OM617 W123 Glow plug power source? (low voltage at relay) - PeachParts Mercedes-Benz Forum

Jarod · #1 12-06-2014, 03:07 PM

Hi again!
Sooner or later I'm sure you guys will get sick of me asking dumb questions.

But until then, here's a dumb question:

The 1982 W123 300d I picked up for my wife to drive, the one that I had tie rod and steering issues that were all my fault...

A new issue has cropped up, I think its been an issue all along, but I didn't notice how hard it starts during the warm weather.

I replaced all 5 glow plugs when I got the car at the beginning of the year, replaced the primer pump and that made the car start great all summer long, no air in the fuel, and 5 working glow plugs.

However the other day, it wouldn't start at all. I checked the glow plugs, all less than 8 months old, ohmed out ok (I know thats not a for sure test)

I then checked the voltage at the relay. 12.3 key off, 9.1 key on. battery is good, battery voltage drops from 12.3 to 11.9 when the key is on, cranks very fast.
The entire fuse panel also drops to 9v when the key is on.

I figure this is the glow systems draw that drops the voltage, but i think there must be a bad connection or possibly a cut wire somewhere.

The thick wire that goes to the fuse in the glow plug relay looks fine where I can see it, but where does it go?

Is there some kind of regulator or place that the relay and fuse box get the power from?

I pulled the kick shield thing off the dash on the drivers side, looked around in there, massive ratsnest of wires, couldn't find the big wire coming from the glow plug relay though.

EDIT: Doing some reading, I hear talk of an OVP fuse/relay near the glove box that might be related, the tach is kinda wonky, bounces around at idle, drops to 0 as soon as rpm is above idle. possible connection?

Where does the wire go? Why would the voltage be so low?
Will I ever stop having silly problems with my cars? Find out after a few people reply.

Thanks for any help in advance!
Tom

funola · #2 12-06-2014, 05:47 PM

Per the FSM wiring diagram, the glow plug relay fuse goes to a junction block C105 on the passenger side top fender well, where it goes to the battery. I think you should get higher than 9.1V at the glow plugs when they are on (I think not much lower than 11 V). I'd check all the connections from battery to C105 to glow relay and also the ground strap under the car.

Jarod · #3 12-10-2014, 03:16 PM

Junction block eh?

Is this perhaps the one with 3 phillips screws? 2 big ones and one small one?
I removed the screws and cleaned it up, reassembled, tested again, I still get 9.3ish volts when glow plugs are energized at the relay, at that terminal block i get 10.9v

I tried swapping batteries just in case i had a weak cell or something.
No go there, same exact situation.

What I'm curious about is if the PO has hacked into the glow plug power wire for the fuse panel, or possibly the radio or something idiotic, as the fuse box on the drivers side also drops to 9.3v when glow plugs are energized.

Do you know if the wire that powers the relay is visible up under the dash anywhere?

I'm going to go run a heavy gauge jumper wire from the battery to the relay and see if i get voltage drop and hard start then (No, I'm not going to leave it like that, this car is for my wife, I'm not doing a hack job like that and leaving her stranded somewhere)

SD Blue · #4 12-10-2014, 05:19 PM

Ditto, checking the ground strap, especially the Chassis-Engine. It may require physical removal and cleaning.

Dropping to 9Vdc is too low. It should only drop to ~11Vdc.

funola · #5 12-10-2014, 05:30 PM

Quote:

Originally Posted by Jarod

Junction block eh?

Is this perhaps the one with 3 phillips screws? 2 big ones and one small one?
I removed the screws and cleaned it up, reassembled, tested again, I still get 9.3ish volts when glow plugs are energized at the relay, at that terminal block i get 10.9v

I tried swapping batteries just in case i had a weak cell or something.
No go there, same exact situation.

What I'm curious about is if the PO has hacked into the glow plug power wire for the fuse panel, or possibly the radio or something idiotic, as the fuse box on the drivers side also drops to 9.3v when glow plugs are energized.

Do you know if the wire that powers the relay is visible up under the dash anywhere?

