W115 300D Glow Plug Relay Issue - PeachParts Mercedes-Benz Forum

cleeves · #1 11-21-2018, 06:21 PM

Hi all,

My glow plug relay ostensibly died last week. The glow light would come on when I turned the crank, but not when turning to position 2. Glow plugs are new. 0V on glow plug 5 when in position 2 and everything under the engine bay seems to check out.

On my old relay board, I found that when turning the key to position 2, one of the relays inside (small one, bottom left in pic) engages. However, the larger relay inside the board sparks initially (single spark on key turn) but does not engage. I confirmed that the ignition switch was sending a signal to the relay board by checking that pin 4 shows 12V when in position 2, and 0 otherwise. Jumping pins 5&6 turns on glowplugs (and indicator) as does manually pushing down the big relay with a screwdriver. When cranking the engine (pos 3) the relay engages.

So, thinking the relay was bad, I bought a NOS relay from eBay for $230. Same problem! Arg. It's hard for me to believe that the NOS relay is also bad. Any suggestions appreciated.

I suppose my next idea is to put an Ammeter on the ignition -> relay input pin and see if the current changes between pos 2 and cranking. Perhaps the ignition switch is bad after all?

Lastly, I am not interested in the manual setup. I would like to get the car back to it's original operating condition.

Thanks!
Henry C.

cleeves · #2 11-21-2018, 06:33 PM

Well,

I just tried to start the car after letting it glow manually for a full 2 minutes. No charm. It stumbled a bit but didn't go. The wires for pins 5 and 6 were hot to the touch (insulation was). Is that normal? This issue is quite frustrating to me!

Thanks,
Henry C.

cleeves · #3 11-21-2018, 09:49 PM

Quote:

Originally Posted by cleeves

Well,

I just tried to start the car after letting it glow manually for a full 2 minutes. No charm. It stumbled a bit but didn't go. The wires for pins 5 and 6 were hot to the touch (insulation was). Is that normal? This issue is quite frustrating to me!

Thanks,
Henry C.

After going through other threads again, I have an idea. In this thread it appears that the GP relay will trip rather than a fuse blow if it's drawing too much current. Perhaps there is a short in the system somewhere that I missed and the relay isn't bad, it's just tripping. Well, I hope I didn't fry anything. I will pull the glow plugs this weekend, check things under the hood, and report back. Any ideas welcomed.

-Henry C.

Diseasel300 · #4 11-21-2018, 10:45 PM

If the relay isn't engaging when it's supposed to, I'd be looking at the key switch, not the relay. You've already swapped it out with another relay and it seems to be engaging at the wrong times. It all strongly points to the switch being your suspect, not the relay.

cleeves · #5 11-21-2018, 11:24 PM

Quote:

Originally Posted by Diseasel300

If the relay isn't engaging when it's supposed to, I'd be looking at the key switch, not the relay. You've already swapped it out with another relay and it seems to be engaging at the wrong times. It all strongly points to the switch being your suspect, not the relay.

Hi,

Thanks for the input. I also suspected this, but I'm not convinced this is the problem because I confirmed that the ignition switch was sending a signal to the relay board by checking that pin 4 shows 12V when in position 2 (and 3, when cranking), and 0 otherwise. So it seems the key switch is engaging correctly. Here is the diagram:

I have no idea what pin 3 does, by the way. Also, as I stated before, 'half' of the relay engages, and the wires seem to get unusually hot. So this seems more like a short somewhere to me. But I could be wrong ...

Warmth,
Henry C.

cleeves · #6 11-21-2018, 11:28 PM

Let me check on pin 3 and dig more into that tomorrow ... maybe that will reveal a problem in the key switch !

Will be making blueberry pie with a hint of grease in my fingernails tomorrow. Yummy.

Henry C.

cleeves · #7 11-22-2018, 12:39 PM

Well, the problem was the glow plugs! I had them replaced less than a year ago, but glow plug #3 was completely shorted out - showed continuity between the outer casing and the leads. No wonder the relay was tripping, performing its function correctly. Either my 2 full minutes of manual engagement or a slow death seems to have overheated one of the resistor wires (plugs 4 -> 5) and it oxidized it heavily. The excess current also burned out glow plug #5, which is still intact but colored white and showing 0.65 ohms (the good ones show 0.22 ohms). I put the shorted plug in hole #1 and the nearly-burned out plug in #2 and the car fired right up. I suppose 3.5 working glow plugs is sufficient.

So much for Bosch Made in India glow plugs. Will order the Germany ones now.

