Can the idle speed on a Cummins QSB be changed?

Mine is set to the standard 600. Alternator does not put out enough at that speed to keep up. Would like to set it to 675 or so. I've called two Cummins service reps, one says no, another says don't know, have to call Cummins. On BoatDiesel there are some old threads that say you need the Cummins laptop software but can be done.

Apparently there was also an option for a C-Cruise panel - a few rocker switches - allowing you to set idle. I don't have that, and the only source I can find has it at about $1250 for the panel and wire harness. I won't comment on the ridiculousness of that.

It seems exceptionally odd that the idle cannot be set somehow.

I believe the QSB electronic throttle is controlled be variable resistance. Raising the base resistance would keep a higher idle . Don’t know if this helps

I have a QSB. No idea if you can adjust the idle speed. I am going to guess it can’t, including by a Cummins Tech. Also, Could a higher speed affect the Trans? If you are still under warranty, tread carefully.

Looked into changing the pulley size if belt driven?

Just raise the throttle a little bit, until you get the charge rate you want.

There are some options for idle speed control, but I don't know the details of such.

Jeez, all these fancy enjin's

From my memory, I believe the idle can be set via the Cummins software on the tech’s laptop. I would call one of the Cummins factory shops (I have used Cummins Northwest) or call Seabar Marine and ask them.

My question is what is the alternator not keeping up with? I don’t understand alternators etc... but it may be that you may want to look at the alternator and see if changing to a high output alternator or as Scott mentioned, look at changing the pulley size.

I would think that increasing the idle speed would put more stress on the transmission as you put it in and out of gear.

^^ +1

It's probably a 8 groove serpentine, you should be able to find a smaller pulley to increase the shaft speed. It's unlikely that you'll over-rev the alternator even at WOT. If you do end up changing the pulley, you'll need an impact wrench to remove the nut from the shaft & re-install as well.

A cautionary note regarding re-installing the pulley- a helical spring washer is often found on an alternator pulley, but isn't a good choice. I had two incidents where an alternator pulley came loose due to a poor stack-up that included a helical washer. I can attest that you DO NOT want your alternator pulley coming loose in a seaway!! Yikes!

The better option is to use flat machined washers/shims in the stack-up, and secure with a prevailing torque lock nut, e.g. Stover locknut, and torqued down with an impact wrench at a mimimum of 100 ft-lb. They are far more resistant to loosening than a helical spring washer. Helical spring washers are fine for static loads, but the alternator is subject to significant temperature swings along with torsional vibration transmitted from the diesel engine's crankshaft pulley, so higher risk of loosening.

The electronic throttle is a CAN node, doubt it is as simple as changing a resistor. The idle is controlled by the ECM. I can bump the throttle up, but only in gear lockout, when maneuvering this is not practical. I could also change the pulley, but there IS danger of overspeed, in fact the ratio is about 2.8:1 so 3000 rpm on the engine is 8400 on the alternator as it is - well above max for the Balmar 94 series (though they assure me that it is OK for limited periods). I don't think the extra 75 or 100 rpm will stress the Twin Disc 5050 swinging a smallish prop - after all many diesels to which it is fit idle at 700 or higher. And in fact the C-Cruise option available then allowed you to bump the idle all the way to 1000.

I did call two official Cummins shops, as I said one said no, one said maybe, and there are postings on SBmar's site suggesting it can be done with the Cummins Insight software. So no definitive answer is yet to be found. I'll try Cummins Northwest.

What I've found is at 600 rpm I get about 12 A from the alternator. 5 is consumed by the field windings. The engine and other electronics needed consume another 10 - 15 A, so I need 20A or the voltage sags. This results in a warning first from the ECM, then from the electronic throttle. That sets off two buzzers which can be separately and temporarily suspended. Just what I need during cross wind docking maneuvers! At about 650 or 675 the output is about equal to consumption and the voltage stabilizes, at 700 I'm charging just a bit, by 1000 it will charge 100A or so.

Makes a stop screw on the governor seem so simple...

DDW said:

Makes a stop screw on the governor seem so simple...

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And old non electronic engines which need only fuel, air, and compression but no electrical current, to run. Unless I idle the Lehmans around 800rpm they come up from their mountings in the ER during docking to see what`s happening.

Getting only net 7A from an alt at idle sounds like a problem with the alt. I have a 100A older Balmar (Ford-Autolite) external reg alternator on my C-series which idles down to about 550. Panel shows I can pull about net 20A there and not get a V dip.

