thud when ac is on while driving.

Post by **RobTheModd** » 23 Apr 2010, 14:42

Hey guys, something new.

When I have my AC on, and driving at low rates of speed, around 30mph and under.
I feel a thud.

usually I do not drive with ac on as it slows my car down substantially, but its Florida and it is getting hotter.

Any clues as to where to look first?

or does anyone know the specs the car should be running with the AC on, and driving.
ie, the load, rpms, and just what ever.
I can measure them all while driving, or parked.

Thanks for your help.

Post by **jblackburn** » 23 Apr 2010, 14:56

More specifics! When do you hear it, getting on/off the gas at low speeds, when the transmission changes gear...?

If so, it's probably a motor mount showing signs of wear under heavy engine load. The top one (as you probably already know) at the firewall, and the lower one under the transmission are usually the suspects for making clunking noises. The subframe ones will cause a lot of engine vibration, but not necessarily noise. Those can be checked, too - you should be able to insert a finger between the bottom of the engine and the front frame bracket.

As far as the AC slowing it down, actually, which was something I always hated about this car - it went from feeling fast to feeling like it had about 100 hp at the touch of that button, I noticed a HUGE improvement once I cleaned the PCV system on my car. It used to feel like a cow with the AC on and just accelerating normally under 3000 rpms, and it would rev past 4 sometimes just driving at my normal pace (not like a maniac!). Since I cleaned that up, it actually doesn't feel like it's lost much power with the AC button on at all!

Then again, it's a turbo. I suspect a lot of the problems traced back to the vacuum on the PTC hose to the turbo, so I don't know whether the same solution is true for the NA cars or not - my 4-cylinder cars were just downright lethargic with the AC on and nothing could be done about it.

Post by **RobTheModd** » 23 Apr 2010, 15:02

Its more so felt when idling.
my upper is brand new.
I will get at changing the lower ones when I can. no job = no money to do stuff...
250k on them, im sure replacing them would be a good idea.

also, im smoking alot due to pcv, it stopped when I changed the flame trap, but its back again.

perhaps a pcv is in order aswell as the motor mounts.

Post by **jblackburn** » 23 Apr 2010, 16:24

I know the feeling. I'd be $200 short of buying the new Macbook I want if it weren't for car repairs this year.

Does it just feel like a heavy vibration at idle? Probably still a mount, just trying to point you in the right direction. For now, you can at least figure out which one(s) it is.

Post by **RobTheModd** » 23 Apr 2010, 16:37

I actually just talked it over with my wallet...
and my next thing is all the mounts, and pcv system.
or saving up for an 854 turbo. then dumping my interior into it since I LOVE my interior!

finally got it like I want it.
would cry to see it sold.

might just save up for a turbo, with lower mileage.

but in the meantime... replacing motor mounts, lower in particular would be a good idea.

I have a new #3 on that diagram. polly upper from IPD.

anyways, ill try to be a bit more specific on whats up.

It feels like the car is shifting into gear, while I am idling with the AC on, when the AC is off there is nothing, in fact the car rides quite smooth without it on.

they are very sudden thuds.
I would say about 10-15 seconds apart from each other. ill try to recreate it tonight, with my vagcom running so I can see if there are any spikes in readings when it happens.

Post by **VCA** » 24 Apr 2010, 06:19

RobTheModd wrote:they are very sudden thuds.
I would say about 10-15 seconds apart from each other. ill try to recreate it tonight, with my vagcom running so I can see if there are any spikes in readings when it happens.

See if the thuds coincde with the A/C clutch applying/unapplying. The compressor might be loose, or something else in the serpentine belt drivetrain is moving from the extra drag.

Post by **RobTheModd** » 24 Apr 2010, 09:46

hmm idk why I didnt think about that.

my clutch is *going* apparently. cycles on and off.
i will check today.

whoa · Post by **whoa** » 24 Apr 2010, 11:14

That could mean you don't have enough gas in there.

HVAC Dude · Post by **HVAC Dude** » 24 Apr 2010, 13:36

RobTheModd wrote:hmm idk why I didnt think about that.

my clutch is *going* apparently. cycles on and off.
i will check today.

Cycling on/off is not a bad thing all the time. It depends on a few things like charge level, ambient temperature, and climate control settings in the cabin. If the weather is in the 90s and you are seeing a lot of sun I would not expect any cycling at all.

Someone already mentioned that you may be low on gas. That could be part of it. The other part is the sluggish power you wrote about. The compressor can take up to 7 hp from the engine. If the compressor is loose, it may clunk. If the compressor is loose, the belt may not line up and take more power to run the compressor. Check that that mounting bolts are tight to the compressor, then check that the fat line back to the compressor gets cold. If it does not get cold, the chances are that you may be losing your refrigerant and it could be due for a recharge.

Of course the fan has to be working too. Make sure that is happening. That could also account for the cycling behavior.

Post by **jblackburn** » 24 Apr 2010, 13:40

Only 7? The AC feels like it drains about 20-30 hp from my dad's V6 Liberty, and used to feel like it drained a ton from mine. I attributed that to the clogged up PCV system with the engine working harder. It's not as noticeable on my girlfriend's Civic, but it doesn't cool nearly as well either with the small engine.