Alternator overcharging on Cummins diesel

[Evidence]

My boat has twin Cummins diesels - model 6CTA8.3-D(M)2 (mechanical not computer managed), fitted with 24V Delco 22Si alternators, one of which, at idle of 600 RPM, delivers highly fluctuating output –cycling from around 15-20 A at 27-28V to 23-24V and 60-70A. Each cycle lasts 5-6 seconds only.

All current goes only to the batteries. No external loads running.

Batteries are 12 month old lead acid - 2 x 12V fully charged- with electrolyte SG ~1.24.

Those readings were with a new Delco. I replaced the original because it displayed same symptoms, and I concluded it had a faulty regulator. This model Delco has an internal regulator.

I have parallel neg and positive cables between the Delco and batteries – in case of loose connections. Clamp meter readings show the cables are OK - low resistance.

I have monitored the voltage difference between the alternator exciter wire – connected to terminal No 2 of the Delco, and to the +ve battery terminal. That difference oscillates from -0.02 to + 0.06V.

That reading from the other, problem free engine’s Delco, is about 0.02V. It delivers a rock steady charge of (approx.) 3A and 27V into the same battery system -immediately after start up.

I’ve started and run the good engine with the instrument cluster from the problem motor- by cross connecting the wiring harnesses at the instrument panel – to check if the fault is in the start system in the cluster– and found no problem.

However, Ive yet to cross connect the harnesses in the engine room to check if the prob is in the wiring harness - between flybridge and engine room.

Presumably connected with this problem is that the gearbox oil pressure gauge display has died. The sender complies with VDO spec ----10-184 ohms.

Cummins have not been able to supply a wiring diagram for this model – and otherwise have not been able to suggests a cause.

The highly fluctuating output should surely help narrow down the cause- eluded me so far. It fluctuates so fast – suggesting – to me at least, that it’s not merely a loose connection.

The high currents are damaging the batteries and I therefore have to conduct tests as fast as possible.

Any help greatly appreciated.

 

[ttmott]

It almost sounds like an intermittent grounding issue to the engine block and batteries. That would be rare though.
The #2 wire terminal on the alternator is the field and it normally (in Cummins applications) is tied to the Bat terminal on the alternator - a short jumper between the two. Intermittent or a bad connection to the Field would not cause surges in voltage other than dropping to 0 then back up to 26 to 28.4 volts. The #1 terminal is usually unused. The Bat wire should terminate directly on the engine's starter Bat terminal which is direct to the battery bank. Check that for loose connections, corrosion, or bad termination - a continuity check isn't good enough.
It might be worth swapping the two alternators to see if the problem moves to the other side.

Does your boat have battery bank isolators?

 

[ZZ13]

Disconnect the air grid heater and try again.

 

[Evidence]

It almost sounds like an intermittent grounding issue to the engine block and batteries. That would be rare though.
The #2 wire terminal on the alternator is the field and it normally (in Cummins applications) is tied to the Bat terminal on the alternator - a short jumper between the two. Intermittent or a bad connection to the Field would not cause surges in voltage other than dropping to 0 then back up to 26 to 28.4 volts. The #1 terminal is usually unused. The Bat wire should terminate directly on the engine's starter Bat terminal which is direct to the battery bank. Check that for loose connections, corrosion, or bad termination - a continuity check isn't good enough.
It might be worth swapping the two alternators to see if the problem moves to the other side.

Does your boat have battery bank isolators?

Hi ttmott. Thanks for prompt reply. My response in bold

It almost sounds like an intermittent grounding issue to the engine block and batteries. (I presume you’re concluding that from the fluctuating voltage difference between the exciter wire at the alternator connection and battery terminal – better analysed by using oscilloscope, I guess) That would be rare though.
The #2 wire terminal on the alternator is the field and it normally (in Cummins applications) is tied to the Bat terminal on the alternator - a short jumper between the two. (Not with this motor / Delco alternator combination) Intermittent or a bad connection to the Field would not cause surges in voltage other than dropping to 0 then back up to 26 to 28.4 volts. The #1 terminal is usually unused. (Correct – with this motor). The Bat wire should terminate directly on the engine's starter Bat terminal which is direct to the battery bank (correct). Check that for loose connections, corrosion, or bad termination - a continuity check isn't good enough. (agreed)
It might be worth swapping the two alternators to see if the problem moves to the other side. (remember this new alternator exhibits same symptons as the original unit)

Does your boat have battery bank isolators? (yes, a single isolator caters for both engines. That part of the circuit is common to both motors)

Not having a circuit diagram is further complicated by Cummins painting over all exposed wiring.