Alternator known healthy? Could one of the six stator diodes have failed?

Batteries know healthy? Any other loads on DC system?

I too would suspect the alternator. I haven't see a situation like your describe since the 60's on a British car with Lucas dynamo.


Also, what gear/throttle controls do you have? Twin Disc? Glendinning? Something else? They often have a fast idle setting that can be programmed in. But I really think you have an alternator problem.

Raising the idle speed can make shifting in docking situations harder on the tranny and everything attached to it.

FF said:

Raising the idle speed can make shifting in docking situations harder on the tranny and everything attached to it.

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That is what I was thinking.

Exactly. If you want to raise the idle for charging do it manually at the controls for alt output and for extended idling I'd sure raise the rpm's just for the oil pressure and motor temperature. You do not want a higher programmed idle for engaging transmission controls more pressure on em when going in and out.

Ski in NC said:

Getting only net 7A from an alt at idle sounds like a problem with the alt. I have a 100A older Balmar (Ford-Autolite) external reg alternator on my C-series which idles down to about 550. Panel shows I can pull about net 20A there and not get a V dip.

Alternator known healthy? Could one of the six stator diodes have failed?

Batteries know healthy? Any other loads on DC system?

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New alternator, nothing wrong with it as far as I can tell - it is doing about what the charts predict, 15A at 1500 shaft rpm is spec. I've measured all the currents going everywhere with a DC clamp ammeter, nothing unusual. Common rail engines take a fair amount of DC power to keep them running.

This is a problem with big alternators - they produce less at low rpm (exceptions are the newer AT style hairspring wound, these will do 70A @ 1500). A 100A alternator will typically produce more at low rpm, a 6 series Balmar 100A is spec'd at 20A @ 1500 so you are in spec. The 6 series 150A only 6A @ 1500 illustrating the problem. I've got a 7 KW alternator on the sailboat (280A @ 24V) that does not break even until about 1700 shaft rpm. I'd have liked to use an AT style, but was advised against it for the Cummins due to the small bearings, short shaft, and offset pulley required.

The throttle is a Gendinning EEC-4, don't see any idle adjustment possible (except shift lockout). For maneuvering the low idle its great, but not going to be much worse with 75 rpm more. I usually bump the throttle up while warming up anyway because it is a bit smoother at 675.

TowLou said:

Exactly. If you want to raise the idle for charging do it manually at the controls for alt output and for extended idling I'd sure raise the rpm's just for the oil pressure and motor temperature. You do not want a higher programmed idle for engaging transmission controls more pressure on em when going in and out.

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I'm not raising it for charging. It is specifically while maneuvering at idle that this is a problem. I'm happy to lock out the shift and bump the throttle up any other time. It is not practical to do that maneuvering with the Glendinning control.

I'm surprised everyone thinks this is death to a Twin Disc. The mechanically injected predecessor to this engine (6BTA) is set to idle at 700 from the factory, and was typically fitted with the same transmission, probably 10s of thousands of installs. Sure slower is always better but these transmissions will not roll over and die at 700 rpm idle.

From the Balmar chart at:

Alternator Output Curves | Balmar



It is a very steep curve. Seems like just a slightly smaller pulley on the alternator would fix the issue with no side effects. How often do you run at max RPM for extended periods of time?

Is your regulator equipped with a sense wire directly to the bank? If not, that might provide a bit more field current to get a few more amps out of it.

The question is, how long can you run at max rpm before the alternator blows up? It is spec'd as 6000 rpm max (they have told me 6500 is safe), max on the Cummins is 3050 or 8500 on the alternator. Max realistic cruise is 2500, 7000 on the alternator, still above the spec, if I go to 3:1 then 7500. And won't get the same effect as increasing the idle to 675. Do I want to be worried about the alternator when running at max cruise? Also, I'd probably have to make the pulley - can be done but a bit of effort. The Cummins pulley is not the same as other engines.

The alternator is externally regulated (MC-614), and the sense wire is on the start battery which is feeding the engine electronics. The regulator shows 100% output but cannot achieve the target voltage.

Setting the idle up seems like the simplest solution.

I can certainly see not wanting to have alarms and bells going off while maneuvering in close quarters unless they really mean something.

