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Topic: Tach vs. Scangauge D (Read 730 times) previous topic - next topic

Tach vs. Scangauge D

I recently installed a ScanGauge D on my FT.  On our recent trip, I noticed that the tachometer reading on the ScanGauge and that of the coach dash don't agree.  Please check my thinking...

There are no Google results for adjusting the tachometer reading of a Scangauge, so it is probably not adjustable.  The information for the ScanGauge comes directly from the engine ECU (1998 U-320 with M-11), so the reading should be accurate.  It seems very unlikely that a poor connection or etc would give a stable (though incorrect) reading - it would likely be erratic and unpredictable.

Sooo...  Does it sound like the dash tachometer just needs to be adjusted to match the ScanGauge?  My reading indicated that there is a small screw on the side of the gauge for adjustment.

Re: Tach vs. Scangauge D

Reply #1 –
I think the only gauge I have that reads correctly is the speedometer.  My coolant gauge caused me several sleepless nights and a very aggravating  2 days of driving trying to keep my coolant temp below 210. Ordered a BlueFire reader after that trip to figure out that the Gauge  was incorrect.  Sleep much better now.

Didn't realize at the time that Roger Engdahl from the forum sells a similar and most on here would say better ECU reader. Think it is VSM

Re: Tach vs. Scangauge D

Reply #3 –
So with the VMSpc, is the tach display accurate, or must it be adjusted to be accurate?  I am still thinking the number from the ECM would be accurate as-is.

Re: Tach vs. Scangauge D

Reply #4 –
VMSpc displays what the ECM reports.  Most VMSpc gauge readings can be modified with an offset and a multiplier.  You would have to have pretty good external measurements to make an adjustment to what is reported by the ECM.  Make changes using the parameter editor.

But road speed, miles, miles per gallon are all dependent on things you could change like tires.  A bigger tire has a longer circumference, more distance per revolution.  Lower air pressure than nominal will get a slightly smaller circumference.

So if your ECM reports 100,000 miles and you think it should be 100,100, you could put in an offset of 100 miles.  A few thousand miles later it will likely be different again by some small amount, so you add in a multiplier to make up for the difference, maybe 1.0002 or 2/100s of a percent. That will get you an extra 26 miles if you had 130,000 to start.  Just how important is that?

How would you know if your engine is turning at 1375 vs 1365 rpm?  If VMSpc displays 1365 RPM as reported by the ECM and your dash tach reports 1400 which one do you think is more correct?  Why?

I rely on air pressure gauges on the dash.  Everything else and much more is on my VMSpc display.  Much easier to see, read and know that critical readings are within my expected ranges at a glance.  Small differences are not something I am going to worry about.

Roger

Re: Tach vs. Scangauge D

Reply #5 –
Quote from: Roger & Susan in Home2 –
VMSpc displays what the ECM reports...

How would you know if your engine is turning at 1375 vs 1365 rpm?  If VMSpc displays 1365 RPM as reported by the ECM and your dash tach reports 1400 which one do you think is more correct?  Why?



Roger

I get what your saying.  At 60MPH, the tachometer vs. ScanGauge difference is 500RPM.  Someone more accustomed to the rig could likely tell by engine sound or shift points which reading is correct.  I am not that good yet.

Re: Tach vs. Scangauge D

Reply #6 –
At 60 mph you are right in the speed range where the transmission will shift from 5 to 6.  If the mode is off on mine normal shift point is about 62. With the mode on down shift is delayed into the high 50s.  My ism11 runs about 1365 rpm at 63.  64-65 mph is just under 1400 rpm.  Every one will be a bit different.

500 rpm difference between gauge and the ECM most likely means the gauge is toast.
 

Re: Tach vs. Scangauge D

Reply #7 –
Capstone

I found no evidence of the tach reading on the ScanGauge D being adjustable, so I think the reading is a valid one.

The reading on the dash tachometer is adjustable.  I finally found instructions, and I have attached them.  There is a button on the back that allows the face reading to be adjusted up or down.  I adjusted the reading down a lot, and now it is at, or very close to, the ScanGauge reading.

My dash tach is OEM style.  The hour reading SUGGESTS that the tach has been replaced at some time.

I hope this helps someone else.

https://j109.org/docs/vdo_programmable_tach_with_hourmeter.pdf