12 volts at glow plug wire when disconnected - PeachParts Mercedes-Benz Forum

rhodes2010 · #1 01-03-2010, 09:42 PM

My 1980 300D won't start all of a sudden.
Been running/starting great all day.
Car starts really easy and fast even in cold weather.

After excluding vacuum issues (to make sure fuel wasn't being cut off)
I went to take all the glow plugs out and test
them, just to be sure they were okay.

This is what seems wierd:

I get 12 volts with key on, at the relay, on the pins for connector.

With all 5 wires off the glow plugs I get 12 volts at each end that
connects to glow plug.

When I connect the 5 ends to the glow plugs, and then connect my volt
meter and turn on the key I get nothing - no voltage with key on at the
glow plug. But I do get 12 volts with the 5 ends disconnected.

I am befuddled. I seem to remember that I used to get voltage at the
glow plugs.

All glow plugs passed the heat up test with 12 volts applied directly.
I do not have an ohm meter that goes below 10 for proper resistance
testing, so I test with full 12 volts off my battery charger.

I know that the glow plug creates heat by resistance, but I still think there
should be 12 volts at the glow plug.

Can anyone please explain ?

Thank you.

kerry · #2 01-03-2010, 09:47 PM

There was a thread about 3 weeks ago with this exact problem. Look it up and see if the problem was ever diagnosed.

Glow plug testing question (w123)

DIESELVOLVO · #3 01-03-2010, 09:55 PM

Just had the same thing happen to me with my wagon. All but one plug was shorting out from the carbon packed around it in the prechamber. Ream out the holes and you should be good.

rhodes2010 · #4 01-03-2010, 09:57 PM

Thank you!. What was happenning in that thread seems
to be exactly what I am experiencing.

Unfortunately the thread has no conclusion.
Many solutions presented, no resolution posted. Dang!.

The suggestion is that there is resistance somewhere sufficient
enough to cause the voltage to drop to zero when connected to glow plugs.

I will check the strip fuse and ground strap for decent clean connection.

Any other specific places to look for resistence ?

rhodes2010 · #5 01-03-2010, 10:00 PM

Hey DieselVolvo, Question about this.

I just had all the glow plugs out.

The center electrode (pencil tip) seems to be connected to ground anyway, so
how does the carbon cause this to short circuit ?

I did feel like the tips were hitting carbon, but did think too much of it.

rhodes2010 · #6 01-03-2010, 11:18 PM

So I cleaned the connector at the body for the negative (ground)
cable. Did not look bad when I took it apart.
Still, took a wire brush to the bolt and cleaned the cable and body.

Then went to the strip fuse in relay. Fuse looked great.
Noticed as I was unscrewing that the screws were not real tight.

After careful inspection reinstered fuse and snugged up screws tighter
than they were.

Cannot say which condition caused the resistance.

But... I started connecting one glow plug at a time to the harness
and turned the key to on after each connection to check voltage
at the glow plug harness. After each connection I continued to get 12 volts
at the glow plug. I was waiting for it all to drop off again, thinking that
maybe one of the glow plugs was grounding/shorting.

Got to the last one and still get 12 volts when key on.

Buttoned it all up and car started just like I am used to.

Thank you everyone.

Diesel911 · #7 01-04-2010, 12:06 AM

Quote:

Originally Posted by rhodes2010

So I cleaned the connector at the body for the negative (ground)
cable. Did not look bad when I took it apart.
Still, took a wire brush to the bolt and cleaned the cable and body.

Then went to the strip fuse in relay. Fuse looked great.
Noticed as I was unscrewing that the screws were not real tight.

After careful inspection reinstered fuse and snugged up screws tighter
than they were.

Cannot say which condition caused the resistance.

But... I started connecting one glow plug at a time to the harness
and turned the key to on after each connection to check voltage
at the glow plug harness. After each connection I continued to get 12 volts
at the glow plug. I was waiting for it all to drop off again, thinking that
maybe one of the glow plugs was grounding/shorting.

Got to the last one and still get 12 volts when key on.

Buttoned it all up and car started just like I am used to.

Thank you everyone.

The bad connection of the scews not being tight was the resistance.

rhodes2010 · #8 01-10-2010, 11:33 AM

So, car started a week ago, let it sit all week.
Now it won't start again.

