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Old 04-13-2021, 03:27 PM   #1
FAC
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Year: 2003
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Engine: Cummins 24valve 5.9
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Shift rpms to high or tach reading high?

I have a ISB with a md3060. It shifts at 3200rpm on the tach, this seems high to me but maybe itís normal? Could it be reading wrong? Or is this the correct shift rpm?

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Old 04-14-2021, 10:32 AM   #2
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Location: Calgary, Alberta
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Year: 2001
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Engine: Cummins ISC 260HP/660Q/MD3060 6spd
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Sounds high to me. My old ISB would shift at 2500 rpm max. Tach might be off like mine currently is, or you have another issue. I wouldn’t keep letting it do that is it’ll damage the engine.
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Old 04-14-2021, 10:56 AM   #3
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Year: 1954
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Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
24 valve 5.9 Cummins

3200 rpm governor limit

Peak torque around 1600 rpm

Peak horsepower around 2900 rpm

Sounds like transmission shift points are too high in the rpm range.

I would expect to shift at 2900 rpm.

The worry point is at 3900 to 4100 rpm. - do not run that engine within those rpm. Odd harmonics tend to break the crankshaft.

William
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Old 04-14-2021, 12:16 PM   #4
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Thanks for the replies,

Bon Voyage. How did you determine your tach is off? Is there a timing light tach or
Something?

Magnakansas, it honestly feels like it’s shifting at 2900 compared to the other 5.9s I’ve driven, the way the power starts to fall off right before it shifts feels like the tach is wrong to me. It will rev past 3200 on the tach in neutral. I eased into it and let off at 3600 just to see if it would stop at a certain rpm, I didn’t go higher because fear of doing damage like you suggest.
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Old 04-14-2021, 01:34 PM   #5
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My tach sometimes seems high but sometimes doesn’t , shift points seemed all over the place and then sometimes when I tap it the needle will drop to lower number.
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Old 04-14-2021, 02:21 PM   #6
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Hmmm mine seems
Very consistent and smooth
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Old 04-14-2021, 03:55 PM   #7
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there was one that i looked into years ago that someone mentioned on here and it was more of an app for your phone and called acoustitach .
might help you check your actual one?
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Old 04-14-2021, 04:59 PM   #8
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Join Date: May 2018
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Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
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verify tachometer....

There are a couple of ways......well three that I can think of...

a mechanical rpm gage.... works from vibrations kinda like a lot of little tuning forks in one little package. I dont see them very often.

Stewar warner makes a hand held mechanical tachometer, you hold it on the center of a pulley and read the gage.... some engines are tough to get onto the crankshaft, so you put on on the center of any pulley you can access and do the math for crankshaft rpm.

optical tachometers.... mark a spot on a crank pulley and aim the optical tachometer at the spot... take a reading.

those are the three test methods I have used myself.....

dodge trucks use a crankshaft fired tachometer sensor. Many others, at least with the 12 valve cummins 5.9 use a sensor in the bell housing and count flywheel teeth. I do not know how your tachometer is driven.

william
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Old 04-14-2021, 05:37 PM   #9
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Thanks jolly roger! I couldn’t find the acoustitach app but found another app and just tried it, my idle rpm was off by 200 and 3200 on my tach was actually 2550 according to the app.

Which means my idle rpm is actually 800 not 1000 and my full throttle shift rpm is actually 2550 which is much closer to what bon voyage says his shifted at.

Im thinking my shift points are actually right on and just the tach is reading high.

Now can it be calibrated? Or just leave it be? It doesn’t matter too much to me to have an accurate tach I guess
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Old 04-14-2021, 06:16 PM   #10
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Join Date: May 2018
Location: topeka kansas
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Year: 1954
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Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
Calibration

Now you have two different rpm readings. Are either correct? I had the same thing. Now you do a third method and verify which is correct. Either look up the tachometer model, or remove the tachometer and see if you have switches on the back side you can change the rpm reading. One of the tachometers I have, there are 8 switches on the rear used in calibration.

William.
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Old 04-14-2021, 10:11 PM   #11
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I get what your saying William, do you think math is good enough as a third reading? I know my tach says around 2600 at 60 but I could take her for a spin tomorrow and get a more precise number at 60 and go from there I know my differential ratio is 5.83-1 and an md 3060 is .75-1 in 5th gear. Or I could spend a few bucks and see if a local diesel shop has an optical tach or can hook up a computer and read the crank position sensor.

I don’t think my tach has dip switches on it. It is the factory Thomas dash tach.
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