Glow plug/wires resistance readings - PeachParts Mercedes-Benz Forum

dcraigk · #1 04-30-2017, 10:19 AM

I'm trying to figure out why I don't have a GP light on the dash on my new to me 300CD.

Can anyone help me figure out these readings? I think I know what they mean but I want to confirm.

GP wire #: reading

1:0
2:3.55
3:0
4:0
5:1

If I understand, GP #2 isn't working and there is a short in GP wire #5. Is that right? Thanks.

Diseasel300 · #2 04-30-2017, 10:26 AM

What are you using to measure your resistances and where are you checking? A good glow plug is typically .7-.9Ω. 0Ω usually indicates a short circuit.

If you have no glow light, check to see if the strip fuse in the glow plug is broken or cracked. Check the bulb in the dash to see if it is any good. It's also not uncommon for the glow plug relay to have issues.

dcraigk · #3 04-30-2017, 10:47 AM

I'm following Dieselgiant's website. Negative probe at the battery negative and positive probe at the plug on each of the 5 glow plugs.

I have good voltage going to the relay, before and after the strip fuse. The bulb was switched with a good bulb and still didn't light.

My meter starts at 1. If it touch the probes together I get 0.

Next he says measure the resistance at the plug for the 5 connections going to the glow plugs.

1,3 and 4 went to 0.
2 went to 3.55
5 had no change at all. It stayed at 1.

I've been reading and watching YouTube but nobody explains the different readings that are possible when reading ohms.

Diseasel300 · #4 04-30-2017, 10:57 AM

It sounds like maybe you aren't using the meter properly? You should be set on a resistance scale. If you have different scales, you should be on Rx1 or 200 depending on your meter.

If you have a digital meter and you're in the right mode, you should usually read something like OL on the display to indicate an "open lead". If you short the probes together, you should read 0. If you have an analog meter, your needle should be full left scale with the leads apart, and full right scale with the probes shorted together.

That cleared up, you should be reading resistance (it will be very low). The best way to check is to put your black lead on the engine block and touch the red lead to the wire connected to the glow plug. You should read just shy of 1 ohm.

If you read a hard 0, you have a short in your wiring harness or a shorted glow plug. If you read OL, you have an open circuit. If you read high resistance (anything above 1) you have a failed or failing glow plug. Generally if you really do have a short in your system, you'll take out that strip fuse (that's why it's there).

dcraigk · #5 04-30-2017, 11:38 AM

From what I've read different meters have different displays for an open lead. My meter doesn't display OL. It's a Klein Tools MM100. Not the best not the worst. I'm using the meter correctly set at 200 ohms. The probes touched together do display 0.

I just tested the relay. Negative to the block, turned the key and touched the clip to each pin and I got 12.xx at each one so the relay is good.

All I'm sure of is this car hasn't been used much in the last 10 years so I'm sorting through things.

I'll just replace all the glow plugs. It looks like I need to remove all the hard fuel lines for the job so I can adjust the valves at the same time since I don't have a set of bent wrenches.

Dr. Sternschnuppe · #6 04-30-2017, 03:38 PM

Quote:

Originally Posted by dcraigk

I'm trying to figure out why I don't have a GP light on the dash on my new to me 300CD.

https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/15-710.pdf

dcraigk · #7 04-30-2017, 04:02 PM

Thank you for that. I'm not great at finding things in the service manual.

I bridged the #1 and #3 and the bulb lights up. After that I can't figure out what I'm supposed to do. It looks like I'm supposed to check the resistance (ohs sigh) from #3 on the plug to the + at the bulb, but the next line says if the bulb lights up, the relay is defective. So that doesn't make sense.

barry12345 · #8 04-30-2017, 04:22 PM

If your meter is digital. You indicate you may have three open glow plugs, one that is too high resitance wise and one perhaps normal.. If the meter indicates the same reading disconnected from the test as well as when connected. That glow plug is open.

Now if your meter is analogue. That is it has a meter movement. Reading in the one ohm area is not too practical. On the basis of what you posted I would expect three good glow plugs and one still partially working and one hardly working if at all.

