Voltage Drop Across Glow Plug Circuit

[Mike and Theresa Vaccaro]

Hi Folks,

Just completed the harness upgrade (there are actually a few boats still out there without it!) and when shooting voltages to confirm a proper hook-up to the panel, found that only 10.5 volts is being delivered by the glow plug switch.  Our boat is an '88, and has the control panel push/hold glow plug and starter switches.  

Note that there is good voltage at the starter switch (i.e., more than 13 volts are being delivered to the glow plug switch), but I'm just curious if this is a characteristic of the glow plug switch, or perhaps the switch is not performing correctly?  Haven't been able to find any schematic info that would explain this.

Cheers,

Mike Vaccaro
"Spirit" '88 Hull 563

[Stu Jackson]

Mike

The reason is that the wire is so small and the distance is so great.  I recommend you install a glow plug solenoid:  reference:  http://c34.org/projects/projects-glowplugs.html.

There's also more on that in either Projects or FAQs, as well as links to Al Watson's website from Projects.

It's a one hour project to do, it's simple, and eliminates all the voltage drop.

Stu

[Mike and Theresa Vaccaro]

Stu,

Concur that a selenoid can be helpful--as is shortening the overall harness.  A 3 volt drop is excessive for a 30' for 10 AWG.  Had a chat with the folks at Seaward and they suspect a loose/corroded connection.  Since all wiring is new except the old engine pigtail, the fault must lie at either the glow plug connection itself or the ground.  Am going to attempt to clean and or replace both connections and then re-measure.

Thanks for the help!

Mike

[Jim Price]

Stu:  I have the harness upgrade and recently replaced two old G27 batteries with new G31's but still have to crank a cold engine on both batteries.  I have voltage drop that will begin to peg(lower) the volt meter when using only one battery & cold engine.  I don't want to have to depend on having BOTH batteries to crank in emergency situations.  I will follow up on the connectivity / corrosion ideas listed here but would like to consider the soleniod route also.  Could you post a couple of photos to go along with your original write-up for the M25XP.  Sometimes pictures help eliminate mistakes - especially with wiring / current.

[burner]

Jim, I'm interested in your situation.  I have a 1988 model and I also have to use both batteries to start a cold engine.  I have ordered the wiring harness upgrade, so I'm not sure if that will help or not.  I've never been comfortable with knowing that I needed both batteries to start a cold engine.  Please keep posting your results, as I'm getting ready to try and figure out my situation as well.

[Ray & Sandy Erps]

I'm in the same boat as the rest of the slow crankers here.  Worse yet, we had to use the compression release to get the motor started at the fuel dock after a half a day of motoring.  We're in an '89 Catalina with the the M25XP.  I've installed the glow plug solenoid.  I've found that the starter switch is wired so I can preheat the glowplugs then release the solenoid relay and still crank the motor.  I've checked for voltage drop to the starter and found nothing appreciable.  Now that the rains have come to the Puget Sound Region, I'm figuring on pulling the starter motor off and checking the starter solenoid contacts, the brushes and cleaning up those little copper segments that the brushes come in contact with.  If that doesn't work, I guess moving a battery closer to the motor comes next, but that looks like too much work.

[Stu Jackson]

GLOW PLUG SOLENOID

For all you slow crankers:

1.  I sent a wiring diagram of the solenoid to Jim Price, since I don't have a picture.  I'll send another copy to Mark Elkins so he can post it on the Projects, or expand the Projects Glow Plug article.  I used #10 wire, although #8 is recommended.  It's what I had in hand, and it's been working fine for three plus years.

2.  For "cold engine" starts, please advise what you have been doing.  We set our switch to the start battery, engage the glow plug (for 15 seconds with a solenoid, 30 seconds before it was installed), release the glow plug and then hit start.  We only switch to the house bank after the engine is off.  The alternator is wired to the house bank with a combiner to the start battery.  I used to have trouble getting the engine started because I held the glow plug button in when I hit start.  Once I released the glow plug first, I've never had a problem.

3.  Ray, instead of moving the battery, just increase the wire size (and ground) or parallel the wires.  This was discussed about a month ago in another post, although I'm sorry I forgot the post, may be able to find it sometime soon.  Or you could search for posts under my name.

4.  Steve L- the wiring harness won't help much with just the start problem, because the wire still goes all the way to the cockpit and back to the engine, that's why the solenoid is recommended.

5.  As always, in general, check the connection, and don't forget the ground at the engine.

Stu

[Jim Price]

:DHow potentially stupid!!! The PO told me to turn switch to far position to heat plugs and then hit start button.  Been doing that for year and a half.  Did not tell me to release switch back to "on" position before hitting start button.  I will definitely try that this weekend!  Boy, do I feel stupid!  Guess holding the "heat" position plus try to crank is killing the voltage available to the starter.  Hope this cures my issue but even so, I will still pursue the solenoid upgrade.

[burner]

Yeah, I really didn't figure that the new wiring harness would help.  I'm going to try the solenoid trick, that has got to help.  I too switch to the house bank after the engine is stopped, then to the starting battery to start the engine the next morning.  However, I'm just like Jim - I've been holding in the glow plug switch while starting.  I'll try it this way - releasing the glow plug - and see if that helps.  It definitely makes sense that it will relieve the load on the battery....

[Mike Vaccaro]

Some thoughts:

First, the harness upgrade won't cure a sluggish cranking motor--that is most likely the result of the wire run between the starting battery and the starter.  The best bet is to either increase the diameter of the wire or move the starting battery closer to the engine.  The purpose of the harness upgrade is to mitigate fire risk.

Second, nothing wrong with using a decompression lever to start the engine.  This technique is recommended by Universal for cold starts or weak battery situations and works like a champ.  The downside is that it requires two folks and you'll have to open the engine hatch.

Third, Ron is absolutely correct about soldering connections.  The solder resists corrosion and greatly improves conductivity at the connection--the crimp provides strength.  Marine grade shrink wrap completes the assembly.

Forth, a solenoid in the glow plug circuit will improve voltage to the glow plug, not the starter.  This will make the plugs more efficient (i.e., you won't have to let them warm up as long, for example 15 vs. 30 seconds).  With the new harness and clean connections, we are only getting 10.5-11 volts through the glow plug circuit.  Typically, we allow 45-60" of plug heating prior to start.  The Universal M-25XP manual recommends 30-60" of plug pre-heating prior to crank with the stock system--additionally, it recommends opening the throttle 1/3 prior to cranking.  Releasing the plug switch just prior to cranking will improve overall system voltage for cranking.

Fifth,  there has been quite a bit written about improving cranking, whether or not you have a dedicated starting battery installed.  But the bottom line is that the stock system is sufficient (not ideal!) if operated in accordance with Universal M-25XP manual.  Ron's suggestion about proper connections is right on the money--and another technique is to simply double up the 4AWG--i.e., add a new wire to the existing circuit.  Will increase the ability to handle amps and the price of 4AWG is much more palitable that large diameter wire (it's also easier to run through existing conduits).

I've concluded that the voltage drop to the plugs is the nature of the system design, although I still have to check the ground connections to ensure that they are not loose or corroded.  Since we just replaced the ammeter with a voltmeter, it's the first time we've been able to quantify the drop.  As stated in the previous posts the solution to this drop is to shorten the circuit (i.e., add a solenoid) or simply hold the switch long enough to ensure adequate preheat.

 I'm curious to find out what type of voltage drop other folks are getting with the glow plugs heating?  You can read this directly off the voltmeter when you engage the plug switch.

Cheers,

Mike
"Spirit" '88 Hull 563

[Jeff]

Interesting discussion. My experience with 1990 hull # 1102 with M35 engine is as follows:

1. Wiring harness upgrade does help glow plug voltage. Shortens circuit length.
2. Glow plug solenoid helps even more.
3. I use SAE 30 oil and find in the Spring and Fall with ambient air temps @ about 50 and water temps in 50s and 60s (am in Tennessee) that engine sluggish to start - requires longer glow plug time and longer cranking time. As air and water temps warm up, starts immediately with less than 10 sec glow plug time. Suspect could improve this by going to 10W40 oil.
3.With regard to Ray's problem with needing both batteries to start after 2 1/2 hours of motoring, have also had this problem. I've seen several written discussions (can't recall sources) that this is caused by thermal expansion of metal parts in engine and is to be expected in small diesel engines - not to worry.

Jeff

[Stu Jackson]

Glow Plug Drop

Mike

In answer to your question:

"I'm curious to find out what type of voltage drop other folks are getting with the glow plugs heating? You can read this directly off the voltmeter when you engage the plug switch."

It's 11 V or less, always at the voltmeter on the cockpit panel.  We also use this gauge to tell us that the solenoid is working, because unless it comes back up when we release the glow plug button or the start button, we've got another type of problem:  the solenoid is stuck.

What else would we expect, voltage-wise?

They're both HUGE DRAINS, and it really doesn't matter.

But the huge drains are very short lived.

It's like starting the engine:

3,575,622 amps at 15 seconds = 2 amp hours for the glow plugs

OR

4,567,342,345,345,798 amps at 1 amp hour for the starter

If the wires and the connections are sized right, they'll hold for the short periods of time.  If they're undersized or loose, they won't be dependable.

Stu

[Mike and Theresa Vaccaro]

Stu,

Thanks for the reply--10.5 is normal.  

Cheers,

Mike

[Jim Price]

I thank everyone also.  Went to boat yeterday and cranked for first time ever on one battery.  Heated glow plug, released switch to "on" position, hit starter button.  Engine turned twice and fired up !!!!  The Universal web site and manual specifically state that you must hold switch in "heat" position while pressing start button.  Obviously they dan't live in the real world!  Thanks again.

[Stu Jackson]

Glad to hear about your success, Jim.

You're also right about the Universal manual, I read it again yesterday.

Guess it's one of those "Don't believe everything you read" stories.

I think if you look a bit harder at their wiring diagram, you may find out why they say that, because the text has references to key switches.  I didn't complete my investigation, probably not worth the time.

In any event, as long as the glow plug button and start button are wired separately, doing them separately won't hurt, and works!  

Think about it: all the glow plugs do is wam up the chamber, much like starting an already warm engine.  In fact, after motoring for a while, with the engine still wam, we don't use the glow plugs and it fires right up.

Stu