If you can't change the idle due to technical restrictions, you are stuck with a different pulley or a different alternator. With the electronics (ECU, throttle) doing the alarming there's no easy way to override that stuff in a different, more convenient way either.

With the 614 already at max field current, there's not much else you can do.

DDW said:

I'm surprised everyone thinks this is death to a Twin Disc. The mechanically injected predecessor to this engine (6BTA) is set to idle at 700 from the factory, and was typically fitted with the same transmission, probably 10s of thousands of installs. Sure slower is always better but these transmissions will not roll over and die at 700 rpm idle.

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It's not. My recollection is that TD says up to 1000 rpm is fine. I would have no qualms with anything up to 800 rpm.


Maybe it's just me, but an alternator that can't keep up at idle is un-fit for the job, in my opinion.

Are you sure the max rom is only 6000?


Seems low....just because the charts stop there doesn't mean a "not to exceed" speed.


Many alts I have heard go up between 12,000 and 20,000 rpm max, the 6000 range sounds like max output.

twistedtree said:

Maybe it's just me, but an alternator that can't keep up at idle is un-fit for the job, in my opinion.

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Or, a motor that can't idle fast enough to keep the alternator alive?

There seems to be no alternative in a high output alternator. A large frame would have done it, but there isn't room to fit one without removing the remote oil filter and its piping, which in retrospect is what I should have done. But well down this road now.

I didn't think there'd be such difficulty in something like idle speed.

psneeld said:

Are you sure the max rom is only 6000?


Seems low....just because the charts stop there doesn't mean a "not to exceed" speed.


Many alts I have heard go up between 12,000 and 20,000 rpm max, the 6000 tange is just measured or max output.

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Just reading from Balmar's spec sheet. Says 6000 max. I've enquired and they say actually it's 6500, and occasional and temporary use to 8500 will be OK.

There are two issues: does the rotor fly apart, and do the bearings take it. One issue unique to the Cummins is that the pulley is offset, sticking out on the shaft further to get belt alignment. This puts some extra stress on the shaft and bearings. This is why Balmar recommended against the AT style. The 94 and 98 series have long, large shafts and large bearings. Trouble with large bearings is they are rpm limited. The AT has as small short shaft and small bearings. You can turn those 18,000 or something.

Believe me when I say I've looked into this, and alternators aren't new to me.

OP, No one said your Trans would roll over and die at a higher RPM. You asked a question on a public forum so some people were just giving their thoughts on the possible longer term aspects of a higher RPM while shifting.

TD’s will engage hard if you set the idle speed to high and yes it does affect their longevity.

I also think the Balmar regulator needs a certain load to kick in. I am going to be at idle for a while I hit Warm Up, throttle up get the charging going and then go back to idle and they keep charging. May not be the same for everybody’s set up but for two AT’s, two 612’s and a CF it seems to work.

Ski in NC said:

Just raise the throttle a little bit, until you get the charge rate you want.

There are some options for idle speed control, but I don't know the details of such.

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I have an old ass Ford Lehman that rattles like it's going to bite at any second, since day 2, I just bump the idle up a bit, 100 rpms, is all it takes to smooth out.
Also have the same issue Alt output. if I'm getting underway soon, I jsut let the batteries deal with it.
But there are times I need a positive flow, I just raise the idle to whatever rpms get the job done. (usually a 1,000)

PaulGel said:

TD’s will engage hard if you set the idle speed to high and yes it does affect their longevity.

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From the Twin Disc manual:

4. ShiftingLimits
For all shifts (NEUTRAL into FORWARD or REVERSE, and FORWARD or REVERSE through NEUTRAL into the opposite direction) limit engagement to a maximum of 1000 RPM or 50% of engine speed which ever is lower.

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As I said above, given that the 6BTA idle was set to 700 and the TD transmissions are neither dropping out the bottom of those boats nor wearing out prematurely, I'm doubting that setting my idle from 600 to 675 will have measurable effect. Lower is better, therefore lowering my idle to 500 would be better still. But unnecessary.

The 614 reports what it is doing. It is supplying max field current, confirmed by clamp ammeter. The alternator is putting out its spec'd current. It's just spinning too slow.

I was hoping someone had some information on the original question.

Can you pull the actual alternator part number off the unit? It's probably a Leece Neville/Prestolite. From there you can find the actual data sheet which will give you max rpm. I would expect 8000 rpm or more. At least that's what I've found for large frame units that I've used.