Glow plug testing question (w123)
 

Hi,

So I have been trying to get my 300d started again and I am having a problem.

I tested the glow plugs per the DIY on the diesel giant page. When I have the harness that goes to the glow plugs disconnected from the relay and I turn the key, all seems fine. I get 12v at the power input, across both sides of the 80 amp fuse and at the male terminals that go to the glow plugs. However when I have the harness to the glow plugs attached and turn the key I don't see 12v anywhere untill the realy clicks off. I am also not seeing any power (I am testing for volts) at the glow plugs. When I test the wire at the female end, I am seeing .6 to 1.0 ohms to all the glow plugs but the terminals that are labled B and TC I am not getting any reading.

What do you all think is going on? The relay is woking untill I hook up the glow plug harness. I am pretty sure that the wire is ok but I don't know what B and TC are. Does this sound like a dead relay or is the wire shorted out and causing the relay to stop working?

Thanks,

Reed

Can you get 12 volts at the ends of the harness without the glowplugs connected?

nope, with the glow plugs disconnected I still don't see any voltage.

At this point, it might be worthwhile to open the relay box and check the condition of the relay contacts. Sometimes, a simple cleaning will restore proper operation.

I took the relay out of the car and opened it up. It looks ok in side but only one side of the relay is making a connection when it's closed. The outside one (closest to the casing) is making a connection but the inside in not. Any one know if they should both be making contact?

Thanks again,

Reed

I have done a little more testing and I was wrong about getting voltage at the harness ends with it disconnected.

When I have the harness completely disconnected from the glow plugs and the sensors I get voltage at all the glow plug and sensor connectors but as soon as I connect any of the glow plug wires the voltage drops to nothing.

Now I am really confused. I replaced the engine to frame ground strap and nothing changed. The glow plugs are Bosch and there brand new. I thought that the relay was bad so I got another one but nothing changed. It seems like the system is grounding out some how but I don't have a diagram.

Thanks in advance,

Reed

You need to trace the circuit back to where you see the 12V again. What's happening is that you have a high resistance somewhere in the line. When there's no current flowing (glow plugs disconnected) then you'll see voltage wherever there's any kind of connection - V=I*R: when current is zero there is no voltage drop across any resistance, so you see the same voltage everywhere in the circuit. The second you start drawing current (glow plugs attached) the most voltage is dropped across the largest resistance. Glow plugs present less than 1 ohm so anything larger than 20 ohms is going to make it look like there's practically nothing at the GPs.

First thing to check is the GP relay fuse. Take it out and make sure its not in two pieces. The GP relay is connected to the terminal strip on RH fender well, same place as the alternator. These voltages are always hot so be careful. Clean all these connections. Make sure the battery has a good connection (does the starter crank okay?). Clean the connection on the glow plug relay where the big wire attaches. When you put it all back together you should have voltage at the glow plugs. If not look for where the voltage goes from 12V to nothing under load, like a broken wire or a bad terminal connector.

You have a high resistance somewhere. Without a load, the voltage is there, but when there is a load, the resistance doesn't allow the voltage to come through.
Think of electricity like water- Voltage is pressure, amps is flow. If you have a garden hose with a nozzle on the end and close the nozzle, you have pressure in the line without flow. This is like potential voltage. Now, if you kink the hose, it still has the same pressure in it until you open the nozzle. Flow (amps) can't get past the kink (resistance). Clear as mud?

Are you sure about that?

I wouldn't bet my life on it, but yeah.... no. No, I'm not. In fact, I was wrong. It's not even close. I stand corrected. I won't even blame fat fingers.

25 years ago I couldn't even spell engineer, now I are one!

mical,
You were originally correct, V=I*R
Ohm's Law

High resistance under load only= dirty connection some place, maybe the contacts in the relay. Good place to start. Then every connection between the battery and the glow plug.
if you put a multi meter on it the resistance may not be high as the multi meter only uses a very small current.
Could turn glow plugs on & then measure voltage down to earth at various points until you find where the break down is occurring.

Wasn't this on Rocky and Bullwinkle, once upon a time?
Remember, with great power comes great current squared times resistance.
OR
Twinkle, twinkle little star E= IR (Eat It Raw)

an old prof. I had liked this one...
Starkle, starkle little twink P=IE, I think? (PIE)

I think the "original" train had already pulled out before you arrived at the station.

If you check my post (#7) you'll note it's been edited to correct the formula.

I remember that I had it I=E/R originally, and wanted to show it wrt E, so I just swapped E and I without thinking. It's something I'd correct on my son's fourth grade homework, but apparently not in my own work.There's a train?

After all that, I hope Reed got his glow plugs to work !!!