I'm going to go run a heavy gauge jumper wire from the battery to the relay and see if i get voltage drop and hard start then (No, I'm not going to leave it like that, this car is for my wife, I'm not doing a hack job like that and leaving her stranded somewhere)

The wire that powers the relay should not go through the firewall/ under the dash if it is factory. Running a heavy gauge jumper wire from the battery to relay is a good test, but that is only half of the equation. The ground strap is the other half, so make sure that is OK also.

Jarod · #6 12-10-2014, 06:57 PM

Quote:

Originally Posted by funola

The wire that powers the relay should not go through the firewall/ under the dash if it is factory.

I will take some pictures tomorrow, it looks factory under the hood, some other things have been a bit butchered though. I'm realizing why it was $1300 with help from my wallet...

I'll also check the ground strap.

With the jumper wire, voltage stayed at the expected 11ish volts at the relay. I will do more with it tomorrow and hopefully the weather will be a tad better.

Thanks for the help guys!
I'll report back with pictures and stuff!

vstech · #7 12-10-2014, 08:46 PM

the 3 screw block is Starter/alternator wires... not GP.

you can follow the red wire from the GP relay all the way back to the battery.

funola · #8 12-10-2014, 09:32 PM

Quote:

Originally Posted by vstech

the 3 screw block is Starter/alternator wires... not GP.

you can follow the red wire from the GP relay all the way back to the battery.

I got the info from the FSM, which says the glow plug relay goes to C105 before going to the battery. The FSM also says the alternator / starter go to C105 before going to the battery. I am assuming it's the same C105 on the fender well. I have never actually physically traced those wires so I could have read the FSM wrong or the FSM itself is wrong. I am not sure if it is possible to physically trace the wire out as they may be buried in a big bundle. If that's the case, disconnect the wires at the terminal block and at the glow plug relay, and buzz them out with an ohmmeter.

Jarod · #9 12-11-2014, 08:23 AM

Ok, I definitely can't follow the wire. I'm quite certain it goes into the car with all the mass of wires going to the fuse box. heres pictures of the steps of the process:

Picture 1 is of the relay itself, the screwdriver is pointing to the wire I followed.

Picture 2 is the wire exiting the relay and looping back around towards the firewall

Picture 3 is the wire entering the loom of other wires. this is underneath the glow relay.

Picture 4 is where the loom goes to. the wires coming out of the loom go to the wiper motor (looks like theres a nick in that wire, I'll check that out while I'm un-looming stuff this afternoon), the brake fluid reservoir, and there is a 2 wire plug like the brake ones that is unplugged.

the rest all enter the vehicle below the fuse panel.

Picture 5 is very blurry, but that is all the wires on the inside under the fuse box that comes through that loom. the ones that still have black insulation like the one coming from the gp relay are tiny wires, the thicker ones all seem to go to the fuse box, but its tough laying on my back half in the door to get in there, so I didn't spend a long time investigating.

I think my next step is going to be removing some loom from under the hood area and seeing if the wire does definitely go in the vehicle. and if it does, finding out what color code it is.

funola · #10 12-11-2014, 09:52 AM

I don't think it is practical to physically trace the wire. Buzz it out with an ohmmeter on the lowest scale..... better yet if it has an audible beeper. Disconnect both battery terminals and all wires at terminal block and glow relay. See where the wire goes from the glow relay onwards, buzz the fuse box, terminal 30 of light switch, all the wires at the terminal block, starter motor wires. Report back what you find.

funola · #11 12-11-2014, 12:35 PM

Quote:

Originally Posted by funola

I got the info from the FSM, which says the glow plug relay goes to C105 before going to the battery. The FSM also says the alternator / starter go to C105 before going to the battery. I am assuming it's the same C105 on the fender well. I have never actually physically traced those wires so I could have read the FSM wrong or the FSM itself is wrong. I am not sure if it is possible to physically trace the wire out as they may be buried in a big bundle. If that's the case, disconnect the wires at the terminal block and at the glow plug relay, and buzz them out with an ohmmeter.

I was curious and had a few minutes and did a test and it turns out the FSM is correct and I did not read it wrong.

I had a volt meter on the fat wire terminal on the glow plug relay and it read 13.5 volts (car was just driven). Turned key on (glow plugs on) and got 11 volts. Turned key off and proceeded to remove the 2 leftmost screws on terminal block C105 and as I lifted the wires off the voltmeter went to zero.

