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  #1  
Old 03-18-2012, 11:19 PM
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1987 w124 300D
 
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Easy Glow Plug Relay mod: better dash indicator

I added a feature to the Glow Plug Relay so the dash indicator now serves another purpose: it shows when the glow plugs are active (relay is closed). Many people think the dash indicator is telling them this already - but that's not the case. It's only a suggested serving of glow, and can go off even when the GP's are powered.

Adding one resistor was all it took. The resistor puts the dash indicator at "half brightness" for showing when the GP's are powered (relay closed).

The dash indicator still shows the original MBenz intentions (suggested glow time is over, and GP fault/s) using its normal full brightness. Any normal full bright activity overrides the mod, so you'll see full bright.

So now, a normal start cycle shows a full bright indicator up to the "recommended dose" and then goes to half brightness and remains that way for up to 30s, or up to and throughout the cranking period, and then the lamp goes off as that's when the GPR releases.

If for some reason the relay sticks (at any time with key in the car or not) the indicator remains lit at half brightness.

Why didn't MBenz think of this?!? Sure beats running a second wire and adding another bulb to the dash.

Here's a picture of the GPR mod showing the resistor added to the solder side of the GPR circuit board. It's so simple it almost doesn't need any explanation. If you don't read resistor color stripes, it's a 47 Ohm resistor, 1/2 Watt is fine. One end of the resistor pokes through a hole MBenz left for an unpopulated part (remember to trim it flush on the other side of the board!), and other end of the resistor gets tacked onto the lamp lug.

This is for a stock 1987 300D GPR. The same idea can probably be applied to other model year GPR, by adding a similar bridging resistor between the relay output and the lamp output.
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Easy Glow Plug Relay mod: better dash indicator-img_3814.jpg  
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Last edited by scottmcphee; 03-18-2012 at 11:39 PM.
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  #2  
Old 03-18-2012, 11:31 PM
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1987 w124 300D
 
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In case you're curious about that unpopulated part "missing" from the board, if you were to place a jumper or say 10 Ohm resistor there instead, it would tell the module to ignore the condition of GP #1 and show the dash light anyway (still evaluating GP 2 through 6, and temperature sensitive timer).

You could add the missing part in combination with the suggested mod in this thread, they can be used in combination.

But I couldn't see the value in omitting GP 1 from all the fun of having it separately evaluated by the GPR.
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  #3  
Old 03-19-2012, 07:59 AM
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That's awesome, thanks for sharing. You're absolutely right, it should have been that way from the factory and I've often wondered why there wasn't a glow plugs active indicator. Thanks again!
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  #4  
Old 03-19-2012, 11:30 AM
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1987 w124 300D
 
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maybe 56 is better than 47

Fine tuning: I suggest using a 56 Ohm resistor instead of the 47 Ohm. This higher resistance yields a brightness level that is closer to the half bright bulb test phase of all the other dash indicators when you're starting the car.

And for full disclosure, safety as your own risk kind of thing: consider using a 3 Watt resistor instead of 1/2 Watt. And for extra bonus points make it a flame proof resistor instead of the common carbon type. Such a resistor would be resilient enough to handle a sustained double fault condition: GPR is sticking closed and your dash glow indicator bulb wire is shorted to ground (no light).

Such a resistor would be too fat to fit on the underside of the printed circuit board for getting the cover back on the device, so you'd have to mount it elsewhere on the component side.

Just to be clear, a common 1/2 Watt resistor is sufficient for the single fault condition of GPR sticking closed. It does not have to be flame proof. Such a resistor will shine the bulb for an indefinitely long period without risk of heating up beyond its design point. It's only if you fear the bulb wire short existing at the same time.

But in my opinion with this mod in, you'd notice a non-working bulb pretty fast because each start cycle you'll be staring at it. If you get "no dash light" your car probably won't start - and it means your GPR has just gone bad, its 80A fuse has blown, or its control circuit wiring has gone faulty. OR the bulb has simply blown and your car continues to start fine down into cold weather, you just need to change the bulb. Acting right away in such a case will tell you what's up, test the bulb first: pull the plug on the GRP and apply +12 to the lamp indicator pin on the harness socket. No dash bulb? Change it, and recheck. Good bulb? Solve GPR issue.
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  #5  
Old 04-18-2014, 09:29 PM
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Update. I went in and changed the 47 ohm resistor, which read 50 ohms... to a 68 ohm. This is a good match for the "dim" mode of the glow plug lamp.

