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Voltage drop across glow plug switch

lnill

Member III
I just completed the re-wiring of my M40. I followed the multiple posts with diagrams to improve current paths etc. everything seems fine...except.....I get almost a 1 volt drop across the glow plug switch. It was cold today (15) and after holding the switch down for about 20 seconds we had 10.7 v at the glow plugs, 10.9 at one side of the switch (grey wire side) and 11.9 at the other side of the switch (violet). There is no solenoid, just the grey #10 wire from the panel. All switches and wire are new with heat shrink ring connectors and Blue Seas buss bars. I checked the switch with an ohm meter and it was a dead short when pressed. So I am pretty sure the switch is ok.
 

supersailor

Contributing Partner
Voltage Drop

I had approximately the same readings on my Universal M25XP. There was too much wire going to the cockpit and back through the ignition switch to the plugs. The current draw is high. I put an a Bosch solenoid and now my glow plug time for a cold engine is 10 to 15 seconds. Wiring the glow plugs the way they were was not one of Ericson's better ideas. The power now comes off the battery and runs 3' to the solenoid and then 18" to the glow plugs. That takes care of the voltage drop.
 

lnill

Member III
I had approximately the same readings on my Universal M25XP. There was too much wire going to the cockpit and back through the ignition switch to the plugs. The current draw is high. I put an a Bosch solenoid and now my glow plug time for a cold engine is 10 to 15 seconds. Wiring the glow plugs the way they were was not one of Ericson's better ideas. The power now comes off the battery and runs 3' to the solenoid and then 18" to the glow plugs. That takes care of the voltage drop.
Supersailor,
what doesn't make sense to me about that explanation is that I have only a .2 volt difference between the lower voltage side of the glow plug switch and at the glow plug. But if I go to the other side of the switch, the voltage is 1 volt higher. That seems to me to eliminate the long wire run as the issue?
 

Randy Rutledge

Sustaining Member
Work the puzzle of the drop or install the solenoid? If you get the present wire to work sooner or later the resistance will build and If you do the solenoid then you will have it solved. Westerbeke uses a solenoid on their newer engines for lift pump and glow plugs. Use your existing wire to power the solenoid and a couple of short pieces of wire and you are done.
 
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Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Supersailor,
what doesn't make sense to me about that explanation is that I have only a .2 volt difference between the lower voltage side of the glow plug switch and at the glow plug. But if I go to the other side of the switch, the voltage is 1 volt higher. That seems to me to eliminate the long wire run as the issue?

I believe that if I found a consistent drop of one volt across a switch, I would replace that switch first. Something is not right inside that supposedly-simple device.
That much internal resistance would make me wonder how much longer that switch will work at all.
Loren
 
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footrope

Contributing Partner
Blogs Author
I agree with Loren. Switch is likely bad, based on the information provided up to now anyway.
 

lnill

Member III
I believe that if I found a consistent drop of one volt across a switch, I would replace that switch first. Something is not right inside that supposedly-simple device.
That much internal resistance would make me wonder how much longer that switch will work at all.
Loren

I must not have said it clearly. When I checked resistance on the switch I got zero ohms when depressed (the switch is identical to the starter switch which checks the same). Both switches new so with zero ohms I don't think its the switch. New glow plugs also. I can put in a solenoid but still would like to understand what is causing the drop. I am going to check resistance of the glo plug circuit and see if the drop makes sense. When I measured at the panel I was connected to ground at the panel. That means on the high voltage side I was only measuring within the panel and on the low voltage side I was measuring what is left after going to the engine and back. Just was looking to see if others had seen such a drop when "nothing is wrong" or if that means there is a big problem. Minus 11 this morning so it's going to be awhile before I get back to the boat.
 

lnill

Member III
I must not have said it clearly. When I checked resistance on the switch I got zero ohms when depressed (the switch is identical to the starter switch which checks the same). Both switches new so with zero ohms I don't think its the switch. New glow plugs also. I can put in a solenoid but still would like to understand what is causing the drop. I am going to check resistance of the glo plug circuit and see if the drop makes sense. When I measured at the panel I was connected to ground at the panel. That means on the high voltage side I was only measuring within the panel and on the low voltage side I was measuring what is left after going to the engine and back. Just was looking to see if others had seen such a drop when "nothing is wrong" or if that means there is a big problem. Minus 11 this morning so it's going to be awhile before I get back to the boat.
Ok, answering my own question here I guess. I just googled an on line voltage drop calculator. See if these numbers make sense to you. 30 feet of 10 AWG, 12 volts supply, 20 amps. The calculator predicts 10.8 volts. If this checks out then solenoid is next....what should I get?
 

supersailor

Contributing Partner
Voltage drop.

Sorry. I didn't make myself too clear. The glow plugs are the closest thing to a dead short that you have in your boat. The amperage draw is fairly awesome. In my case, the momentary switch welded itself in the on position causing the glow plugs to operate 24/7. The resulting low voltage did the fuel pump in. The result was getting pushed into the harbor by a friend with a dingy and its 2hp outboard on a windless day. as an aside the outboard managed to push us up to 3.1 knots.


Wiring the glow plug load through the ignition switch to the glow plug switch and then to the start switch creates a long path for a high amperage and that means voltage drop and wear on switches. In my case, the PO had changed the wiring harness and I had 45' for voltage dissipation. He had also changed the ignition switch for a Honda car switch. Carrying the high amperage that far and through so many switches doesn't make sense. A Bosche relay is inexpensive and you can use the glow plug wire as the activating wire. I now have a five foot run for the high amperage and my active glow plug time before start has dropped from one minute to 10 seconds.
 

lnill

Member III
Sorry. I didn't make myself too clear. The glow plugs are the closest thing to a dead short that you have in your boat. The amperage draw is fairly awesome. In my case, the momentary switch welded itself in the on position causing the glow plugs to operate 24/7. The resulting low voltage did the fuel pump in. The result was getting pushed into the harbor by a friend with a dingy and its 2hp outboard on a windless day. as an aside the outboard managed to push us up to 3.1 knots.


Wiring the glow plug load through the ignition switch to the glow plug switch and then to the start switch creates a long path for a high amperage and that means voltage drop and wear on switches. In my case, the PO had changed the wiring harness and I had 45' for voltage dissipation. He had also changed the ignition switch for a Honda car switch. Carrying the high amperage that far and through so many switches doesn't make sense. A Bosche relay is inexpensive and you can use the glow plug wire as the activating wire. I now have a five foot run for the high amperage and my active glow plug time before start has dropped from one minute to 10 seconds.

Yep. I made myself a separate drawing of just the glow plug circuit including the battery. Battery cables go from the battery (SB side in front of the nav station) to the engine compartment (under companionway). That's about 8 feet of battery cables so not so bad... But then you go to the panel and back with 10AWG to include the switch. Harness is about 15 feet long (so 30 total) and all that current has to make the whole trip. Knowing this, I am amazed it lasted 30 years.....
 

Pat O'Connell

Member III
Glow Plugs

Wow! I'm only about 60 miles Northwest of SalemMa and the temp is 18f. Last night was -10f.
Two ft of snow on the boat cover and three ft of snow on the mast cover. I can't even think of a glow plug issue. I hope our boat hauler has some dates in July. Good luck!
Best Regards
Pat O'Connell
1981 E28+
 

Tom Metzger

Sustaining Partner
Inill - You didn't mention why we are having this discussion. Is it safe to assume the engine didn't start? At 15* you need a real good battery, well charged, to start your diesel. Hopefully you have a charger running while you are doing this. The glow plugs will have to be run for a longer time to get enough warmth for the engine to fire up. You are also probably having fuel problems because regular diesel fuel will be gelled at 15* F and won't pass through filters.

I don't know it as fact, but I expect that the glow plugs will work at 10.7 volts. Just more slowly. Do you really have an electrical problem?

I get almost a 1 volt drop across the glow plug switch.

Both switches new so with zero ohms I don't think its the switch.

Let's see; 1 volt divided by 0 ohms. That's lots of current. BIG, BIG battery. Not in this lifetime. Hint: the resistance is not zero. At 20 amps 1 volt requires 0.05 ohms. It's unlikely you can measure that reliably.

Based on your demonstrated electrical knowledge I would go back and check the wiring changes and connections you made and then, not finding anything wrong, I would change the bad switch. You could just change the glow plug switch with the start switch and see what happens.

The best switch to use is the commonly available Cole-Hersee M-490, or with a rubber cap M-626. They are rated at 35 amps and should not have a problem with engine currents. Make sure the drain hole is down.

A relay in the glow plug circuit will help, but most people don't find it necessary, and it frequently masks other problems. That said, they do put them on new engines.

Good luck.

BTW, isn't Salem the home of witches? Could this be significant here? :)

Loren - Did I just agree with you again? Sorry.
 

lnill

Member III
Inill - You didn't mention why we are having this discussion. Is it safe to assume the engine didn't start? At 15* you need a real good battery, well charged, to start your diesel. Hopefully you have a charger running while you are doing this. The glow plugs will have to be run for a longer time to get enough warmth for the engine to fire up. You are also probably having fuel problems because regular diesel fuel will be gelled at 15* F and won't pass through filters.

I don't know it as fact, but I expect that the glow plugs will work at 10.7 volts. Just more slowly. Do you really have an electrical problem?





