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

vwbuge · #1 08-27-2002, 03:43 PM

ihad this posted before but...When I turn the key the glow plug light goes out almost immediately. I wait the usual 4-5 seconds and she starts fine. i really don't think my glow plugs are bad. Where is the glow plug relay, what does it look like and how much do they usually cost. Thanks.

engatwork · #2 08-27-2002, 07:33 PM

The GP relay is located on the drivers inside fender at the front of the engine compartment. Looks a little larger than a pact of cigs and has a black plastic cover over it. If the light is coming on and the car is starting fine then you do not, in my opinion, have a GP relay issue. If you have a volt meter you can measure voltage from the relay to the gp's or measure resistance through each gp. Do a search and you will find alot of information.

#3 08-27-2002, 07:56 PM

hey folks,

There does not seem to be any problem from what you are saying. The glow plug light in the dash is a suggestion of how long to wait before starting the car. For example, if it is warm outside the glow plug light on your dash will stay on for a second or so, if it is30 degrees maybe 8 seconds. The glow plugs will continue to glow between 15 and 40 seconds depending on the temp outside and will discontinue once the car is started.

How did I learn this?

When I got my old benz I put new glow plugs in and I thought that the glow plugs went out when the indicator light did so I would turn it over right away. She would start but it was real smokey for about 2 minutes.

Soon after that I got the cd manual and in there it goes into detail about the functional system of the glow plugs

Now, I glow plug for about a ten count everytime, even when it is warm outside. reason being it is easier to change glow plugs than a starter motor on the 300d and it smokes and idles alot better.

Personally speaking, if you dont know how old the glow plugs are change them anyway. I have seen other opinions on the casual replacement of glow plugs on this forum.

good luck

bob c

P.E.Haiges · #4 08-27-2002, 10:00 PM

Vwbuge,

Change your glow plugs (GP) because they are old even if your engine starts easily: RIDICULOUS. The best test of GP is how easy the engine starts. If it ain't broke, don't fix it! (and save yourself about $60).

I have 2 300SDs. The GP light stays on much longer on one than the other. There is a timer that controls the GP light and one is out of calibration (that's what is wrong with yours). So for the one with the short light on time, I just wait a little longer before engaging the starter. This allows the engine to start easily. I'm not about to change the GP relay for such a small imperfection.

P E H

sublettd · #5 08-28-2002, 12:35 AM

Speaking from experience here, if your Gp relay goes bad, you will have other problems. The problems I have encountered are a dead battery. When my GP relay went bad, twice now, it was staying on all the time and drawing a truckload of current from the battery, thus killing it. I do not know what caused my Gp relay to go bad twice, but it was so frustrating that I decided not to fix it again and am ready to get rid of the car. I just picked up an 84 300D, and am ready to dispose of the 80 300D.

Anyway, just my two cents on Gp relays.

Mike

P.E.Haiges · #6 08-28-2002, 01:04 AM

Sublettd,

That's like amputating your whole leg because of a broken toenail. Now if your crankshaft went bad twice, I could understand.

You could put in a manually operated relay and that would eliminate the problem completely. That's what the PO did to my Diesel pickup truck. So I have to hold a switch for 10 seconds when the engine is cold. When hot, it starts easily without need to use the glow plugs.

P E H

sublettd · #7 08-28-2002, 11:14 AM

I know it sounds extreme to get rid of the car just because of the GP relay, bu that is not the only reason. I also have an 85 300d in extremely good shape and now I have this 84 300d in even better shape. I don't need three cars. The 80 has a little bit of body cancer that will only get worse with time. The AC also died on it and I ahve to have AC since I live in Houston. The AC servo on it went about two years ago and it would cost way too much to fix. If you total up all of the repairs needed to make this a daily driver, I could pay for a good used one. There are a lot of good parts on the car though. I have put new axles on it, brake systems, shocks, and various other parts. I would like to part this one out or sell it whole to someone who needs some good parts including an engine. The engine has about 276,000 miles on it, but it is still strong as an ox. The seat springs are also still in good shape. Believe it or not, the car is 22 years old and the drivers seat spring is not sagging to the floor board.

To sum all of this up, I am not putting this one out to pasture just because of a bad glow plug relay. If anyone out there needs a good parts car, shoot me a message and we can talk.

Mike

P.E.Haiges · #8 08-29-2002, 12:28 AM

Mike,

OK now I understand why you are parting out your 300SD. I have an '80 300SD with 236,000+ miles and its like a new car. The '79 I have I bought for a parts car 4 years ago and it ran so good, I started driving it and now has 40,000+ since I bought it and still going strong. I use it for my winter car. Here in the NE, if you use a good car in the winter it won't be a good car for long because of the road salt.

The last car I bought ('87 300SDL)is from TEX and it doesn't have a spec of rust. How did your 300SD get rusty in TEX? Was it originally from the north?

P E H

sublettd · #9 08-29-2002, 01:47 AM

It was originally from Florida. That salty air can really do a number on a car if you don't take care of it.

Mike

P.E.Haiges · #10 08-29-2002, 02:25 AM

Mike,

I bought a 22 year old Plymouth Valiant from Tampa Fl and it was completely rust free. I remember taking off the bumper: I loosened the bolts with a ratchet and screwed them out with my fingers. Maybe your MB was driven in the surf. That will really kill a car.

P E H

vwbuge · #11 08-29-2002, 03:49 PM

I'm thinking.....it starts pretty easily but idles rough for a minute or so.....bad glow plug right?......Could that bad plug cause the light to go out quickly?

lrg · #12 08-30-2002, 01:29 AM

PEH
It makes a big difference if a car near salt water is stored outside or inside. My parents lived near the water on the West Coast of Florida and had two cars. One was kept in a closed garage and the other out in an open carport. Quess which one rusted. The good news is that most Benz owners, at least those who bought them as "young" cars, could also afford to properly garage them. Salt air can be hazardous to your cars health.

P.E.Haiges · #13 08-30-2002, 05:03 AM

Vwbuge,

I don't think a bad GP will affect the timing of the GP indicator. There is a built in detection system that cause the GP indicator to blink after starting if there is a open (burned out) GP.

Do a resistance check on the GP. You need a sensitive Ohmeter, better if it is a digital one. Readings of cold GP should be about 0.5 Ohm but it more important that they are all about the same. If one reading is much different than the others, I would check the wire and connection to that GP first and then replace that one GP if the wire and connection are OK.

The easiest way to do this is to pull the cable connector that goes to the GP out of the GP relay and plug your Ohmeter test lead into the holes in the connector (one at a time) and the other lead to a ground on the engine. This will also detect if there is any high resistance between the relay and the GP.

P E H