 

[ttmott]

Hi ttmott. Thanks for prompt reply. My response in bold

It almost sounds like an intermittent grounding issue to the engine block and batteries. (I presume you’re concluding that from the fluctuating voltage difference between the exciter wire at the alternator connection and battery terminal – better analysed by using oscilloscope, I guess) That would be rare though.
The #2 wire terminal on the alternator is the field and it normally (in Cummins applications) is tied to the Bat terminal on the alternator - a short jumper between the two. (Not with this motor / Delco alternator combination) Intermittent or a bad connection to the Field would not cause surges in voltage other than dropping to 0 then back up to 26 to 28.4 volts. The #1 terminal is usually unused. (Correct – with this motor). The Bat wire should terminate directly on the engine's starter Bat terminal which is direct to the battery bank (correct). Check that for loose connections, corrosion, or bad termination - a continuity check isn't good enough. (agreed)
It might be worth swapping the two alternators to see if the problem moves to the other side. (remember this new alternator exhibits same symptons as the original unit)

Does your boat have battery bank isolators? (yes, a single isolator caters for both engines. That part of the circuit is common to both motors)

Not having a circuit diagram is further complicated by Cummins painting over all exposed wiring.

1 - Not the "exciter" wire bur rather the actual grounding of the alternator case to the engine and batteries.
2 - Some configurations have the field wires routing to the battery isolator so the alternator's do not "compete" to charge when things are combined through an isolator. Check the isolator to see if there are alternator field connections. If the isolator is failing then the symptoms could happen.

@ZZ13 has a very good point - if the grid heater (assuming you have them) is cycling on and off, it will cause the alternator to change in voltage and current.

 

[Evidence]

Disconnect the air grid heater and try again.

Hi ZZ13. Thanks for yor interest.
I’m in south west of West Australia, temperature rarely gets below freezing here. No heaters installed on these motors.

 

[Evidence]

1 - Not the "exciter" wire bur rather the actual grounding of the alternator case to the engine and batteries.
2 - Some configurations have the field wires routing to the battery isolator so the alternator's do not "compete" to charge when things are combined through an isolator. Check the isolator to see if there are alternator field connections. If the isolator is failing then the symptoms could happen.

@ZZ13 has a very good point - if the grid heater (assuming you have them) is cycling on and off, it will cause the alternator to change in voltage and current.

Thanks for your interest ttmott. My reply in bold.

1 - Not the "exciter" wire bur rather the actual grounding of the alternator case to the engine and batteries. (The Delco has a dedicated ground connection. I have used it and have installed a back up neg/ground cable to cater for poor grounding of alt. to engine – as well as back up pos cable back to battery.)

2 - Some configurations have the field wires routing to the battery isolator so the alternator's do not "compete" to charge when things are combined through an isolator. Check the isolator to see if there are alternator field connections. If the isolator is failing then the symptoms could happen.(The isolator -Blue Seas, Remote Battery Switch – is a single pole switch - has no such connection. That is connected to a +24V bus –from which +24V is delivered to each engine’s starter system)

@ZZ13 has a very good point - if the grid heater (assuming you have them) is cycling on and off, it will cause the alternator to change in voltage and current. ( I’m in south west of West Australia, temperature rarely gets below freezing here. No heaters installed on these motors)


I believe you’re on the money with your comment “The #2 wire terminal on the alternator is the field and it normally (in Cummins applications) is tied to the Bat terminal on the alternator”.

I suspect the “exciter” wire - which quickly disappears out of view into the engine, is connected to the starter solenoid and it is that connection which is loose. When I was trying to trace it back to its source, its flexibility was such / consistent with being loose at a terminal. Before removing the solenoid to check, - not easy – because it’s on the “bastard” side of the engine, maybe I could do - as you essentially suggested, - momentarily “short” the exciter wire to the +24V bus – and observe impact on volts and current?

