Bad Tachometer?

[IceHammer]

Does anyone know how the tach on the 3.4L v6 works? Is it a mechanical linkage? Or is it sensor voltage based?

My car seems to be reporting an engine speed about ~400 rpms slower than it should. I say this because with the A4 when I'm going 45 and overdrive engages the tach reads ~1100 when it should be more like 1500. Same deal at 75, it reads 2000 when i would expect closer to 2300-2400. Finally, pedal to the floor it shifts at 5100 rather than 5500 yellow line.

So the main question here is if the ECM is getting a low voltage reading from the tach, will it cause my car's issues (low rough idle, high idle, reset w/ battery disconnect)?

[cam98aro]

Re: Bad Tachometer?

not sure, but im guessing it is a sensor/motor that controls the tach

and that sounds about normal, its pretty tough to find someone with a 4th gen f body and have the tach, or any gauge for that matter accurate, mine is about 300rpm off.

the best thing you can do is find a friend with a good OBD scanner (if OBD supports that), or a tool that hooks to the plug wire, and see if they can get the real RPM of the engine

[whitdog54]

Re: Bad Tachometer?

Agreed, I dont think anyone has a stock tach thats completely acurate. Mines off by about 150-200 rpms according to aeroforce gauge. I think your ecm should be fine though. Unless you actually start to get a low rough idle, but if you cars running fine, then you just got a crappy gauge like most of us.

[IceHammer]

Re: Bad Tachometer?

No it does have a low rough idle a lot of the time. When it doesn't it stalls then has a high idle. I'm wondering if the tach reading is related. When I bought the car a year ago it seemed to be around 200rpm low, now it seems to have risen to 400 around the same time these issues came about so im looking to see if they can be related.

[insaneABOT]

Re: Bad Tachometer?

The tach is electrical is comes from your coil pack wiring into the car.

[whitdog54]

Re: Bad Tachometer?

I dont think you have an issue with your tach, your engine is just running at low rpms. I would check vacuum lines.

[insaneABOT]

Re: Bad Tachometer?

Does the car sputter or stall because it idles so low?

[IceHammer]

Re: Bad Tachometer?

It stalls occasionally, not positive its attributable to the revs, but it does get a wicked vibration if in gear and stopped when its doing the low idle.

[whitdog54]

Re: Bad Tachometer?

how many miles are on your car? Could be fuel system related? Does it hesitate when accelerating?

[IceHammer]

Re: Bad Tachometer?

A little shy of 140,000. I replaced the gas tank last year which included all of the lines, the FPR and the fuel filter. So that leaves injectors and pump which i suppose is possible but I think its unlikely because of how well the car appears to run when it isn't idling. It does have a slight hesitation when the car is cold and you touch the gas ever so lightly, but once its warmed up that disappears as well.

What could cause the extreme rough idle other than a misfire?

[whitdog54]

Re: Bad Tachometer?

vacuum leak