Your Diagnostic Input, Please

37649 Views 144 Replies 35 Participants Last post by jsvette, Mar 23, 2017

NightRaceRick

Discussion Starter · Nov 11, 2014

I have a service appointment and have always found that I get better results when suggesting what I think the problem is, so here's what's up. At interstate speeds of between 74-78 MPH, I get a howl from over my right shoulder in M8 and D8. If I slightly accelerate, the howl diminishes above 78 and goes away as I top 80, and obviously, it goes away if I lift and drop below 74. My initial reaction was the right rear tire, and then the non-functional ZF1 right side vent, but what has changed my mind is that I can also make the howling stop by downshifting to M7, and make it come back just as quickly by upshifting back to M8 while maintaining the same speed in both gears. This scenario makes me think it may be in the drivetrain. What has me worried is that the Service Mgr. says she has never heard of this even though they sell about 500 Corvettes a year. I also mentioned lag between 1st & 2nd gears, which I was also told that they haven't heard of that, which can be expected with a new model year tranny. Fortunately, BOBSZ06 posted a Service Bulletin and I followed the instructions for "Active Learning" for the A8 and it cured the lag. The last thing I want to hear from the the Service Dept. is we couldn't find anything wrong. The howl has been going on since delivery to currently 400 miles, so anything you guys think I should suggest to the Service Dept. would be greatly appreciated.

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rdslon01

· #2 · Nov 11, 2014

Well, it is not in your differential or rear axle or wheels. If it were, it would not go away when you shifted to 7th and maintained the same speed.

It is not in your torque tube or engine. If it were, it would be in a certain RPM range, not a certain MPH range.

That only leaves one thing: the A8 transmission, which, yes, is in the rear over your right shoulder.

People may get confused and lose sight of facts which logically rule out certain things. You have to keep repeating the facts and steer them back to what the facts and symptoms are.

It is in your transmission.

Reactions: 5

jsvette

· #3 · Nov 11, 2014

Have you tried to duplicate it with the roof panel removed?

Reactions: 1

mjw930

· #4 · Nov 11, 2014

Do you have NPP (dual mode exhaust)? If so, can you change the sound by switching modes?

Reactions: 1

NightRaceRick

Discussion Starter · #5 · Nov 11, 2014

Appreciate the help Rodney, Jeff, Mark! To answer the questions, I haven't had the top out of the car and my NPP is defaulted to Track. Will give those things a try.

mjw930

· #6 · Nov 11, 2014

Switch the exhaust so it changes with the mode, I do that on the highway because I get what could best be described as drones at certain speeds with it open all the time.

Reactions: 1

NightRaceRick

Discussion Starter · #7 · Nov 11, 2014

I'll give these suggestions a try this evening when traffic has died down on I-81. Since I'm still in the break-in period, I've stayed out of V4 mode, so I'll give the D/V4 combo a try and see if that makes any difference too.

Chip

· #8 · Nov 11, 2014

Rick; First off, don't let that service manager's comment put you off. The C-7 A-8 is different from every other Corvette that has ever been produced. You mention the noise comes and goes and appears to be speed sensitive. Is it RPM sensitive as well? When the noise comes and goes, is that growl the same in intensity? What I mean is; Is there any increase or decrease in sound as you approach the threshold speeds that you have indicated? One way to duplicate the highway experience is to place the car on a lift and run it to the same indicated speeds and get the mech. to use a stethoscope and listen to points along the drive line. The driveline will not be loaded as it would be on the road, but maybe the issue can be isolated none the less. Good luck!

Reactions: 1

jsvette

· #9 · Nov 11, 2014

Chip said:
Rick; First off, don't let that service manager's comment put you off. The C-7 A-8 is different from every other Corvette that has ever been produced. You mention the noise comes and goes and appears to be speed sensitive. Is it RPM sensitive as well? When the noise comes and goes, is that growl the same in intensity? What I mean is; Is there any increase or decrease in sound as you approach the threshold speeds that you have indicated? One way to duplicate the highway experience is to place the car on a lift and run it to the same indicated speeds and get the mech. to use a stethoscope and listen to points along the drive line. The driveline will not be loaded as it would be on the road, but maybe the issue can be isolated none the less. Good luck!

I don't think that they would put your (or any car) on a lift and run it up to some indicated speed around 78 to 80 mph. They would be fairly dangerous. If they were going to try and duplicate the road situation they could put it on a dyno which will safely allow bringing it up to speed.

