcomes.

the vibration is still there with the fan on, but at a higher rpm. There is no vibration at any rpm with the fan off.

Therefore, I'd have to conclude that the damper is not the culprit, yet. Unless you have other reasons for thinking this not the case.

Paul

[Edward J.]

Re: pulsing vibrtion at about 2500 rpm

Paul, I myself have seen a situation that the fan clutch hub and water pump shaft dia. are different and this does not let the fan clutch center on the water pump. Also the fan clutch holes were also slotted, Just food for thought. Ed

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

Thanks Ed.
I'll take a look again, but I think this is OK re mine.

Paul

[Timothy B.]

Re: pulsing vibrtion at about 2500 rpm

Paul,

I had a water pump break and spray fluid all over me one time when helping a friend work on his 1967 L-79. Seems someone installed a fan clutch with a 3/4" diameter hole over a water pump with a 5/8" pilot shaft. That must of beat like a excentric until something gave up, I was lucky my face was not closer and was pretty pissed off after looking at the fan clutch hub hole.

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

Tim,
you have me a little confused. From memory the clutch just had a small disc that just attached to the water pump pulley with 4 bolts. The pulley didn't have any type of pilot shaft sticking out of it. Should it?

Paul

[Timothy B.]

Re: pulsing vibrtion at about 2500 rpm

Paul,

The area I want you to check is where the water pump pilot shaft goes into the fan clutch hub. The W/P shaft and clutch hub should both be 5/8" diameter.

[Paul P.]

Re: pulsing vibrtion at about 2500 rpm

Tim,
the C1 and C2 designs must be different, as my C1 doesn't have the fan clutch connecting directly with the W/P at all. The pulley connects to the WP and the clutch connects to the pulley.