Also, after tinking under the dash, I seemed to have messed up the gauges. The fuel gauge, water temp gauge, and brake light are now in-op. The oil pressure and battery lights still perform correctly. I see two possibilities on this issue:
1. I pulled out a wire or shorted something while handling the relay.
2. The high current of the shorted glow plugs possibly melted through and shorted out the gauge wires when I manually connected the relay yesterday for 2 minutes.

This issue is reason to make a strong case to not convert to a simple manual switch for the relay. Or if you do do it manually, make sure to use a fuse. I don't know if Kent Bergmas's conversion kit has a fuse on it, but the original system performs just fine if the parts or good, and it's not overly complicated. Any manual non-fused conversion kit is simply dangerous. If plug #5 had shorted, rather than plug #3, it would have been a direct route from the relay to ground through the wiring harness, and this wire would have certainly melted apart and taken other things with it. The W115 was engineered well; this case gives me further reason to question any re-engineering of original functions.

If anyone has tips on getting the gauges working again, please chime in!

Take care,
Henry C.

cleeves · #8 11-22-2018, 12:57 PM

I just got a set of 12 Made in France Bosch NOS glow plugs from eBay. I paid $160 including shipping. I will sell half the set (6 of them) to anyone here for $80 + $10 shipping. I can do paypal/venmo, send me a message if you are interested and want half of them.

Henry C.

cleeves · #9 11-23-2018, 05:06 PM

Quote:

Originally Posted by cleeves

Well, the problem was the glow plugs! I had them replaced less than a year ago, but glow plug #3 was completely shorted out - showed continuity between the outer casing and the leads. No wonder the relay was tripping, performing its function correctly. Either my 2 full minutes of manual engagement or a slow death seems to have overheated one of the resistor wires (plugs 4 -> 5) and it oxidized it heavily. The excess current also burned out glow plug #5, which is still intact but colored white and showing 0.65 ohms (the good ones show 0.22 ohms). I put the shorted plug in hole #1 and the nearly-burned out plug in #2 and the car fired right up. I suppose 3.5 working glow plugs is sufficient.

So much for Bosch Made in India glow plugs. Will order the Germany ones now.

Also, after tinking under the dash, I seemed to have messed up the gauges. The fuel gauge, water temp gauge, and brake light are now in-op. The oil pressure and battery lights still perform correctly. I see two possibilities on this issue:
1. I pulled out a wire or shorted something while handling the relay.
2. The high current of the shorted glow plugs possibly melted through and shorted out the gauge wires when I manually connected the relay yesterday for 2 minutes.

This issue is reason to make a strong case to not convert to a simple manual switch for the relay. Or if you do do it manually, make sure to use a fuse. I don't know if Kent Bergmas's conversion kit has a fuse on it, but the original system performs just fine if the parts or good, and it's not overly complicated. Any manual non-fused conversion kit is simply dangerous. If plug #5 had shorted, rather than plug #3, it would have been a direct route from the relay to ground through the wiring harness, and this wire would have certainly melted apart and taken other things with it. The W115 was engineered well; this case gives me further reason to question any re-engineering of original functions.

If anyone has tips on getting the gauges working again, please chime in!

Take care,
Henry C.

Got the gauges working. Fuse #5 broke at a coincidental time. That's right - it didn't really melt, part of the metal was bent/weakened on one of the ends.

All good now !

Henry C.

maclaveau · #10 11-23-2018, 07:06 PM

Glad to hear you fixed it!!! You should keep those plugs because they are getting very hard to find.

cleeves · #11 11-23-2018, 08:22 PM

Quote:

Originally Posted by maclaveau

Glad to hear you fixed it!!! You should keep those plugs because they are getting very hard to find.

Thanks! I may just do that, I was reconsidering my post as I looked at it again and saw your comment.

Take care,
Henry C.

cleeves · #12 11-23-2018, 08:30 PM

Also, it's interesting you have a diesel way up north. I suppose 3.5 glow plugs wouldn't suffice on a cold day there! Any pictures of your car?

-H

maclaveau · #13 11-24-2018, 10:35 AM

I put her away the first of October and take her out of storage the first of May, because they still use salt up here on the roads. I thought I had some pictures on this computer but I cannot find them.

cleeves · #14 11-29-2018, 03:28 PM

Replaced with Bosch French glow plugs. The light has never gone out so fast before! Even when the last knock off set was new it took a good minute and a half. Now? 20-30 seconds. And it was 40 degrees this morning.

Seems unnecessary to change to a pencil/ parallel style setup when these work so fast!

Henry C.

maclaveau · #15 11-29-2018, 03:30 PM

Found some pictures: