Curious glow plug relay behavior? - PeachParts Mercedes-Benz Forum

OldFred · #1 02-28-2011, 02:56 PM

I have a well kept '84MB300SD with 90K miles. Starts and runs well as long as the temps are above freezing. I think I have some weak glow plugs and plan to replace them. My questions is about my glow plug relay: When I start the car the glow plug light comes on as it should and the car starts. What is curious is that the glow plug light comes on intermittently but often once the car is running. It has done this since I purchased the car Sept'10 and the only change is that the frequency of the on/off cycle has now increased to about 3Hz...the noise coming from the relay on the front fender is annoying. Now get this: turning the headlights on turns the glow plug light off & stops the noise reliably!

I read somewhere that the glow plug light can come on while the car is running, but the problem can be with the plugs & not the relay. Does this make any sense? Shouldn't off be off? My big question is should I replace the relay as well as the plugs? I have thought about just replacing the plugs to see if the above behavior changes, but do not want to damage my new plugs with a defective relay. Any advice will be appreciated.

rhodes2010 · #2 02-28-2011, 04:09 PM

Remove and test all the plugs. My test always involves a source for 12 volts
and wires to watch them glow ( a visual check).

Many people here like the book method of using an Ohm meter.

Doubt you can damage the plugs with a bad relay. Not likely at all.

Also doubt the relay is bad, because every time I thought it was the relay it was not. There are built with tremendous durability.
If you take the relay apart you will see how well built it is.

Even the weird headlight being on thing makes me think bad glow plug and a ground/shorting issue inside a plug.

tangofox007 · #3 02-28-2011, 05:36 PM

Quote:

Originally Posted by OldFred

I read somewhere that the glow plug light can come on while the car is running, but the problem can be with the plugs & not the relay. Does this make any sense? Shouldn't off be off?

Once the light goes off, only cycling the key switch should cause it to illuminate. You might have a poor electrical connection in the switch circuit, causing the timer to "think" the key has been cycled off and back on. Or the solid state device that controls the light could have failed.

layback40 · #4 02-28-2011, 07:52 PM

Hi OldFred,
Welcome to the forum. We all hope you get all the help you need for your car. When you get a chance, please put your details in your signature so you dont need to continually type them in your posts.
It would be worth your while to check all earth connections that you can find. They often cause "strange" behavior like you see. Its not good to have your GP's cycling on while the car is running, it shortens their life.
Good luck with your investigations. Please let us know what you find.

OldFred · #5 02-28-2011, 09:16 PM

I haven't actually tested anything yet, but I ordered a Bosch glow plug relay kit from PeachParts.com. The price was so good that I am worried about what will be in the package.

layback40: I'm learning about how to do the profile, signature, and avatar things. I'm looking for a picture of my MB.

layback40 · #6 02-28-2011, 11:15 PM

Quote:

Originally Posted by OldFred

I haven't actually tested anything yet, but I ordered a Bosch glow plug relay kit from PeachParts.com. The price was so good that I am worried about what will be in the package.

layback40: I'm learning about how to do the profile, signature, and avatar things. I'm looking for a picture of my MB.

Your doing just fine Fred, many on here have been around for years & still havent done that stuff !!

I have no idea what will be in that kit. Good luck!!

There are lots of threads about GP's on here. Probably one of the top 10 topics!! You may wish to try a search, on them. Make sure you have plenty of time on your hands.
There is a member ; whunter , its worth looking at his posts. He has more knowledge on these cars than most of us combined!! He may tune in & give you a list of links.

Good Luck !!

OldFred · #7 02-28-2011, 11:28 PM

Thanks for the suggestions. I read a lot of the glow plug entries on this and other forums and gathered a good feel for the usual issues, tests, and remedies. I just felt that my car's behavior had not been talked about yet, but I've got more reading to do.

I put a picture in my profile, but I guess I need to shrink it more to put it in my signature. Too late to do it tonight...time to turn in.

layback40 · #8 02-28-2011, 11:31 PM

glowplug light comes on randomly

I think this is about your problem.

OldFred · #9 03-03-2011, 08:25 AM

layback40: Thanks for finding that thread for me. I read it last night and the answers seem almost evenly split between "replace the relay" and "there is some other problem". I have ordered a new relay & plugs and will measure plug resistance this weekend while waiting for delivery. If the plug resistance values are reasonable, I plan to replace the relay first once the relay&plugs kit arrives. If the plug resistance values are all OK and the relay replacement stops the plugs from coming on while the car is running, I think it will put the plugs on the shelf and celebrate. If the plugs still come on while the car is running, I will take the plugs out, see if they light up like a new one and, pending those results, maybe replace all plugs. If the plugs still come on (with new relay&plugs) while the engine is running then I will try to track down some of the other ideas (like bad ground connections and/or ignition switch) that I have collected.