I have cleaned the ground strap, thinking that resistance was
from frame to engine and that this resistance was allowing
voltage to drop at the glow plugs. Did not help.

One poster said to ream glow plugs to stop this from happening.

Everything I read about reaming talks about better starting and
longer glow plug life. None of the posts refer to not starting.

I found this post on reaming glow plugs, have never reamed out gp's.

Glow plug reaming

Can anyone confirm that the loss of voltage at the glow plugs
can be caused by carbon build up ?

As posted earlier in this thread, I have voltage at the glow plug wires
when the wires are not attached to the glow plugs. The relay appears
to be working as it should.
When I attach wires to glow plugs and turn on key the voltage is zero.
Even if I am attached to only one glow plug. And it does not matter
which glow plug. Any one plug causes voltage to drop to zero.

micalk · #9 01-10-2010, 09:22 PM

Check the heavy wire from the glow plug relay to the terminals on the opposite fenderwell, make sure all those terminals are clean and tight. Those terminals are from the alternator and the battery and they feed to GP relay. So to be clear, there is a direct connection from the battery to the terminal strip, and from the terminal strip to the gp relay. If the terminal is loose (just like the fuse) it will show up as a high resistance and cause your problem.

rhodes2010 · #10 01-10-2010, 11:53 PM

Wish I could change the main title of this post. It seems confusing now.

Thank you for the advice about the pre-relay connection causing resistance.

I guess what is happenning is that although I have voltage, I do not have enough amps to keep the glow plugs happy.

Has anyone ever bypassed/replaced the hot wires that feed the relay ?

I hate having a non-standard/stock car, but at 30 years old the wires
are getting hard, probably breaking down a bit.

The wires themselves could be part of the problem.

Anyone ever run a bypass line to feed the relay ?

Thanks everyone.

FormulaReed · #11 01-11-2010, 01:23 AM

I had this same issue with my car. I replaced the glow plug relay, engine to body ground strap, all glow plugs and nothing changed. I started to check out the wiring in the rest of the car and I found some really scary stuff, like a half wired in car alarm, lots of wiring that went nowhere and the red wire that leads from the headlight switch to the ignition looks like it had been on fire and was shorting out.

I cleaned a bunch of it up, removed the alarm, and like magic, the glow plugs started working again. I really don't know if anything that I did fixed the car, tried undoing the work that I have done but nothing that I do will make the glow plugs stop working. I have been driving the car for about a week and the glow plugs are still working great.

I checked the voltage regulator yesterday and it was completely worn out. I doubt that has anything to do with the glow system, but now the stereo works and the headlights don't go dim and bright. Maybe that was causing something to short out, but I really don't know. If you find out what is broken on your car, please post it. I would love to know that I really fixed my car.

Cheers,

Reed

micalk · #12 01-11-2010, 10:46 PM

The insulation may break down, but the wires would still be there. As long as the connections are good and the wires haven't been mangled the copper should be fine. Age shouldn't affect the ability of the metal in the wire to carry current; broken strands will have a much greater affect on the current carrying capacity. Broken insulation may lead to corrosion of the wires which would also lead to reduced capacity.

That said, one way to check if you have a problem in this area is to measure the voltage to ground at the different points while the glow plug relay is energized. Measure at the battery post, battery terminal, fender terminal and glow plug post where the large wire terminates. Ideally, these should all be at the same voltage. Voltage drops will tell you if connections are bad or wires are bad or battery is bad.

Another thing you can do is run a wire directly from the battery to the glow plug terminal. You could rig it up temporarily to see if that solves your problem.

And check the fuse again. The nuts may have loosened up. Ya never know.

babyjames · #13 01-11-2010, 11:35 PM

Are they clean and tight? You just replaced the battery? Did you clean the posts w/emery cloth or a wire brush, and do the same to the inside of the clamps, before you put the clamps on the posts?

You checked the ground strap from the bell housing to the frame, but how about the one from the battery to the inner fender?

Are the clamps firmly affixed to the cables at the battery? Check the hot wire to the starter? I've had the POST (stud, call it what you will) on the solenoid that serves as the connecting point for the hot wire from the battery work its way loose before. DISCONNECT THE BATTERY BEFORE YOU TRY DOING ANYTHING AT THE SOLENOID END!!!

Jay.