The way you check the glow plug circuit on these cars from about 1981 to the 1985 123 models. The glow plug harness plug disconnects at the glow plug relay. You measure resistance between ground and the individual glow plug wire feeds there. If you leave the plug installed all the glow plugs are bridged together at the relay I would think.

This is not high tech so keep it simple. I just looked up your meter it is digital and has a 200 ohm scale. Far from a poor quality meter. On that basis verify and I would change all the plugs as 3 of them look to be open. With another showing excess resistance but intact. This is what is keeping your dash light off as well.

Also it may now be good practice to check the new ones with your ohm meter before installation. Bosh is the only brand of replacements to consider as well. There are a lot of junk brands in the market place. Beru is also a good brand but not common to find.

Last but not least. After you have changed the plugs out. let the car run for five minutes. Then reach in with a meter lead to any glow plug. With your meter set at volts dc. You want to make sure the glow plugs are not staying on as you have had a major glow plug failure. You want to see zero volts.

This could have happened by someone just ignoring the glow plugs as they failed one after the other or the glow plug relay is constantly on. Chances it is the first situation but it only takes a minute to verify it was.

Why do I do things like this? Burning out new glow plugs is not going to happen. Plus all of a sudden I am not going to be dealing with battery issues from the constant drain of the plugs on all or most the time.

dcraigk · #9 04-30-2017, 06:15 PM

What I was trying to figure out is what a high and low reading represents.

I know <1 is good. So when they are burning out they read high and when the die they read 0?

Like I said in an earlier post, to test the relay I jumped the 1 & 3 on the 4 wire plug and the light came on. I'm not sure what the next instruction is telling me to do.

Junkman · #10 04-30-2017, 06:46 PM

Buy 5 new Bosch glow plugs. Keep 2 of the old plugs for emergencies. Measure resistance of the old plugs when they are in your hand or connect them to a battery via car jumper wires to test.

Don't plan on the old plugs being more than a stop gap. If you ever install them, order another set immediately as you will need them. Consider ordering the glow relay fuse if inexpensive in case you need it unless you frequent the yards and can get a few extra for free.

(I get the entire relay when at the yard along with door bumpers and mono valves. I need to replenish.)

barry12345 · #11 05-01-2017, 11:15 AM

I got out a digital meter. After reflection I realized that at one time I used them so much. That part of the usage was automatic. So for an explanation.

When I turn on this meter it presents a large number 1 at the left side. If I touch the probes together the meter indicates .7 ohms. The actual calibration at this low ohm reading is off a little. If the meter just remained at the number I when checking a glow plug I would know the plug was open. Called infinite resistance basically. The meters not indicating any resistance.

Now if I wanted to check a glow plug. I would have to reduce the .7 ohm from my reading. In this meters case say I read a glow plug and the meter indicated 1.6 ohms. I would reduce the reading by the .7 ohm giving me a truer reading of .9 ohms. I think I might have been a liitle wrong earlier or your meter reads a negative ohm value just when you short the leads together. It may not indicate this though. Even though your meter is reading 0 ohms on three plugs. The first glow plug that shorted would have taken the fuse in the relay out. So I cannot believe that you have three shorted glow plugs in reality.

More likely is your digital meter is also off somewhat like mine at very low ohm readings. If it reads in a negative fashion when you short the probes together this probably will not be indicated. I have never had the experience of a digital meter being slightly off in that direction to the best of my knowledge still it is possible. Actually the meter I grabbed should not have been off that much. I have never used that meter much but it may have aged somewhat siting around over the years. It is not a five dollar special but even those probably would have indicated better than what this one did.

Just for interest I got out one of my auto ranging meters. It indicates 0.1 ohm with the probes disconnected. With the probes shorted together it indicates 000.0 ohms. When measuring resistance it is so seldom you get into these low ohm value situations. The manufacturers probably are not fixated on accuracy values that low. Or I doubt they are testing after manufacture. Instead testing on a fixed 100 ohm resistance and if It reads between 99-101 ohms they would consider the meter to be within one percent of accurate. I like all people make misteaks. Even with more than twenty five years of heavy electronics service well behind me. I never noticed when turning a digital meter on there was not a universal standard display. That is why I got out the other digital meter for turning on comparison and accuracy check.