Let's see; 1 volt divided by 0 ohms. That's lots of current. BIG, BIG battery. Not in this lifetime. Hint: the resistance is not zero. At 20 amps 1 volt requires 0.05 ohms. It's unlikely you can measure that reliably.

Based on your demonstrated electrical knowledge I would go back and check the wiring changes and connections you made and then, not finding anything wrong, I would change the bad switch. You could just change the glow plug switch with the start switch and see what happens.

The best switch to use is the commonly available Cole-Hersee M-490, or with a rubber cap M-626. They are rated at 35 amps and should not have a problem with engine currents. Make sure the drain hole is down.

A relay in the glow plug circuit will help, but most people don't find it necessary, and it frequently masks other problems. That said, they do put them on new engines.

Good luck.

BTW, isn't Salem the home of witches? Could this be significant here? :)

Loren - Did I just agree with you again? Sorry.

Tom,
Had hard starting last fall. was taking a lot of cranking to start. I was busy with work at the time so had the yard check it out. They (unfortunately chose to check compression first). Found it to be 400 lb on all four so then checked glow plugs (replaced the previous season) and found very low voltage.

I had rewired the house wiring two seasons back so decided to just redo the engine and instruments per the posts on this site.

I am certainly not an expert in electrical circuits but also not a total novice. I have triple checked the wiring and have had the yard mechanic check as well. It matches my wiring diagrams. I can post those if you like.

By zero ohms, I mean the meter does not detect any resistance in the switch when the button is pushed. The switches are Cole-Hersee M-626. I can swap the switches or even just buy a third one but if your meter showed zero ohms on a (disconnected) switch when closed and infinity when open, what can be wrong with it?

Yes, Salem is the witch town....don't want to be there the week of Halloween.....

Lee
 

Tom Metzger

Sustaining Partner
then checked glow plugs (replaced the previous season) and found very low voltage. [snip]

By zero ohms, I mean the meter does not detect any resistance in the switch when the button is pushed. The switches are Cole-Hersee M-626. I can swap the switches or even just buy a third one but if your meter showed zero ohms on a (disconnected) switch when closed and infinity when open, what can be wrong with it?

You should have more than 12 volts on the engine panel.

A lot of people have had problems with the connectors in the engine harness. Check the voltage drop from solenoid to the engine panel with a load on it (glow plugs or blower for example). You can do it by comparing the voltages to ground on the solenoid and the engine panel (red wire). The difference should be very low - a couple of tenths of a volt at most.

Alternative: run a parallel #10 wire and see if the voltage improves.

The switch has a volt across it when closed. That means it's toast. Unless you have a very good meter you can't read the resistance reliably. The ohm meter just shows you that it is closing, not that it is making a good contact.
 

lnill

Member III
You should have more than 12 volts on the engine panel.

A lot of people have had problems with the connectors in the engine harness. Check the voltage drop from solenoid to the engine panel with a load on it (glow plugs or blower for example). You can do it by comparing the voltages to ground on the solenoid and the engine panel (red wire). The difference should be very low - a couple of tenths of a volt at most.

Alternative: run a parallel #10 wire and see if the voltage improves.

The switch has a volt across it when closed. That means it's toast. Unless you have a very good meter you can't read the resistance reliably. The ohm meter just shows you that it is closing, not that it is making a good contact.

I did not use connectors. Used Blue Sea buss bars and ring connectors. I'll get another switch and see what happens.
 

Cory B

Sustaining Member
Bad switch or Connection?

I'm with most of the above posters...

You should see very little voltage drop across the switch when engaged. If you are seeing 1 volt then the problem is likely a bad switch, or possible a bad wire or crimp connection to the switch.

I know you said you have 0 ohms resistance across the switch, but things behave differently as loads increase. Many of the momentary switches available aren't rated for the current glow plugs may draw. Our 3 cylinder can draw up to 18 amps.

Solenoids, etc. are all things to consider, but I think your particular problem may lie elsewhere.

Good luck!
 

lnill

Member III
I'm with most of the above posters...

You should see very little voltage drop across the switch when engaged. If you are seeing 1 volt then the problem is likely a bad switch, or possible a bad wire or crimp connection to the switch.

I know you said you have 0 ohms resistance across the switch, but things behave differently as loads increase. Many of the momentary switches available aren't rated for the current glow plugs may draw. Our 3 cylinder can draw up to 18 amps.

Solenoids, etc. are all things to consider, but I think your particular problem may lie elsewhere.

Good luck!
Based on responses so far, I'm inclined to think either switch (and then I will wonder about every other switch that I haven't checked) or as you say, a bad crimp. Bad news is I can't get back to check for over a month.
 
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