Read RPMs from OBD2 Port?


#1

Is it possible to hook up something to the OBD2 port that would read the RPM’s? I ask because I am trying to figure out if the tachometer is showing the true RPM’s - which I think it is not. Is there a portable tachometer that is cheap or possibly something that attaches to a usb for a laptop?


#2

An engine analyzer will do that. Try any parts store. Bring your wallet.
They’ll probably be hanging from the same rack as the lesser OBD2 scanners.


#3

You can buy an inexpensive OBDII display for your instrument panel;

https://ultra-gauge.com/ultragauge/ultragauge_wired.htm?gclid=CPGcpKqH38sCFRVsfgodeHAPwQ

For less than $20 you can buy an OBDII cable and software for a laptop computer.


#4

Mtnbike said it… He is correct. Depending on what make and model car you have there could be other options. For instance if it was a VW/Audi product you could use your laptop and VAG-COM software and OBD2 cable made for the Vag-Com. You can do amazing things with that setup… Can see every sensor…troubleshoot…disable functions of the car or security system…its uses are limitless.

For most other makes however you need to take Mtnbikes advice…and like he said…bring serious cashish with you. What you are getting into with that is Professional Tool territory.

Blackbird


#5

Why do you think the tach is off? BTW, if you read the RPMs from the OBDII port, you are getting the exact same info the tach in the dash is getting.


#6

Nevada_545 thanks for the link, that looks pretty cool.

Keith: basically once I am in fourth gear the tach sits at around 2000. Even when accelerating the tach barely moves at all.


#7

Do you have a CVT transmission? How fast are you going when the tach is sitting on 2000?


#8

The last description sounds like the tach is needing replaced. Find out if it can be fixed before buying an instrument you might only use once. If this unknown vehicle is an older vehicle and it runs fine why even worry about the RPMS. Not all cars have RPM tachs and people use them just fine.


#9

Its a 2001 dodge grand caravan with a 3.3L V6 engine.
I am going about 60mph and when I press on the gas to go to 75 it might move from 2000 to 2200 rpm. It seems to accelerates fine


#10

Sounds perfectly normal to me.
The engine at that vehicle speed is hard-linked to the driven wheels. It will not gain RPM faster than the car gains speed, and it doesn’t have to change RPM much to get the GC from 60 to 75.

And I seriously doubt if it’s going to have the poop at that speed to downshift. If it did, it would probably be beyond any useful torque curve… so it doesn’t. Since it’s accelerating okay, I can safely say that it’s already in a useful part of its torque curve. It knows these things, so it stays in the same gear.

I love to invest in stuff like engine analyzers. But for you it might be a waste of money. It might make more sense to be sure the engine is properly tuned up and running right. A compression test kit might be a better investment. You can use that to find out if the engine is getting tired.

How many miles on the ol’ puppy anyway?


#11

umm… the math. 2000 RPM at 60 MPH = 2500 at 75. close… but the OP might want to try for an accurate reading.


#12

Here’s something for $36.


#13

The tachometer reading sounds a little low. The gauges in that instrument cluster can be adjusted with the calibration feature in the DRBIII scan tool but I don’t think it is worth the effort. For less money you can buy a display that shows most of the engine values.


#14

“An engine analyzer will do that. Try any parts store. Bring your wallet.
They’ll probably be hanging from the same rack as the lesser OBD2 scanners.”

“For most other makes however you need to take Mtnbikes advice…and like he said…bring serious cashish with you. What you are getting into with that is Professional Tool territory.”

What? What do you consider “serious cash” ? I have had some Actron “Auto Scanner” code readers for a few years (In fact I put one in my son’s, wife’s, and daughter’s car in case they have a problem it lets me help from afar by cell phone) that gives a lot of good information, including Engine RPM. I bought them with discounts or coupons and I wouldn’t consider them expensive. They sell several models and some will indicate RPM for sure.

CSA


#15

This is a 15 year old van, if it runs and the speedometer is reasonably correct why even worry about the RPM’s. It is not like you are going to manual shift the automatic transmission on track days.


#16

There is an add-on engine meter that used the data port to get info. It can display lots of different things, like oil temperature. I’m pretty sure that RPM is one of those.

The thing is, this is not an engine analyzer, but is designed to look good attached to your dash somewhere.

But, I can’t find it online now, although I saw it a few months ago from a link posted here. There were several brands.


#17

One of the more well known multifunction displays is Linear Logic’s Scan Gauge. The hypermilers swear by it.
http://www.scangauge.com/


#18

BLE: thanks


#19

Your RPM at 60 should be 1866 and 2177 at 70 for a top gear ratio of 0.69:1 and a 3.62:1 rear. that seems close enough to your reported values given your OEM tachometer. Don’t sweat it.


#20

CSA…Serious cash for me starts around $7K ? Some of those snap on Pro Scan tools exceed this… that is what i was referring to. I’m not too familiar with what the lower end simple scan tools can do…but it is certainly possible that some could monitor this…I dunno?

I thought you were talking about a Sunnen Pro or Snap on Scan tool / computer. Ha

Blackbird