Engine RPM's look okay?

Topic: Engine RPM's look okay? (Read 802 times) previous topic - next topic

Engine RPM's look okay?

September 21, 2021, 04:36:09 pm

I want to show pictures of what my tachometer shows at different driving speeds, and ask whether these look normal.
The first is normal idle, 800-900 RPM.
The second is fast idle, 1,200, set through the cruise control buttons on my smart steering wheel.
The next two show cruising on flat ground at 55 (2,150 rpm) and 65mph (2,500 rpm).
Photo 5 is WOT acceleration at 52mph: 3,100 rpm.
Last is climbing a hill in the truck lane with flashers on. Speed 49mph, water temp has started to rise, and tach is at 2,900.
So, do others see numbers like these with their Cummins C8.3?
When I posted earlier this summer about water temp rising to over 200 when climbing big hills, people said to downshift and keep rpm's UP to 2,000 or so. I'm already up around 3,000 without downshifting!

Re: Engine RPM's look okay?

Reply #1 – September 21, 2021, 04:43:46 pm

Since your engine is still running, you have absolutely PROVEN that the tach is not accurate.

Re: Engine RPM's look okay?

Reply #2 – September 21, 2021, 05:45:15 pm

Your tach is reading too high. Looks like the error gets bigger as the RPMs increase.

Our engine idles at 650 - not 800-900. My coach would not even shift out of neutral if the idle was at 900 RPM.
At 65 MPH cruise we see about 1900-1950 RPM - nowhere NEAR 2500.
During full throttle acceleration our coach up-shifts at about 2300 RPM - it never gets above 2500 RPM in any normal driving situation.

You can easily find out the correct RPM numbers for your specific engine - just register your engine at Cummins QuickServe Online. Registration is free - all you will need is your ESN (Engine Serial Number). Once registered, you will have access to a wealth of info about your specific engine, including a list of RPM numbers.

Cummins QuickServe Online

The RPM numbers for our C8.3 (copied directly from QuickServe Online) are listed below. Our engine loves running between 2000 and 2400 RPM. The numbers for your engine should be very similar.

ADVERTISED HP: 300 @ 2200 RPM

GOVERNED HP: 285 @ 2400 RPM

TORQUE PEAK (lb-ft): 820 @ 1300 RPM

HIGH IDLE: 2760 RPM (New coach power train check out)

GOVERNOR BREAK RPM: 2450

Re: Engine RPM's look okay?

Reply #3 – September 21, 2021, 05:53:28 pm

Tach, if like mine, is adjustable. I used a Scangauge D for an accurate reading reference.

From my notes:

Press button on lower back 2x and hold. Reduce number (like 5.5) to decrease, increase to increase.

Re: Engine RPM's look okay?

Reply #4 – September 21, 2021, 07:22:11 pm

Quote from: oldmattb – September 21, 2021, 05:53:28 pm

Tach, if like mine, is adjustable. I used a Scangauge D for an accurate reading reference.

I know the tach is adjustable, but Scangauge D, BlueFire, Silverleaf and the like don't work with these mechanical engines. How do I know what the actual RPM's are so I can calibrate the tach?

Re: Engine RPM's look okay?

Reply #5 – September 21, 2021, 07:30:15 pm

Buy one of these for $10-$20 and away you go. photo tachometer | eBay You can also check fan or alternator rpm too.

Pierce

Re: Engine RPM's look okay?

Reply #6 – September 21, 2021, 09:08:30 pm

Learn to back down on throttle, down shift never go over 1800 RPM or under 1500 RPM. 3^rd gear 30mph, 4^th gear 40mph. never WOT to save engine and all parts from overheating. Add Pyrometer & Boost gauge.

Re: Engine RPM's look okay?

Reply #7 – September 22, 2021, 07:26:38 am

Where do you find the RPM standards on Cummins Quick Serve Online? I have my engine serial number registered but I guess I do t know the correct term to find this info (see Reply #2 above).

Re: Engine RPM's look okay?

Reply #8 – September 22, 2021, 08:37:08 am

Quote from: Alan & Gerri Ortiz – September 22, 2021, 07:26:38 am

Where do you find the RPM standards on Cummins Quick Serve Online?

1. Go to QuickServe - log on (sign in) with ESN to enter site
2. Click on "Parts" tab at top left
3. Click on "Option View" tab
4. Scroll down, click on "Fuel Rating" FR9455
5. Under Remarks, click on "Fuel Rating...(More)"
6. Read the list of interesting info pertaining to your specific engine!

Re: Engine RPM's look okay?

Reply #9 – September 22, 2021, 10:12:08 am

If your tach is not reading correctly, it's either the sender or the tach. I'm betting on the tach. Send it in to a VDO speedo shop like Hollywood Speedometer. Not an expensive fix if you can't adjust it yourself.

Pierce

Re: Engine RPM's look okay?

Reply #10 – September 22, 2021, 10:21:19 am

I'm still trying to figure out where these tacos 🌮 are that the tacometer is metering ?

Re: Engine RPM's look okay?