Presumably that would be safe??

What you think?

 

[Evidence]

I did post this item before, in early March 2024.

I’ve done extra testing – details of which Ive included below -and so posting again.
All discussion greatly appreciated. Thanks in advance.
My boat has twin Cummins diesels - model 6CTA8.3-D(M)2 (mechanical - not computer managed), fitted with 24V Delco 22Si alternators. The starboard engine, at idle of 600 RPM, delivers highly fluctuating output – cycling from around 15-20 A at 27-28V to 60-70 amps at 23-24V. Each cycle lasts 3-5 seconds only.
Both engines use (and charge) the same lead acid batteries for starting.

Batteries are 12 month old lead acid - 2 x 12V fully charged- with electrolyte SG >1.24.
The port engine exhibits no problems.
Cummins have not been able to supply a wiring diagram for this model – and otherwise have not been able to suggests a cause.

RE THE PROBLEM ENGINE:

All alternator output current goes only to the batteries. No external loads running.
The above readings were with a new Delco alternator. I replaced the original because it displayed same symptoms, and I concluded it had a faulty regulator. This model Delco has an internal regulator with just a single field / exciter wire and the -ve ground and + 24v terminals.

The exciter wire originates from somewhere within the engine’s wiring harness – which I’m currently attempting to locate.
As a precaution, I installed parallel neg and positive cables between the Delco and batteries – in case of loose connections. However, clamp meter readings show the parallel cables are not required.

I have monitored the voltage difference between the alternator exciter wire – connected to terminal No 2 of the Delco, and to the +ve battery terminal. That difference oscillates from -0.02 to + 0.06V. Thus, the exciter wire voltage also fluctuates between 24-27V with current about 1 A.

That voltage variation reading from the other, problem free engine’s Delco, is about 0.02V. It delivers a rock steady charge of (approx.) 3A and 27V into the same battery system - immediately after start up.

I’ve started and run the good (port) engine with the starboard instrument cluster (incl its starter function)- by connecting the port engine wiring harnesses at the instrument panel – to check if the fault is in the starboard instrument cluster – and found no problem. I’ve also connected the port instrument and port harness to the problem motor – same problem symptoms displayed.
So, the problem appears to be in the engine room - within the battery/motor electrical system.
I cannot detect any shorting / grounding of the exciter wire or the ignition/running wiring.

The highly fluctuating current/volts – probably faster than the response times of my meters and faster than normally associated with a loose connection, is surely a key to
sleuthing the cause of the problem (diodes??). However, its eluded me so far.
Unfortunately overcharging problems, esp these symptoms, appears to be unreported.

The high currents are damaging the batteries and risk damaging the alternator. So, I have to conduct testing as fast as possible.

Any help greatly appreciated.
Thanks E

 

[ttmott]

It sure looks like there are issues with the field control wire. If you disconnect the field wire does the voltage go to battery voltage and current go to zero? Did you happen to disconnect the existing field and run a separate jumper directly to the battery's positive?

 

[Evidence]

It sure looks like there are issues with the field control wire. If you disconnect the field wire does the voltage go to battery voltage and current go to zero? Did you happen to disconnect the existing field and run a separate jumper directly to the battery's positive?

Hi ttmott,
thanks for your interest & replying.
No I havent been game to do that yet. I recall you suggested that connection previously.
However, after today's internet searching I see that arrangement on Delco 22Si is used by many people and your comment reinforces the merit of doing that. I'm thinking that's gotta be my next test.
I've got the original Delco which I believe is prob OK, So I can test diodes on that

However if that makes the overcharging problem go away, im left with wondering why Cummins use the sensor/excitor wire and, also, knowing there's a fault somewhere on the engine - which I need to find and fix - to be comfortable.

Nevertheless, the result of doing that provides info - which always helps in solving the problem.

