Volts drop at Stop solenoid during start up

[superheat6k]

My Cummins 6BT5.9M engines have a Lucas CAV DPA distributor type injection pump. This incorporates a 12 volt stop solenoid. Energise to open / run, de-energise to stop.

Recently the engine simply stopped shortly after starting which led to me mis diagnosing a faulty injection pump most due to the load click / snap sound that emanated from the pump as it stopped. Anyway the pump has been fully inspected and re-tested and all is fine, and its calibration has been adjusted, but shortly after the pump was reinstalled and after initial re-start, the following day the original problem recurred. So I measured the voltage at the solenoid connection. From 13 volts pre start the voltage dropped to ~ 7.5 - 8 volts when the engine was refusing to start.

However, the engine was spinning freely on the starter, and this is a big 5.9 litre 6 cylinder engine. The battery is 125 AH.

I had found the engine battery isolator switch was not working so I replaced this, and all the connections on the start circuit are good.

My suspicions now are that the crank battery is simply losing too much voltage at start up, but it spins the engine with ease.

What is the minimum the terminal voltage should fall to during the start up ?

BTW I did run a separate feed direct from the port engine and this allowed the engine to start without a problem.

I was intending to run a series of voltage drop checks, but after the first check of the stop solenoid the voltmeter decided to go for a swim in the oily bilge water I was about to clean up from the works I had just concluded and is still drying out ! I will be checking the battery charge status with an hydrometer, but fear it is new battery time.

Or should I change the stop solenoid first ? This works fine at 12 volts, but doesn't not sty open at 8 volts.

 

[rotrax]

I renovated a Westerbeke 8KW genset last year and installed it in our boat in April this year.

On the Mitsubishi powered Westerbeke the pumps stop solenoid uses two windings-one to throw it open which takes a high current, and one to hold it open which takes a much lower current.

The second winding will hold the fuel shut off piston open when the engine is running, but is not powerfull enough to operate the fuel shut off piston against its spring when starting. Both windings need to work in unison, the heavy winding while cranking, energised by the starter motor and the lighter winding when the start has been made.

I wonder if the system you have is similar, and you have a winding out in the solenoid?

 

[Heckler]

My Cummins 6BT5.9M engines have a Lucas CAV DPA distributor type injection pump. This incorporates a 12 volt stop solenoid. Energise to open / run, de-energise to stop.

Recently the engine simply stopped shortly after starting which led to me mis diagnosing a faulty injection pump most due to the load click / snap sound that emanated from the pump as it stopped. Anyway the pump has been fully inspected and re-tested and all is fine, and its calibration has been adjusted, but shortly after the pump was reinstalled and after initial re-start, the following day the original problem recurred. So I measured the voltage at the solenoid connection. From 13 volts pre start the voltage dropped to ~ 7.5 - 8 volts when the engine was refusing to start.

However, the engine was spinning freely on the starter, and this is a big 5.9 litre 6 cylinder engine. The battery is 125 AH.

I had found the engine battery isolator switch was not working so I replaced this, and all the connections on the start circuit are good.

My suspicions now are that the crank battery is simply losing too much voltage at start up, but it spins the engine with ease.

What is the minimum the terminal voltage should fall to during the start up ?

BTW I did run a separate feed direct from the port engine and this allowed the engine to start without a problem.

I was intending to run a series of voltage drop checks, but after the first check of the stop solenoid the voltmeter decided to go for a swim in the oily bilge water I was about to clean up from the works I had just concluded and is still drying out ! I will be checking the battery charge status with an hydrometer, but fear it is new battery time.

Or should I change the stop solenoid first ? This works fine at 12 volts, but doesn't not sty open at 8 volts.

I had a 16 cylinder gas fired Cat Generator on a platform. Had similar issues, go to start, it would spin over good but all the logic relays droven off the 24v system were "failing" we spent days chasing the prob, changing the relays etc then spotted the voltage drop on the batteries when the starter was spinning, the relays werent holding due to the voltage drop. New batteries sorted it. This little anecdote is to confirm that failing batteries can do that sort of thing. You have answered your own question really with the lead from the other engine.
Stu

 

[PaulRainbow]

What is the minimum the terminal voltage should fall to during the start up ?

Circa 10v, not much less. I'd suspect it's new battery time.

 

[lw395]

The cold cranking current spec for batteries is often quoted at 1.2V per cell. Suggesting 7.2V.
The only diesel solenoid I've fiddled with was a two coil thing which drew about 30A while the starter motor was energised and about half an amp while running. I've found the spec for that, it wants 9V for the hold circuit to hold, it doesn't say what the start circuit should need.

The specs for these things are often not very 'joined up', we'd never get away with it in avionics.
Your battery should give its CCA at 0degF for 30 seconds before getting down to 1.2Vpc when new.

A new battery may make the problem go away, but it doesn't seem right when the engine still cranks nicely.

 

[Heckler]

The cold cranking current spec for batteries is often quoted at 1.2V per cell. Suggesting 7.2V.
The only diesel solenoid I've fiddled with was a two coil thing which drew about 30A while the starter motor was energised and about half an amp while running. I've found the spec for that, it wants 9V for the hold circuit to hold, it doesn't say what the start circuit should need.

