Tach not reading correctly

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vt solo2

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Hi everyone, I tried searching for this, but didn't find anything. So about a week ago I noticed that my idle has been relatively low. Usually it idles (in gear) right around 650 to 700 give or take. Lately I've been noticing 400 or so. The tach will read a little higher (like 450) when in park. The car's been running great lately (knock on wood), definitely doesn't seem like the rpms are that low. But anyway, I got home tonight and put the scanner on it, no codes, but while looking at the data, the RPMs showed up at around 700 rpms. So from what I can gather the tach seems to be around 300 low. I first noticed it about a week ago, it was low only a couple of times, but over the past day or two it seems to be low all the time, so it's becoming more frequent. Has anyone come across this before? And better yet, any idea how to fix it?

Some background, it's a 2000, V6, automatic and 108k miles on it. I know I have a coil on it's way out on the passenger side, it's not bad yet, just got the 40-50 mph stutter right now. I also just replaced the IAC and TPS not quite a month ago. Thank you in advance, I appreciate any help!

Vinnie
 
I have a similar problem, but the opposite. Mine reads about 200 rpm high. Mine is a v6 manual and doesn't have any codes for the idle either. Although mine idles at about 880 rpm fully warmed up with no loads.

I'm not sure what the problem is, hopefully we can find something out.
 
Silver_LS said:
Not sure if this is possible, could the Tach Sensor whatchamacallit, be dirty?
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No.
The tach value comes from the engine computer. The engine computer not only knows exactly how fast the engine is running, it also knows exactly where the crankshaft is turned to at any given instant. If it didn't the engine couldn't run because the computer couldn't fire the plugs and the injectors at the right times.

The OP's instrument cluster is defective.
 
gotcha

joegr, is the sensor for the speedo in the trans or at the wheels, or neither? Mine reads that I'm going 4mph faster than I really am. I have stock wheels and tire size.
 
Silver_LS said:
gotcha

joegr, is the sensor for the speedo in the trans or at the wheels, or neither? Mine reads that I'm going 4mph faster than I really am. I have stock wheels and tire size.
Click to expand...

The ABS controller averages the wheel speed sensors (one at each wheel) to create the vehicle speed. Speedometers typically read a little fast so that no one can blame the manufacturer when they get a speeding ticket. If your tires were brand new and fully inflated, the speed would be about right. As the tread wears off of the tires, the actual speed of the car decreases from what the speedometer says.
 
joegr said:
No.
The tach value comes from the engine computer. The engine computer not only knows exactly how fast the engine is running, it also knows exactly where the crankshaft is turned to at any given instant. If it didn't the engine couldn't run because the computer couldn't fire the plugs and the injectors at the right times.

The OP's instrument cluster is defective.
Click to expand...

I was worried that it could be the instrument cluster. I was so relieved to see the scanner showing the RPM's to be where they should be, at least the car isn't really running at 300 rpms. I was planning on disconnecting the battery this weekend with the hopes that it might reset and zero out the tach. Am I wasting my time?
 
vt solo2 said:
I was worried that it could be the instrument cluster. I was so relieved to see the scanner showing the RPM's to be where they should be, at least the car isn't really running at 300 rpms. I was planning on disconnecting the battery this weekend with the hopes that it might reset and zero out the tach. Am I wasting my time?
Click to expand...

Probably [a waste of time], but I suppose that stranger things have happened.
The only instrument cluster failures that I have heard about were caused by someone removing needles to put an overlay on the gauge face, but there's a first time for almost everything.
 
joegr said:
Probably [a waste of time], but I suppose that stranger things have happened.
The only instrument cluster failures that I have heard about were caused by someone removing needles to put an overlay on the gauge face, but there's a first time for almost everything.
Click to expand...

Yay! Glad I can be a first! And of course this morning the gauge was reading correctly. I honestly don't get this car sometimes. :Bang

Thank you so much for your help, I really appreciate it.
 
vt solo2 said:
Yay! Glad I can be a first! And of course this morning the gauge was reading correctly. I honestly don't get this car sometimes. :Bang

Thank you so much for your help, I really appreciate it.
Click to expand...

I imagine that the servo that drives the tach needle is sticking/binding or otherwise failing. You may find that it works at some interior temperatures and not at others.
 
vt solo2 said:
I was worried that it could be the instrument cluster. I was so relieved to see the scanner showing the RPM's to be where they should be, at least the car isn't really running at 300 rpms. I was planning on disconnecting the battery this weekend with the hopes that it might reset and zero out the tach. Am I wasting my time?
Click to expand...

I reset mine several times and it's the same thing.
 
joegr said:
The only instrument cluster failures that I have heard about were caused by someone removing needles to put an overlay on the gauge face, but there's a first time for almost everything.
Click to expand...

That's what I was thinking. I've thought about pulling the cluster apart and resetting it.
 
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