I see also that dead diodes could explain the data I'm getting - so I'll check those first - the alternator and those in the harness - which I can find and are accessible.
Unfortunately Cummins say they have no circuit diagram for this engine

Thanks again. Cheers

 

[ttmott]

Hi ttmott,
thanks for your interest & replying.
No I havent been game to do that yet. I recall you suggested that connection previously.
However, after today's internet searching I see that arrangement on Delco 22Si is used by many people and your comment reinforces the merit of doing that. I'm thinking that's gotta be my next test.
I've got the original Delco which I believe is prob OK, So I can test diodes on that

However if that makes the overcharging problem go away, im left with wondering why Cummins use the sensor/excitor wire and, also, knowing there's a fault somewhere on the engine - which I need to find and fix - to be comfortable.

Nevertheless, the result of doing that provides info - which always helps in solving the problem.

I see also that dead diodes could explain the data I'm getting - so I'll check those first - the alternator and those in the harness - which I can find and are accessible.
Unfortunately Cummins say they have no circuit diagram for this engine

Thanks again. Cheers

Shorted rectifier diodes are an easy test to find out. With a good DVM (Fluke for example) set it on AC voltage and measure the output at the alternator with everything normally connected and running. If the voltage is greater than 0.1 volts (100 millivolt) AC your alternator's rectifier bridge is failing.

 

[Evidence]

Shorted rectifier diodes are an easy test to find out. With a good DVM (Fluke for example) set it on AC voltage and measure the output at the alternator with everything normally connected and running. If the voltage is greater than 0.1 volts (100 millivolt) AC your alternator's rectifier bridge is failing.

Thanks for that suggestion. I'll do that end of week when next at boat.

What about using diode function on a multimeter.
Today I checked the original 24V Delco 22Si - with it uninstalled - sitting on bench - with the red probe on the casing and black on the Batt terminal and got a reading of 1290millivolts. Nothing with the probes reversed.

Is that a healthy reading?
I read that 500-600 mv is the expected reading from a healthy 12V alternator.
Thanks again.

 

[ttmott]

Thanks for that suggestion. I'll do that end of week when next at boat.

What about using diode function on a multimeter.
Today I checked the original 24V Delco 22Si - with it uninstalled - sitting on bench - with the red probe on the casing and black on the Batt terminal and got a reading of 1290millivolts. Nothing with the probes reversed.

Is that a healthy reading?
I read that 500-600 mv is the expected reading from a healthy 12V alternator.
Thanks again.

I think a bench test will only look for direct shorted diodes. An operating generator will reveal diodes that are actively failing. Remember a diode has a breakdown voltage that is probably greater than what a DVM/multimeter provides.

 

[Evidence]

I think a bench test will only look for direct shorted diodes. An operating generator will reveal diodes that are actively failing. Remember a diode has a breakdown voltage that is probably greater than what a DVM/multimeter provides.

Thanks fo reply.
Planning to test the voltage of alt in AC mode tomorrow.
Also if Alt warning light comes on when I turn ign key - & efore I start the engine.

 

[saopm]

Here is a wire schematic I have from Seaboard marine. I had similar issues and ended up replacing both alternators at the end of last season. All was well when I put the boat away for winter.

 

[Evidence]

Here is a wire schematic I have from Seaboard marine. I had similar issues and ended up replacing both alternators at the end of last season. All was well when I put the boat away for winter.

Thanks for posting.
I also have that diagram. However it doesnt correspond to my engines.

The local Cummins branch here in West Australia concede they have no diagram for my engines.
Worse, they have my engine numbers listed as the computer managed model!
I did further testing yesterday on alternators of both engines. The diode test was done after removing the cable attached to the batt terminals - & done with a multimeter. Bothe engines gave an initial reading of about 800 millivolts and drifted down over a minute or 2 to a fairly steady 540 mv.
Actually, writing this now reminds me that I left the sensor wires attached when I did that test. So I need to redo that test.
Maybe that explains the reading drift.
I've not seen any info re mv I should expect from a 24V Delco 22Si. Nevertheless that both engines gave same value suggest their both OK.
However, conflicting data is that checking the AC content of the output of the starboard alternator showed 1.5 V - suggesting faulty diodes.
So, Ive now got further data to use to help sleuth the cause of the highly fluctuating alt. output.
If anyone has suggestions I'd be grateful to hear from you.
Thanks

 

[Evidence]

Thanks for posting.
I also have that diagram. However it doesnt correspond to my engines.

