no glow plug light... - PeachParts Mercedes-Benz Forum

1980sd · #1 08-25-2011, 02:22 PM

I have 12 v at all 5 GPs for about 45 seconds after switching to "run". The relay makes a light "thunk". I used a test light.

I was having trouble starting but I chased it to a loose ground at the GP relay. It starts fine now.

It seems that the timer part of the relay is working but I get NO voltage to the dash light. I thought it was supposed to blink if the GPs were dead? I know the glow plugs are doing something because it fired up from dead cold right after I restored the ground. Before that it was a bugger to start...

It looks like part of the relay goes through the park switch.

Ether posted this pic:

I suspect the purple wire #5 is the problem. Shouldn't I have voltage there with the ignition in the "run" position?

It's showing the wire going to the "A/C START RELAY". Surely the GPs aren't tied into the air conditioner...

kerry · #2 08-25-2011, 02:45 PM

Check plug #1.

tangofox007 · #3 08-25-2011, 02:57 PM

Quote:

Originally Posted by 1980sd

It looks like part of the relay goes through the park switch.

Note the word "input." Start voltage goes to the timer device, not the other way around.

Quote:

Originally Posted by 1980sd

I suspect the purple wire #5 is the problem. Shouldn't I have voltage there with the ignition in the "run" position?

No, only in the "start" position.

Phil · #4 08-25-2011, 03:08 PM

On the 83 you get no light when 2 or more plugs are not working. With two plugs out you probably could still start it. Does it run ruff when first started?

1980sd · #5 08-25-2011, 03:08 PM

Quote:

Originally Posted by tangofox007

Note the word "input." Start voltage goes to the timer device, not the other way around.

No, only in the "start" position.

Thanks, that makes sense. So when it starts the GPs stop glowing? Engine start being signaled by releasing the key? No "start" (Switched to run) position then it glows the whole cycle of 45 sec?

I'll check #1 but I have the pencil type GPs and voltage at all of them.

kerry · #6 08-25-2011, 03:21 PM

Check the resistance of plug #1 with an omhmeter and the plug harness disconnected from the relay. The system self-checks by comparing the draw on #1 with the other plugs. A failed #1 plug does odd things to the light.

tangofox007 · #7 08-25-2011, 03:24 PM

Quote:

Originally Posted by 1980sd

Thanks, that makes sense. So when it starts the GPs stop glowing? Engine start being signaled by releasing the key? No "start" (Switched to run) position then it glows the whole cycle of 45 sec?

Later models worked that way for sure. (Actually, whether the engines starts is irrelevant; what matters is that a start was attempted.) Yours might be a little different. Looks like the start signal prevents a "timeout" during start.

Quote:

Originally Posted by 1980sd

I'll check #1 but I have the pencil type GPs and voltage at all of them.

Check resistance.

1980sd · #8 08-25-2011, 03:47 PM

I had changed out what I thought was the temp gauge sending unit. I changed out the coolant temp sensor in the head...

With the old sending unit in the circuit the light comes on. It stays on for about 15 sec and goes off. The seatbelt light is the same as the GP light and they are grounded through the dash.

I'm going to let it cool off and try starting. It fired right up when I grounded the GP relay.

I found a wad of ground wires that weren't tight. One bundle between the radiator and the headlight. The ground for the GP relay had been relocated. I cleaned up the metal and put washers on the nuts...

I feel confidant that it will fire right up. I drive one every day and IT fires right up.

They are awesomemobiles!

I just wanted to add that the strip fuse gets really warm so there must be pretty decent current draw.

1980sd · #9 08-25-2011, 07:12 PM

An update...

After being parked for over a month it was VERY hard to start. It started but my guess is that the GPs weren't doing anything. After starting it was pretty rough running until it warmed up.

I changed the coolant sensor and refilled with water and it started right up. It runs a lot smoother when first started now.

Still no GP light...

I checked the GPs with my little VOM set at 200 ohms and got a low of 1.0 at #1 and highs of 1.3 at #4 and #5.

micalk · #10 08-26-2011, 03:52 PM

Resistance should be less than 1 ohm for all plugs. If you can, set ohmmeter to a lower range. 1.4 ohms is twice the normal resistance. May still be okay but might be a contributor to hard starting.
Also, having no voltage at the dash light is different than no voltage at the GP relay plug. You could have a disconnect in the circuit so check both places. You might try wrapping pin 5 (?) with a small wire or a single strand so that you can check the voyage out of the relay assembly with the plug connected. (Don't short it out!) Now you will only see that light for the first 5 to 10 seconds of glow. The glow plug relay will continue to glow the plugs until the relay times out or the key is turned to start.
The temp sensor is an input to the relay and determines in part how long the dash light stays on. This may be resolved with the new sensor.

1980sd · #11 08-27-2011, 03:37 PM

I let it sit overnight with the charger on the battery. Still no light with the ignition on

It did fire up immediately after about a 15 sec glow but it did run a little rough until it warmed up.

I put a test light on one of the GPs and the light went out after the ignition "start" was released.

I really hate to fool with the glow plugs when it starts so well...

It needs a manual glow with a salt shaker...

I just wanted to add that my other car glows for several seconds and blinks after the engine starts. It starts fine as well...