The specs for these things are often not very 'joined up', we'd never get away with it in avionics.
Your battery should give its CCA at 0degF for 30 seconds before getting down to 1.2Vpc when new.

A new battery may make the problem go away, but it doesn't seem right when the engine still cranks nicely.

My anecdote, my personal experience, engine was cranking fine by the SOUND of it, thats what threw us to go chasing faulty relays. The only true test is the meter on the solenoid whilst caranking.
Stu

 

[burgundyben]

I told you V drop was the problem! Lol.

BTW - that doesn't sound a lot of battery, mine are same as yours, but 24v, I have a pair of 110 Ah in series for each engine. I think 800CCA.

Check the manual what it asks for. Do you have the manual? I have it at home, I'll check tonight.

 

[lw395]

My anecdote, my personal experience, engine was cranking fine by the SOUND of it, thats what threw us to go chasing faulty relays. The only true test is the meter on the solenoid whilst caranking.
Stu

Measuring a voltage isn't a 'test' without a 'test limit'.
The volts will drop when you crank an engine.
The question is, what is the limit of acceptable volts.

If you look at the data for a typical starter motor, it seems that they expect cranking volts to drop a lot, e.g.:
http://www.newindo.com/delcoremy/images/28MT-12V_pc.jpg
http://www.newindo.com/delcoremy/28MT-DelcoRemy-Starter.htm
Peak power seems to happen around 8V.

A starter motor on a real engine will initially draw huge current until it gathers speed. This inrush may be too fast for the solenoid to drop out.

Sounds a bit like the people who specify motors, batteries, solenoids and relays don't always talk to each other?

 

[superheat6k]

I told you V drop was the problem! Lol.

BTW - that doesn't sound a lot of battery, mine are same as yours, but 24v, I have a pair of 110 Ah in series for each engine. I think 800CCA.

Check the manual what it asks for. Do you have the manual? I have it at home, I'll check tonight.

Thing is Ben I did measure the voltage before I removed the fuel pump, and I could not get the click / snap sound out of my mind. I did not measure it during the start phase though. Even PB Asher agreed inspecting the pump was the correct thing to do - hey ho its done now and it was out of calibration anyway.

The batteries are standard issue for these engines on this boat, but I suspect the battery is on its way. I am going to take my hydrometer to the boat on Saturday and check everywhere for volts drops along the supply and return cables during start up. I will also speak to Barden and see what they think the terminal voltage should drop to. I might simply double up the bank.

 

[earlybird]

.
I will also speak to Barden and see what they think the terminal voltage should drop to. I might simply double up the bank.

If by doubling up the bank, you mean adding a new battery in parallel with the old one, then I suggest that's not a good idea.

 

[lw395]

If by doubling up the bank, you mean adding a new battery in parallel with the old one, then I suggest that's not a good idea.

I can't see that it's a bad Idea.
It might be more informative to power the solenoid from a second battery and see if that allows the engine to start?
Or is it possible to manually hold the solenoid in the 'go' position?

 

[superheat6k]

If by doubling up the bank, you mean adding a new battery in parallel with the old one, then I suggest that's not a good idea.

No - if I do this it will be two new batteries of identical type / make / AH / CCA. Due to space limitations this might be 2 x (2 x 110AH) rather than 2 x 140 AH as are currently fitted.

 

[superheat6k]

I can't see that it's a bad Idea.
It might be more informative to power the solenoid from a second battery and see if that allows the engine to start?
Or is it possible to manually hold the solenoid in the 'go' position?

I did think about a special circuit to hold a higher voltage on to the run solenoid during start up, but then the boat is 26 years old and hasn't needed such a circuit until now, so a cure for a problem that does not normally exist.

The more I think about this the more this points to the battery, but I am amazed it could spin the engine, which is not a small thing, so freely.

 

[lw395]

If the battery is spinning the engine 'freely', in my book there's not much wrong with the battery.
I'd be concerned that somehow the solenoid needs more volts than it used to, and while a new battery will cover that for a while, sometime next year it might fail altogether?
I'd suggest finding someone familiar with the actual Lucas thing you have.

 

[PaulRainbow]

Absolutely no reason why the battery cannot be spinning the engine but dragging the voltage down to a level where other equipment will not function. If there is only 7.5-8v at the solenoid i'd say that's too low and the battery is on its last legs. Try another battery, can one of the domestics not be used as a test ?

It should also be possible to fit a DC-DC convertor to give the solenoid a stable 12v, but as it's been working without one for decades.................

 

[PaulRainbow]

If the battery is spinning the engine 'freely', in my book there's not much wrong with the battery.

Perfectly possible that the battey could be turning the engine over, but be on its last legs.

I'd be concerned that somehow the solenoid needs more volts than it used to

We have no information regarding the voltage required by the solenoid in the past, when it was working as expected, so we don't that it is requiring more volts. It works now with 12v.

7.5v to 8v during cranking is low.

 

[Heckler]

Perfectly possible that the battey could be turning the engine over, but be on its last legs.



We have no information regarding the voltage required by the solenoid in the past, when it was working as expected, so we don't that it is requiring more volts. It works now with 12v.

7.5v to 8v during cranking is low.

I agree, go back to my anecdote, the spinning freely was leading us down a blind alley. Anyway, the OP said it worked ok when he put leads from the other engine, surely that proves the point?
Stu