The local Cummins branch here in West Australia concede they have no diagram for my engines.
Worse, they have my engine numbers listed as the computer managed model!
I did further testing yesterday on alternators of both engines. The diode test was done after removing the cable attached to the batt terminals - & done with a multimeter. Bothe engines gave an initial reading of about 800 millivolts and drifted down over a minute or 2 to a fairly steady 540 mv.
Actually, writing this now reminds me that I left the sensor wires attached when I did that test. So I need to redo that test.
Maybe that explains the reading drift.
I've not seen any info re mv I should expect from a 24V Delco 22Si. Nevertheless that both engines gave same value suggest their both OK.
However, conflicting data is that checking the AC content of the output of the starboard alternator showed 1.5 V - suggesting faulty diodes.
So, Ive now got further data to use to help sleuth the cause of the highly fluctuating alt. output.
If anyone has suggestions I'd be grateful to hear from you.
Thanks

Further t

 

[Evidence]

Thanks to those who have offered me advice on how to solve - what appears an unusual problem of massive overcharging by an alternator on one engine in my twin engine boat.

Briefly recapping: my boat has twin Cummins diesels - model 6CTA8.3-D(M)2 (mechanical - not computer managed), fitted with 24V Delco 22Si alternators.

I’ve done more testing- to sleuth reason why a new 24V Delco 22SI on the starboard engine is delivering highly fluctuating volts 22-28V and current 70A–15A respectively (ie., high current at low voltage) at idle RPM: The duration at each of those voltages is about 4 -10 seconds and it cycles repeatedly.
I’ve established the cause of the problem is in the engine room – both the instrument cluster and harness are good.

Further testing showed: (done with battery charger turned off, and the 24VDC/24V AC inverter ground cable removed from battery)

• The Diode test function on my multimeter – when used on alternators of both engines - done with batt+ cable and sensor wire removed, gave steady 880 and 870 (mv?) – which may indicate that rectifier diodes are OK – opinions needed on significance of that result.
• However, the AC content of the alternator’s ~24V DC output is a massive ~ 1.5V – and fluctuates wildly – compared to - what I understand is the max acceptable level – 100mv.
• Similarly, the diode in the engine harness labelled “Alt diode” read 0.589 – which I understand indicates it is functioning properly. I presume that diode is the one which controls the Alt warning light on the dashboard.
• The dashboard warning light comes on - but only briefly (1/2 sec), after the engine starts – same as Port engine.
• I’ve verified that all engine harness wires are not grounding and, also, are “open circuit” with respect to the alternator sensor wire – except for the red wire which shows <0.1 ohms – all those results agree with those on the good engine.
• All resistance checks on the Aux magnetic switch and associated connections agree with the good engine
• Because the alternator heats up quickly when delivering these high currents – and at idle, I cannot risk running the engine for more than about 3 minutes at a time
• Removing the sensor wire from terminal No. 2 of the Delco made no difference to the output.
• The alternator Batt + terminal is shorted to the sensor terminal – marked 2 on the housing – which is not the case with the port engine
• Another test I need to do is to connect the sensor terminal on the alt directly to the battery and check impact on output.
• All suggestions, hints, comments greatly appreciated.
• Thanks, E

 

[ttmott]

First the AC ripple, if that is actual, is way too much which indicates the alternator's rectifier bridge is bad. Second, all of this (hot alternator and fluctuating voltage/current) almost indicates a shorted battery cell. Quite possibly the battery resistance is so low that the alternator doesn't know how to behave and is failing - again.

 

[Evidence]

Hi ttmott,
Thanks for those comments.
The fluctuating AC possibly simply mirrors the fluctuating DC value.
I'm thinking the closed circuit (short) between the sensor terminal (No.2) and the Batt connection is a certain indicator that the Alt is bad- despite being new.
Is that (short ) also a symptom of a failed rectifier bridge?
Also that result argues its pointless installing this alt on the port engine to help determine if its the cause of the high output.
Your comment about the battery, appears improbable bc the same 2 x12V wet cells service both engines - and do not cause a prob with the port engine.
Thanks
Cheers
E