I was working in electronics at the dawn of the digital meter. A five dollar one today probably has more functions than the first ones I brought for over a hundred dollars apiece. Before the transistor swept into the market the analogue meter had some advantages. It still had some operational advantages after. Even that is pretty much history as well.

Also with a meter long out of daily service. Most probes probably have a protective layer of metal on them. Al lthe common ones will oxidise a little with time probably. A non abrasive cleaning of them is probably not wasted time. For example if you touch the probes to each other in different areas and get different meter indications I would clean them.

spark3542 · #12 05-01-2017, 12:08 PM

Quote:

Originally Posted by barry12345

. On that basis verify and I would change all the plugs as 3 of them look to be open.

I disagree that 3 look to be open.

Based on the OP's lack of experience with ohmmeters, I would bet the meter was on 200k. Having said that, plugs 1,3 and 4 are good, 5 may be good, and 2 could possibly be bad.

A shorted plug is unlikely, so the zero readings are more likely 0.9 ohms. A plug actually reading 3.5 ohms is also unlikely, so it may be 3.5 Megohms, or if his hands were touching the probes it's going through his skin.

barry12345 · #13 05-01-2017, 12:14 PM

Quote:

Originally Posted by Junkman

Buy 5 new Bosch glow plugs. Keep 2 of the old plugs for emergencies. Measure resistance of the old plugs when they are in your hand or connect them to a battery via car jumper wires to test.

Don't plan on the old plugs being more than a stop gap. If you ever install them, order another set immediately as you will need them. Consider ordering the glow relay fuse if inexpensive in case you need it unless you frequent the yards and can get a few extra for free.

(I get the entire relay when at the yard along with door bumpers and mono valves. I need to replenish.)

Picking up a spare glow plug relay from a wreck or junk yard when the opportunity presents is an excellent ideal. They are expensive new. If the pick and pull type place only wants about five dollars for one grab two if you can.

Many of our old cars may still have the originals and they all eventually fail. The contacts alone deal with perhaps 80 or more amps when they close. As the glow plugs heat their internal resistance increases. Still this will leave perhaps a forty amp disconnect. It is never a really clean contact break.

The relay points take damage. Over the years with use. I have never taken one apart to examine it. I doubt there is any form of current limiter on contact breaking.

dcraigk · #14 05-01-2017, 12:21 PM

Yes the meter was at 200K. What should it be at? I am inexperienced at testing with a multimeter.

I just retested at 20K and the readings didn't change.

Plugs 1,3 and 4 start at 1 and drop to 0 when I touch the contact. Plug 2 was still 35.5 (I think). Just the decimal moved. Number 5 stayed at 1 when I touched the contact. There was no change in that reading.

barry12345 · #15 05-01-2017, 12:48 PM

Quote:

Originally Posted by spark3542

I disagree that 3 look to be open.

Based on the OP's lack of experience with ohmmeters, I would bet the meter was on 200k. Having said that, plugs 1,3 and 4 are good, 5 may be good, and 2 could possibly be bad.

A shorted plug is unlikely, so the zero readings are more likely 0.9 ohms. A plug actually reading 3.5 ohms is also unlikely, so it may be 3.5 Megohms, or if his hands were touching the probes it's going through his skin.

It actually drove me to fish out a couple of meters. One shorted plug would have taken the glow plug relay fuse usually. I was having a very hard time interpreting what he thought he was reading. On infinity or an open glow plug my two meters produced different results. One would have stayed at 0.1 ohms and the other at a large I on the left of the display. I was going to go get another to compare then remembered it was buried in storage. I knew I had yet another but it was out at the cottage.

I was also going to mention that a manual for his meter is on the web. I neglected to mention this. Or look at it. The 200 ohm setting is very clear on his model though I noticed from a picture of the meter. When you are an infrequent users of meters the more you understand them the better.

If there were only one thing to pass along. It is never accidentally measure voltage. When the meter is in the amperage or resistance settings. In all probability you will damage the meter or burn out an internal fuse.

The resistance of the body in parallel is so high that for all practical purposes it will not effect any low value ohm reading. I do agree that he probably has three good glow plugs. I have too much racked up in my memory bank. It actually is a conditioning. I do many things so automatically that I do not often consider the how or methology. Until something like this comes along.