Reply #11 – September 22, 2021, 11:05:08 am

Maybe find out how it works and fix it. It may be a voltage generator with a magnet and spring. Check to see if the spring is intact.

Re: Engine RPM's look okay?

Reply #12 – September 22, 2021, 11:36:13 am

The '81 I have uses the alternator for the tach signal.

Re: Engine RPM's look okay?

Reply #13 – September 22, 2021, 12:33:34 pm

Quote from: Protech Racing – September 22, 2021, 11:05:08 am

Maybe find out how it works and fix it.

Well, your comment got me curious, so I had to consult my 12V Wiring Diagram.

My tach has 3 input wires. One is power lead, the other two (both labeled T3) come from "SENDER TACHOMETER IN FLYWHEEL HOUSING".

I don't actually know where that "sender" is physically located, cuz I never looked for it. If accessible, might be worth checking for damage, or a loose/corroded wiring connector.

Re: Engine RPM's look okay?

Reply #14 – September 22, 2021, 02:27:21 pm

Does the power input come from a voltage stabilizer? Often some gauges use a controlled voltage input. If it supplies too much/too little volts, the gauges will all read off .
Mine seems to not have any voltage controll input .
Edit , if the trigger comes from the flywheel, it is a counter style. If it comes from a little tiny generator , it is a magnet /spring style . Generally if the tack counts at all, the trigger is fine . But if it gets too many counts m maybe there is a false trigger making a count. Like a bolt head or rust. etc.
Not very likely IMHO. . The tach is most likely the issue . If the rest of the gauges are close ..

edit of the edit . There is a slim chance that the tach will work with the power lead disconnected . That would show that the trigger is a 2wire VR trigger . Swap the trigger wires, check for a ground on the mounting studd.

Re: Engine RPM's look okay?

Reply #15 – September 22, 2021, 03:09:47 pm

An 8'3 Cummins tach signal usually comes from the alternator. When I
had a Dynasty with an 8.3 the tach was out but I can't remember
what I did to fix it.

Re: Engine RPM's look okay?

Reply #16 – September 22, 2021, 03:27:04 pm

I went out and got a photo laser non-contact RPM meter. The dashboard tach is definitely off.
950 on dash reads 690 on laser;
1,200 on dash reads 870 on laser;
1,300 on dash reads 942 on laser;
1,600 on dash reads 1,160 on laser;
2,000 on dash reads 1,450 on laser.
Off by 38% consistently.

Instructions for calibration of the VDO tach can be found here. I need to know the pulse-per-revolution of my engine. Any idea what that number is, or where I find it? I looked on the QuickServe site and couldn't find it.

Re: Engine RPM's look okay?

Reply #17 – September 22, 2021, 04:15:07 pm

Try 1.

Re: Engine RPM's look okay?

Reply #18 – September 22, 2021, 04:21:38 pm

Quote from: Protech Racing – September 22, 2021, 04:15:07 pm

Try 1.

The instructions use 14.7 and 16.5 as examples.

Re: Engine RPM's look okay?

Reply #19 – September 22, 2021, 04:23:14 pm

Worse case is experiment with pulses/revolution until VDO tach reads same as photo tach.

Re: Engine RPM's look okay?

Reply #20 – September 22, 2021, 08:42:39 pm

Our tach sensor is visible on the top engine bell housing with a wire connected to the end of sensor. No gauges connected to alternator.

Re: Engine RPM's look okay?

Reply #21 – October 11, 2021, 03:59:26 pm

Update to my attempt to calibrate the new tachometer:
The simplest method is to set the "pulse per revolution" number into the tach, but no one was able to provide me that number. Not Cummins, who told me the chassis manufacturer would know it. Not Foretravel in Nac; who never returned my call. And not MOT, who should know it since they put the tach in last year. The value in the tach is currently set at 120.45, so me "guessing" at a new number and hoping to get close seems unreasonable.
Instead, since I now have the digital photo laser tach, what I did today was set a brick on the accelerator to hold the rpm's up, have the wife point the laser at the crankshaft pulley and live stream video of the display to me at the dash, where I matched the tach dial to the reading on the laser. That seems to have worked, as readings now agree through the range of idle through 2,000 (at least in no-load testing.)
One last question: I'm pointing the laser at the large pulley in the photo below. That is the right one to give me engine RPM, yes?

Re: Engine RPM's look okay?

Reply #22 – October 11, 2021, 04:03:25 pm

Quote from: Dan & Annie – October 11, 2021, 03:59:26 pm

One last question: I'm pointing the laser at the large pulley in the photo below. That is the right one to give me engine RPM, yes?

Yes, that is the crankshaft pulley, with the harmonic damper right behind it. That pulley turns at engine RPM.

Re: Engine RPM's look okay?

Reply #23 – October 11, 2021, 04:03:50 pm

Quote from: Dan & Annie – October 11, 2021, 03:59:26 pm

One last question: I'm pointing the laser at the large pulley in the photo below. That is the right one to give me engine RPM, yes?

Yes, the lower pulley is bolted to the crankshaft.

If turning at a different speed that the engine, some REALLY bad things would have happened.

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