BTW: I have noticed another behavior...it seems that the plugs are LESS likely to come on when I'm steadily cruising over 30MPH. My best guess about this is that I'm generating more juice from the alternator at speed and maybe sending a "stronger signal" to the relay letting it know that the engine is running. (BTW, there are no other signs, like dimmer headlights at idle, that the battery might be getting a bit weak. In fact, I'm amazed at how long and strong it will crank the engine when it is not starting at sub-freezing temperatures.) If the "stronger signal theory" makes sense, unfortunately this interpretation seems the opposite of the fact that the turning the headlights on will turn the plugs off (because the headlights should DRAIN electricity from the system). I'm still confused and hoping that replacing the relay will just fix it.

Finally, mhunter has not chimed in yet. I will wait until I do the stuff above and if that fails, I'll send him a message and beg him for his opinion.

charmalu · #10 03-03-2011, 11:07 AM

There is his GP list Whunter put together up under the resources tab in DIY Links.

glow plug conversion kit thread links

(quote) the battery might be getting a bit weak. In fact, I'm amazed at how long and strong it will crank the engine when it is not starting at sub-freezing temperatures.)

This sounds like the Glow Plugs are not all working to supply sufficient heat.

I had the GP light on our 80 240D flash every once in a while as we were driveing along. It would come on 3 to 4 times them randomly a dozen times or more. may not see it for 1/2 hour, then flash again.

My indy told me over a yr ago, sounded like to him, it had a bad GP. so last month I finally got around to replacing them with Bosch 80006 plugs. at first no flashing, then it did come back a time or two. but haven`t seen it now the past few weeks.

With the old plugs, it always started before the GP light would go out or hear the Relay clink off.

Charlie

tangofox007 · #11 03-03-2011, 05:40 PM

Quote:

Originally Posted by charmalu

I had the GP light on our 80 240D flash every once in a while as we were driveing along. It would come on 3 to 4 times them randomly a dozen times or more. may not see it for 1/2 hour, then flash again.

My indy told me over a yr ago, sounded like to him, it had a bad GP.

Like the timer device decides to check the glow plugs in the middle of a drive? I don't think so.

charmalu · #12 03-03-2011, 08:04 PM

I don`t know if the timer device is checking anything or not.

Thought I had it fixed. today coming back home from errands, it started flashing again. maybe 20 times. hasn`t done it for a while.

Don`t have any problem with it starting, but hope it isn`t something in the GP relay that is on it`s way out. the relay for the 80 240 is $450 on Fastlane. this is the first yr for this style with the legs.

Looks like this was use for 80 & 81 then 82 & 83 they went to this style. It is $227. think I saw it for $157 on Allpartsexpress

Charlie

micalk · #13 03-03-2011, 11:11 PM

The GPs are on the side of the relay that gets energized (relay contacts). The light is controlled by the electronics in the relay assembly. As tangofox007 said, cycling the key cycles the GP relay assembly. After starting, the GP relay continues to receive a 12V signal from the ignition to tell it the the car is running. If that signal goes away and comes back intermittently, the GP thinks the key has been turned off and back on again. You can check this but you need to string a sense wire to pin one of the four prong small plug to the relay and check for voltages. If you do this, make darn sure there are no shorts. While the light is acting up you should see that voltage come and go.

OldFred · #14 03-06-2011, 11:27 PM

I measured the resistance of my glow plugs and got 1) 0.85 2) 4.08 3) 0.80 4) 100k 5) 0.80. It appears that I have a bad plug or two. I will replace them once my reamer comes in. I also intend to watch each glow plug glow to see where they glow, as was suggested. Should I clean the good plugs (with the possible exception of #5 depending upon how difficult it is to get out)? Any wagers on whether these two are the cause of the glow plugs coming back on after the engine starts? Also, what is the best way to clean any carbon off the "good" glow plugs before re-installation OR should I just replace all five plugs?

I also measured the time that the glow plug relay stays closed when I turn the key to make them glow (but not start the engine)...both when the engine is "cold" 15C and when warmed up 80C. I got measures of 24 seconds when cold and 22 seconds when completely warmed up (after running around for a bit). Does this mean that my temperature sensor is not communicating well with my glow plug relay? What times should I have expected from this test? The temp gauge on the dash works well.

BTW, the glow plug relay in my car is a Hella (Made in Germany). I ordered a Bosch (Made in ???) from FASTLANE, but received a HUCO (Made in Germany). My local MB guy told me to buy Bosch. Should I send the HUCO back and try to get a Bosch OR just put the HUCO in if the new and cleaned glow plugs do not stop the GPs glowing once the engine has started?

As always, if you know the answers to any of the above questions, I'd appreciate hearing about it.

vwnate1 · #15 03-06-2011, 11:42 PM

I'm a poor boy and I still replace all of them any time I have them out ~ they have a finite lifespan so if one goes , the rest are right behind it .

FWIW , I have two sets of good used glow plugs and NO ONE wanted them so I wound up tossing them out .