Glow plugs and Relay - PeachParts Mercedes-Benz Forum

anhydrous7 · #1 02-02-2004, 03:54 PM

I have the Mercedes CD repair manual for my 82 300 SD. I also have reviewed the postings concerning the glow plugs and the relay associated with them, but have still come up with some questions pertaining to the glow plug/relay system. This is my first time asking for help from this forum. With that being said, I will describe my situation.

On cold mornings I am having difficulty starting my car if I do not plug it in overnite. I have measured the resistance of each individual glow plug with the connector disconnected at the relay on the driver side wheel well. According to the spec. in the manual, I should be seening 1 ohm or less for each of the glow plugs. My measurements ranged from 1.2 ohms to 2 ohms. I concluded that the glow plugs are suspect and I have ordered a new set. I also checked for voltage at each glow plug and found that I have approximately 10.5 volts at each glow plug. While performing this check, I realized that the relay is only staying engergized for about 2 to 4 seconds total. This is verified by the voltmeter going to 0 volts after I hear the relay open up. I have been noticing over time that the glow plug indicator light on the dash panel has been going out more quickly over time (i.e. staying on for a shorter and shorter time).

My question is this: Is there a relationship between the glow plugs degrading over time and the glow plug indicator light illuminating for a shorter amount of time.

Also, is this an indication that the relay is starting to fail, or does the relay have a hard failure, as in it works or does not work.

Any help will be appreciated.

Thanks,

Rob

anhydrous7 · #2 02-03-2004, 01:35 PM

Has anyone noticed their glow plug light staying on longer (along with the relay) after they replaced their glow plugs? It sounds like #1 glow plug is tied to the relay and is used (sensed) to determine how long the relay is engergized.

I have a new set of glow plugs coming. I plan on replacing them one at a time and taking resistance and voltage readings as I go. I will post the outcome when I am finished.

psfred · #3 02-03-2004, 03:06 PM

Time function is controlled by a temp sensor (thermistor) in the relay, but I believe current draw is also invovled.

2 ohms GP resistance is way too much -- won't get hot enough at that resistance, to say nothing of only having 4 sec of current!

If they don't stay on properly (20-90 sec cold) with new plugs, it's relay time.

Check the strip fuse on the relay, too -- it may be cracked and opening up with heat.

Peter

#4 02-03-2004, 03:18 PM

Rob, I don't think there's any relationship between glowplug condition and the relay's time, but of course, I could be wrong. A couple of years ago, my '84 300D's glowplug relay wouldn't stay on, and I was able to fix it by replacing a bad electrolytic capacitor (100uF, 16v) on the relay's circuit board. An easy, cheap fix and it's been working well since. Don't know if this is the source of your relay's problem, but at least it's something to consider. Good luck.

Jerry

janko · #5 02-03-2004, 03:22 PM

my glow plug experience. last august with the temperature in the high 80s, my 300dt would not start. replaced the glow plugs and the bugger started. now on to january of this year. 8 degrees. car sat overnight. glow plug light on for about 10 seconds. hit the starter and immediate response. i be a believer in fresh glow plugs. oh, car has 290k miles. and a fresh battery.

anhydrous7 · #6 02-03-2004, 07:57 PM

I received the new glow plugs today. I installed 4 of the 5. The fifth one is going to take some more time due to location behind the oil filter and near the firewall. Ohm readings were between 0.1 and 0.3 ohms on the four new glow plugs. I am satisfied with that. I checked the glow plug light and found that the time it stayed on did not change from before. It still only illumitates for about 2 seconds. However, the time the glow plugs stay engergized has increased from about 2-3 seconds prior to glow plug replacement to now after the four out of five have been replaced, has increased to 15 seconds. This is with the engine still warm after the drive home. I think tomorrow morning after the car has had all night to cold soak, the relay will stay energized for a longer period of time. I will report back on my findings.

I am still puzzled as to why the glow plug light is only staying on for about 2 seconds.

Also, I thought that since my car is a 1982, it uses a thermocouple to sense the tempeture of the engine (or the surrounding air temp) to determing how long the relay will stay energized. While replacing the glow plugs, I noticed that there was a sensor on the side of the block between glow plug 3 and 4 that appears to be in the same wire bundle as the glow plug wires. Is this sensor used to deterimine the block fluid temperature, rather than the thermocouple that is called out in the manual (CD format)?

This is really getting interesting.

Thanks for the responses and help.