I then hooked a test light to all the hanging wires and this one feeds the terminal block from the battery (lone fat white/ red stripe wire).

I then buzzed out which wire goes to the glow relay and it is this one (a fat white wire with another smaller diameter red wire)

You can try to measure the wires resistance but don't bother trusting the absolute accuracy of the result. Your ohmmeter is not capable of a true reading at such low resistance.

Jarod · #12 12-11-2014, 04:56 PM

OK, my brother got me this awesome little extech multimeter lead set, man that's handy. Clips on the leads kinda like in your picture. I clipped one lead to the relay, and went around poking with the other lead.

I got 0.3 ohms at the fuse labelled c, for the aux fans it said.
I got 0.4 ohms at the headlight switch terminal.
I got 0.4 ohms at a red wire on the terminal block with them disconnected.
I got 0.4 ohms at the battery positive with the terminal block connected.

That seems like reasonable figures to me.

I tried again with my test wire strung across the top of the engine, 12.4 key off, 10.7 key on glow active.

Without the test wire, 12.3 key off, 9.2 key on glow active.

Should I just make a permanent jumper wire?

funola · #13 12-11-2014, 05:13 PM

Did you isolate (remove) the glow relay wire at the terminal block (like in my pic) when you took those measurements? If you did, then those measurements indicates that wire has been messed with.

masc243 · #14 12-11-2014, 06:12 PM

Quote:

Originally Posted by Jarod

OK, my brother got me this awesome little extech multimeter lead set, man that's handy. Clips on the leads kinda like in your picture. I clipped one lead to the relay, and went around poking with the other lead.

I got 0.3 ohms at the fuse labelled c, for the aux fans it said.
I got 0.4 ohms at the headlight switch terminal.
I got 0.4 ohms at a red wire on the terminal block with them disconnected.
I got 0.4 ohms at the battery positive with the terminal block connected.

That seems like reasonable figures to me.

I tried again with my test wire strung across the top of the engine, 12.4 key off, 10.7 key on glow active.

Without the test wire, 12.3 key off, 9.2 key on glow active.

Should I just make a permanent jumper wire?

Your resistance values are fine. However the glow plug system draws lots of current. Take a small wire brush and clean the surface of the terminals as well as the terminal blocks that they connect to at the connection near the coolant resevoir as well as the battery terminals themselves. If you can get to the grounds; clean them too. When you check resistance, you are effectively getting continuity from one pin sized spot at the terminal, but you need a good clean surface to transfer all of the current to the wire efficiently. If the contacts aren't clean, you will get a voltage drop and not get your desired voltage at the gp relay or the glow plugs. I wanna say my glow plugs were at 11.5 or better recently after changing my glow plugs.

If all of the wiring is unaltered, I believe that barring major corrosion within the terminal to wire contact area, shining up the surface areas should help tremendously.

On another note, you say the car won't start. In your jumpering effort, you might consider jumpering an easy to get to glow plug; probably #3; straight to the battery for 20 seconds or so and then cranking the motor. At least you'll then know your no start condition is definitively glow plug related....

Hope this helps.
Mike

Jarod · #15 12-11-2014, 06:59 PM

Funola: I removed one set of wires, the other set seems unwilling to move, I am afraid of rounding the screw out, I might see if I have a terminal block on my spare motor and swap them tomorrow morning.

I highly suspect wiring has been tampered with. Most of the car was tampered with. Wish I'd been a little more careful when looking it over when I bought it.

But, bent tie rods, damaged undercarriage, bearings, hidden rust, sunroof leaks, improperly attached air filter housing, flex discs, trashed cooling system and now glow plugs aside, it wasn't that bad of a deal...

Masc:
The car starts and runs good, when its warm outside, but the colder it has gotten, the worse it starts, glow plugs were my first suspect as that's what always did it on my 116.

I haven't been able to start the car for a week or so now, all whilst making an attempt at diagnosing this issue.

With a jumper wire attached, the voltage is much better and the car started earlier, which makes me want to just permanently install the jumper wire...

But it obviously the entire car had been a hack job before, and since my wife has claimed it as hers now, I want it to be reliable and solve the problems rather than patch them.