Reason I went into the glow relay was checking the whole glow circuit out... was getting rough starts, and no glow indicator when key was in glow position.

The relay checked out fine, 80AMP fuse fine..

Glow plug #6 was found to be bad. Failed open (no continuity). When this particular plug goes bad, the relay senses it and shows the fault as "no glow lamp". It's designed to do this. But with this resistor mod, there's actually a "dim" glow lamp which says power is being applied to all plugs. There's just no "full bright" period. Only dim.

Found it is possible to change glow plug #6 on om603 with minimal yanking of things off the engine to get at it. I took off the engine lift mounted at the rear top of the engine, it straddles the air intake manifold on hole 6. With that out of the way, a flex head ratchet and deep 10mm takes off the glow plug wire, then a deep 12mm takes the plug out. Can just move the ratchet about 20 degrees, 1 or 2 clicks at a time, slow going...
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Last edited by scottmcphee; 04-18-2014 at 09:42 PM.
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  #6  
Old 04-18-2014, 09:57 PM
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Nifty mod, I'm surprised it hasn't gotten more attention. Does this also work if you have afterglow by cutting the purple wire?
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Old 04-18-2014, 10:18 PM
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Quote:
Originally Posted by tjts1 View Post
Nifty mod, I'm surprised it hasn't gotten more attention. Does this also work if you have afterglow by cutting the purple wire?
Yup!

The mod tells a simple story: if there's power going to the plugs, the dash indicator will be dimly lit. So if you've started the car and are after-glowing, it will be dimly lit. And it would be a really good thing to know if you've done an after glow mod... because you'd want to see the indicator go OFF at some point... If it stays dimly lit... you'll be burning out your glow plugs sooner than you'd hoped.
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Old 04-19-2014, 01:47 AM
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Clever! I did what Jeremy did - run a wire from any glow plug to the dash and use the blank space between the glow indicator which has a green tint. Jeremy used a labeler to make a decal labeled "AFTER GLOW." Green means the plugs are still glowing but it's ready to start, though it glows green even when the orange glow indicator is lit.

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  #9  
Old 04-19-2014, 04:12 AM
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I guess I have go be the devils advocate. I don't think you need the Glow Plug Light at all.

If you have trouble starting you know it should start OK you know need to check the Glow Plugs.

The Push Button Switch and Ford Type Starter Solenoid that subs for a Glow Plug Relay has worked without a hitch since around 1992 and no Glow Plug Light although the stock set-up had a GP Light.

If My Stock Mercedes Relay goes out I am replacing it with a Ford Starter Solenoid. But, this time I think I would wire it so the Solenoid activates in the Pre-glow position instead of a separate Switch.
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Old 04-19-2014, 10:12 AM
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Quote:
Originally Posted by tee51397 View Post
You're absolutely right, it should have been that way from the factory and I've often wondered why there wasn't a glow plugs active indicator.
They should have mounted that indicator right next to the altimeter and fuel pressure gauge!!!
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Old 04-19-2014, 10:33 AM
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Thanks for the tips on both the glow plug light and getting to #6. 87 300td is my kids DD, I want to make sure they wait the correct amount of time before starting. I did change #2&3, but the dash light goes out quickly, +/- 5 sec, so I tell them to wait another 15sec to get a good start. A "truer" GP light will be helpful. Also, removing the washer tank makes getting to the GPs a lot easier too w/0 R&Ring the intake manifold as another member suggested, just make sure its drained 1st.

Treetops
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  #12  
Old 04-19-2014, 11:15 AM
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Quote:
Originally Posted by treetops View Post
A "truer" GP light will be helpful.
If your engine won't start when the light goes out, it could be that other items need attention. IP timing, for example, has a significant influence on how quickly the engine starts.
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Old 04-19-2014, 11:43 AM
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Quote:
Originally Posted by tangofox007 View Post
If your engine won't start when the light goes out, it could be that other items need attention. IP timing, for example, has a significant influence on how quickly the engine starts.
In general Your IP Timing and Your Valve adjustment if you have an Engine that needs that should not suddenly go out on You. So you have to ask Yourself what is the most likely cause.
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Old 04-19-2014, 11:46 AM
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Hm, I wonder if this works for the later afterglow style relays? I'll have to open mine up.

-J
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Old 04-19-2014, 12:08 PM
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Quote:
Originally Posted by Diesel911 View Post
In general Your IP Timing and Your Valve adjustment if you have an Engine that needs that should not suddenly go out on You. So you have to ask Yourself what is the most likely cause.
And the "sudden onset" is mentioned where?
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