Rob

anhydrous7 · #7 02-04-2004, 07:45 AM

82 300SD was good and cold soaked this morning. I timed how long the relay was energized this morning. Total time was 10 seconds. The car fired up fine.

I am starting to think that the relay is suspect. I plan to go to a u-pick this weekend and find a used relay. By trying the boneyard relay, it should accomplish two things. First, I should be able to pick it up for around $5 and second, I won't have too much invested in it if I still have a problem with the relay. Is there any part number or identifying mark to look for to make sure I match up correctly? The only idenification I found was on the under side of the relay cover. Is it safe to assume that any 3.0 litre Mercedes Diesel of that year or one year newer will have the same relay? I guess what I am trying to say is, can I pull one from a 300D if I can't find a 300SD?

I also ohmed the one of the four glow plugs that I took out of the car yesterday and it read .6 ohms. I am going to hang on to the old glow plugs as spares. I am thinking that they are not altogether bad.

anhydrous7 · #8 02-04-2004, 09:14 AM

According to this website below, the glow plug relay has the same part number for 300SD 81-85 and the 300D 81-85. The glow plugs also show the same part number. I feel that I can find a realy from either a 300D from those years or an SD.http://www.adsitco.com/newdesign/default.asp?CID=85

DieselAddict · #9 02-04-2004, 12:44 PM

You're right, you can use a GP relay from a 300D as the engine is the same. I replaced the relay on my 240D after being unable to repair it or find a cheap used one. New ones are only $99 and they have afterglow, making any cold diesel run smoother, plus they come with 4 GP's as a bonus. I had the same problem as you. The relay would only energize for barely 10 seconds which isn't long enough.

anhydrous7 · #10 02-06-2004, 03:57 PM

I went today and found a used relay from an 85 300D. Installed it in the 82 300SD. The results are good.

The glow plug light stayed on for about 15 seconds and the relay stayed energized for about 35 seconds.

The old relay would only illuminate the glow plug light for about 2 seconds, and the relay would stay energized for about 10 seconds. I was able to pick up the relay for $5.00. I thought this was a pretty good price. I also picked up a cruise control amplifier for $5.00. I will put that on over the weekend.

I still have number 5 glow plug to install. This last glow plug may increase the lenght that the relay is energized if understand the circuit correctly.

One last question. It appears that there is a sensor on the side of the block in between #3 and #4 glow plugs and its electrical connector joins the glow plug wire bundle. Does this sensor feed info to the relay?

Rob

romansek · #11 02-06-2004, 04:35 PM

I checked Electrical Wiring Diagram for Preglow System and it
shows temperature sensor connected to preglow time relay.
Also the manual shows graf indicading relation of preglow time to coolent temperature.

Roman
------------
87 300SDL 145K
86 420SEL 225K
86 300SDL 210K
83 300D 220K
81 300D 140K

240Joe · #12 02-08-2004, 04:19 PM

Can you give us a few sample times versus temp off the graph?

For example, what is the delay at 0 f (-17c ), 32 f (0 c), and 70 f (21c).

thanks
Joe

romansek · #13 02-08-2004, 05:48 PM

The graf relation of preglow time to coolent temp. indicates:
-20C=15s
0C=5s
30C=3s

Roman

dmorrison · #14 02-08-2004, 11:10 PM

Rob
Service Manual, Service Manual, Service Manual, and if you had one I know you would be using it. Sooo!!!

http://skinnerbox.steaky.org/123_DISK2/program/123-cd-index.html

Scroll down to

15 - Electrical system engine (ignition system, preheating system, starter, alternator)

And enjoy the reading. It should be able to answer all your questions.

Dave

anhydrous7 · #15 02-09-2004, 01:23 PM

Dave, I have a service manual in CD format. Comparing the data that is in my CD version to the link you posted, the content is different somewhat. I don't know how to explain the difference. Your link shows some interesting charts and graphs that are not in my version.

I guess I should have been more specific with my question about the temperature sensor. I have the manual on CD and in it I found that for 1981 and newer, a they reference a NTC resistor temperature sensor that is used to determine how long to pre-glow. I am assuming that since I have a 82 300SD, that my car should have this. It appears there is a sensor that threads into the block in between number 3 and 4 glow plugs. Just wondering what this sensor is for. It appears that it is part of the wire bundle for the glow plugs. The manual (mine in CD format) makes no reference to this.

Hope this clears up my question.

Thanks,

Rob