Chip

· #10 · Nov 11, 2014

jsvette said:
I don't think that they would put your (or any car) on a lift and run it up to some indicated speed around 78 to 80 mph. They would be fairly dangerous. If they were going to try and duplicate the road situation they could put it on a dyno which will safely allow bringing it up to speed.

I just had my 4X4 truck up on a lift for a rear end noise and they did exactly what I suggested. No problems, other than the bad axle bearing they found.

NightRaceRick

Discussion Starter · #11 · Nov 11, 2014 (Edited)

Thanks guys! Well, I guess it starts out as being a speed sensitive issue as it comes on quickly and at it's loudest when it starts at 74 MPH. If I lift, it immediately stops, but if I continue with slight acceleration, it gradually diminishes after 78 and is gone by about 82. If I want to reproduce the noise, I'll usually have to drop down to 74 to get it to start again. Then there's the RPM sensitive part of it. If I drop into M7 it immediately disappears, and when I bump up to M8, it immediately returns. My speed is constant, so the only thing changing is the RPMs. This all takes place in V8 mode as I'm trying to keep it out of V4 mode for the 500 mile break-in period. I'm going to take her out in a bit when the interstate is less busy and try a few things with the different modes for the tranny, engine & exhaust, and check the top too.

Petercwelch

· #12 · Nov 11, 2014

Take the service manager for a little ride!

Reactions: 3

NightRaceRick

Discussion Starter · #13 · Nov 11, 2014

Petercwelch said:
Take the service manager for a little ride!

Oh, I'm going to offer that up for sure. If they decline, and then tell me they can't duplicate the noise, someone is definitely going for a ride!

Reactions: 1

NightRaceRick

Discussion Starter · #14 · Nov 12, 2014 (Edited)

Back from a variety of test runs taking into consideration every suggestion mentioned. What I have found is that regardless of V4 or V8 mode, exhaust setting (touring, sport, track), drive mode (touring vs. sport), transmission (D vs. M), top on/off, the noise remains the same. What I was able to determine is that the "howl" starts when the car is traveling at 73-74 MPH at 1500 RPMs. The speed may vary slightly depending on if the car is going slightly up or down a small grade, but the RPMs are always dead on 1500 in 8th when it starts. When the howling is evident, I can blip back and forth between 7M @ 1900 RPMs and 8M @ 1500 RPMs and the howl stops in 7M and restarts in 8M like a light switch as long as I maintain the same 74 MPH speed. I also tested at 1500 RPMs in M7 and M6 and was unable to duplicate the howl the car makes at an identical 1500 RPMs in M8, so I guess the howl is speed sensitive as well as RPM sensitive. ...Wait...what

The info provided and suggestions made have been invaluable in helping me narrow down and pinpoint what I need to communicate to the Service Mgr., and I really appreciate it. I'd buy you guys a drink or a soda if near

Post-test run thoughts/suggestions?

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Reactions: 1

rdslon01

· #15 · Nov 12, 2014

It still consistently points to your transmission's 8th gear.

Reactions: 2

NightRaceRick

Discussion Starter · #16 · Nov 12, 2014

rdslon01 said:
It still consistently points to your transmission's 8th gear.

Thanks Rodney!

Chip

· #17 · Nov 12, 2014

Rick; Is this sound loud enough to record on your phone or other device? It might be worthwhile to get a passenger to shoot a video with you doing a narration containing the info. you have posted above. It would make for good documentation.

Reactions: 1

NightRaceRick

Discussion Starter · #18 · Nov 13, 2014 (Edited)

Chip said:
Rick; Is this sound loud enough to record on your phone or other device? It might be worthwhile to get a passenger to shoot a video with you doing a narration containing the info. you have posted above. It would make for good documentation.

Thanks chip! That could be a possibility. It's loud enough that I can hear it over the radio, so with the radio off.....

NightRaceRick

Discussion Starter · #19 · Nov 16, 2014

Petercwelch said:
Take the service manager for a little ride!

Dropped the car off Saturday, but the Corvette Service Mgr. was off. Because of the volume of Corvette sales Criswell does, they have a dedicated Corvette Service Mgr. that will being handling my car, so I didn't ask the Saturday guy to go for a ride.
Also, I was finally able to duplicate the 1500 RPM/8th gear howl at 1500 RPM/7th gear, although it was only about half as loud. Just trying to provide as much info as possible for the techs to work with.
Talked to forum Supporting Vendor Mike Furman while there, as he was getting ready to deliver a DSOM vert. My first in-person look at a DSOM, nice!

rdslon01

· #20 · Nov 16, 2014

Ah, you have now produced the sound in two different gears at the same RPM... That makes this issue not so clear cut now...

Reactions: 1

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