Wierd observation - glow plug voltage question

[TWeatherford]

This weekend I've been busy putting new battery cables and upgrading to a set of Group 31 batteries. While I was at it, I decided to give my glow plug system the annual once over. All the plugs ohm out good, and since my harness is in not so great shape I wanted to verify that I'm getting 12 volts to them. They're on a momentary toggle switch, so I had my wife run the switch while I used the meter. Before she hit the switch, I was reading 11.5 volts. When she hit the switch, I'd get the expected 12.5 volts.

I haven't done anything to the glow plug system since replacing the plugs and converting to push button over two years ago. I was under the impression that there should be no voltage at all going to the plugs with the key on until I hit my manual toggle switch. I haven't hooked them up yet, as I'm afraid at 11.5 volts they'll burn out in short order besides draining my batteries and putting a big load on the alternator. Of course if I hadn't done that check I'd have no idea about it at all.

Anyone care to comment if I'm correct or not? If anyone could run out and check their truck I would be much appreciative.

 

[Exekiel69]

Cant remember right now but I believe the relay triggers ground not voltage. Like I said, have not looked that far in a while.

 

[icanfixall]

The plugs can't have power to them all the time even if its a reduced amount of volts like you found. I will be watching this so I can learn more about thisissue you seem to have.

 

[dgr]

Doesn't matter if a relay uses hot or ground or magic on the trigger. If it isn't triggered, there should be zero volts on the load terminal.
I would back track to the relay and see what is up.
Are you ONLY using a manual switch or do you still have part of the solid state system in there? It does use a reference voltage through the z bar to check resistance through the gps to see if the engine is running,

 

[TWeatherford]

I think that the controller is completely bypassed, but I'll have to double check now. I do know that the WTS light never comes on unless I push the button.

 

[rhkcommander]

Cut all the wires from the controller. Make sure your momentary switch isnt internally shorting or that there arent loose wires coming from it arcing.

Could be a failing-on relay too

 

[sle2115]

The solid state system is switched to ground, the relay itself. Keep in mind, there is an afterglo built in as well, so you may be seeing some residual control from the relay. If hooked up correctly, you will take away the ground to the relay, and when you apply the switch, you will put it back. Otherwise, you may still be allowing the relay to partially function, and it's seeing the need for voltage because of low resistance...my .02 worth.

 

[sle2115]

Well, then again, looking at your sig, I'm not really sure what you have...are you using the 1994 glow system or the 85? Are the glow plugs spade or bullet connectors? (unless someone changed them, which is possible, the bullet is the later (87-94), the spade is the early system(83-86)

 

[TWeatherford]

I have a 6.9 but swapped all of the 7.3 glow plug related stuff over: controller, relay, wiring harness, and (bullet) glow plugs.

Thanks for the replies, this is a real mystery to me.

 

[dgr]

If all your stuff is bypassed, you should have a trigger wire to your relay, power to your relay, power out of the relay to your GPs and power out to your WTS light. That is the minimum needed to activate your GPs and see they are activated.

 

[LARRY RANALLO]

some meters when your checking ohms,and then switch to volt test,you need to put the test leads in a different port.

 

[TWeatherford]

Thanks for all the replies.

When I was checking for voltage at the glow plug connectors, with the key on, I was doing so with none of the connectors connected. I decided to try connecting one of them to a glow plug, and checking the voltage on another. When I did this I got 0 volts. My guess is that the controller is sending a tiny amount of current to the plugs, enough to read with a volt meter, but not enough to matter. Not enough to light up my WTS light, and certainly not enough to hurt any glow plugs. I considered going ahead and doing away altogether with the controller, but its dark and cold out and I didn't feel like it.

Fired it up, man the new batteries and cables are nice sure make it start quick. And I was pleased to note that I have 14.5 volts across the batteries with it running, indicating my 5 year old alternator is still doing fine (I was worried the alternator had died and took out the batteries with it).

 

[franklin2]

I believe we had a thread about this some time ago, and came to the same conclusion. The "brain" of the controller monitors the voltage across the flat piece of metal that sends current through to the glowplugs. The flat piece of metal can be considered a shunt, and it has a very very small resistance, so there is a small voltage drop across it when the current is flowing to the plugs.

Anyway they have a small wire hooked from the brain to this circuit(which goes to the plug harness) and we figured like you mentioned, there is a very small current coming from this wire that will appear to have 12 volts on it.

 

[rhkcommander]

My guess is that the controller is sending a tiny amount of current to the plugs, enough to read with a volt meter, but not enough to matter. Not enough to light up my WTS light, and certainly not enough to hurt any glow plugs. I considered going ahead and doing away altogether with the controller, but its dark and cold out and I didn't feel like it.

I'm glad its running good for you, when it gets nicer out you might consider disconnecting the controller fully - even low power will drain the batteries as a parasitic draw. Could still be a sticking relay too

 

[franklin2]

Most newer vehicles have a draw on the battery at all times, the newer they are the worse they are. You can have up to around a .07 amp draw and still be considered "normal". No, you usually can't let a newer car set for over 6 months without possibly having problems with it draining the battery.