Help diagnosing glow plugs - Page 2 - PeachParts Mercedes-Benz Forum

babymog · #16 10-19-2010, 09:28 AM

Happy that it starts normally when you wait.

As others have mentioned, the light is a reminder, and an indicator that the glow-plug system is functioning normally.

If the light doesn't come on, it is indicating a fault, probably one or more bad plugs. Ditto with coming on after the engine is running: a fault. Your symptom is different:

If the light comes on, but goes out fairly quickly, it is not a fault in the glow-plug system, but in the light timer which is a separate function of the relay. I have had two of my relays fail in this way, an annoyance but I usually use my seat-belt routine as the timer anyway, turn the key on first and once buckled I start the car, this is usually adequate for all but the really cold mornings as the light is out by then. An extra-long glow period will shorten the life of the glow-plugs, but nothing else, and the smoother the start the better for the rings and cylinders as well as other parts of the engine so err on the long glow if given a choice.

The glow-light circuit on the '86/'87 OM603 is not dependent on the coolant temp (unlike the afterglow system on the other 60x engines), only a temp-sensor in the relay box itself. I'd imagine that this could be repaired, I've never opened a relay box to figure out what part causes this failure, but there are some of us who have the time and energy for these repairs, ... and might be able to give you the information you need to do so. My repair has been to replace the relay, you can also simply glow based on your feel and experience with the car and the outside temps and live with the relay as-is, no harm in that IMO.

turbo97se · #17 10-20-2010, 11:43 PM

I actually did open up the relay. There is an LM3202 OP AMP in there with a bunch of circuitry and a decent size coil relay ... some old school stuff! There is also a temp sensor ... I was in a hurry so didn't really play a lot with it. I thought about converting to a microcontroller setup. It wouldn't be hard to do really .. just the time to do it! It hasn't gotten really cold yet here, that will be the true test but it certainly is an improvement so far. Cheers!

babymog · #18 10-21-2010, 10:40 AM

Best is to upgrade to the '90-'91 SDL relay and the 3-pin coolant temp sensor, gives you glow-time based on coolant temp and "afterglow" where the plugs stay on after starting to help cold-idle quality and emissioins/smoke.

turbo97se · #19 10-21-2010, 03:49 PM

Thanks for the advice! I will do a search on how to do that!

Quote:

Originally Posted by babymog

Best is to upgrade to the '90-'91 SDL relay and the 3-pin coolant temp sensor, gives you glow-time based on coolant temp and "afterglow" where the plugs stay on after starting to help cold-idle quality and emissioins/smoke.

Beastie · #20 10-21-2010, 05:11 PM

Babymog, Will the 90-91 SDL glow plug relay work on my 85 300SDL ?

babymog · #21 10-21-2010, 07:05 PM

The '85 SD is a 5-cylinder, and so are the '90-'93 300Ds. They are set up for 5 plugs in parallel, not sure whether the SD is a series arrangement nor what other wiring differences there might be, but it sounds like a great idea!

vstech · #22 10-21-2010, 07:18 PM

Quote:

Originally Posted by Beastie

Babymog, Will the 90-91 SDL glow plug relay work on my 85 300SDL ?

first off, you have an 85 SDL?

second off, yes, it'll work fine. all the relays have 6 connectors, the harness will have a blank spot in it for the 6th connector. go for it!

tangofox007 · #23 10-21-2010, 07:26 PM

Quote:

Originally Posted by vstech

all the relays have 6 connectors

????????????

vstech · #24 10-21-2010, 07:33 PM

go look at your 617 or 616 pencil glow relays...
6 connectors.

tangofox007 · #25 10-21-2010, 09:22 PM

Quote:

Originally Posted by vstech

go look at your 617 or 616 pencil glow relays...
6 connectors.

Somebody needs to look at a first grade arithmetic book. Review the chapter on counting.

digieditny · #26 10-21-2010, 09:34 PM

I did a real power test on two of my glow plugs (used a jump box. Gnd to body.Pwr to tip). The first one glowed marvelously but the other one... it smoked for about 3 seconds and then boom! All these cinders shot out of it. Scared the poop out of my cats. Why did this happen?!

digi

micalk · #27 10-21-2010, 11:02 PM

Quote:

Originally Posted by tangofox007

Somebody needs to look at a first grade arithmetic book. Review the chapter on counting.

now, now, boys. don't get all huffy over the semantics.

6 pins/sockets in the connector.

tangofox007 · #28 10-22-2010, 12:06 AM

Quote:

Originally Posted by micalk

now, now, boys. don't get all huffy over the semantics.

6 pins/sockets in the connector.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1991-Mercedes--Benz-300d-Body--Electrical&yearid=1991%40%401991&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6179%3AED%7C10000095%3AMBC%7C1501%40%40300D&catid=241959%40%40Body+Electrical&subcatid=242005@@Preglow+Time+Relay&mode=PD

layback40 · #29 10-22-2010, 01:33 AM

digi, what you saw was a GP failure. Just as well it happened with it out of the motor.

While the children are arguing over counting techniques you may wish to get on with replacing your GP's with good new bosch ones.

Beastie · #30 10-22-2010, 08:58 AM

VSTech, Sorry too little sleep this week. 85 300SD not SDL.