glow plug light timer circuit? - PeachParts Mercedes-Benz Forum

snorider · #1 08-11-2014, 01:42 PM

I did a little experiment with glow plug timing on my 79' 300td this morning. Morning starts have been a bit rough and I was getting ready to start checking for air leaks. The glow light is on about 10sec and if I start the engine it runs rough. If I glow for 20-25sec it fires right up and runs fine. I assume the timer circuit is in the glow plug relay? I guess I can just wait a bit longer to start it up but...Is there a good way to clean /adjust the relay or will a new one help.
Thanks

mach4 · #2 08-11-2014, 02:32 PM

Quote:

Originally Posted by snorider

Is there a good way to clean /adjust the relay or will a new one help.

Many people, myself included, have opted to go with a completely manual system for glowing. The strategy is to use a momentary switch in the cabin connected to a Ford solenoid that actuates the glow plugs. By going this route you have complete control over your glow cycles using only the amount of glow and after-glow needed for each situation. Since you're not using unneeded glowing your plugs will last dramatically longer...and less to fail.

I gutted a glow plug relay so I could use the stock fuse and wiring harness. The installation looks completely stock and works really, really well.

Not for everyone, but I prefer the setup.

JamesDean · #3 08-11-2014, 03:15 PM

I think that behavior is normal though? Light out after 10 seconds but glow plugs remain on for some time after that...

001 545 35 32 - Glow Plug Relay - Imgur

cooljjay · #4 08-11-2014, 05:20 PM

So you have a 79? Can you tell us the year it was built?

early 79's and pre 79's use a glow plug relay under the dash and a temp sensor in the head. 79 uses a rare three legged relay under the hood...

If you have loop plugs still, upgrading to bosch 80035 pencil plugs needs to be done.

JamesDean · #5 08-11-2014, 06:48 PM

I believe the rare three legged guy is this:

http://imgur.com/a/HkfLj#0

I have a customer with one and the PO wired the battery in backwards...blew out the damn thing.

I convinced him to convert over to the "newer" unit found on 81+ cars.

charmalu · #6 08-11-2014, 07:13 PM

Quote:

Originally Posted by cooljjay

So you have a 79? Can you tell us the year it was built?

early 79's and pre 79's use a glow plug relay under the dash and a temp sensor in the head. 79 uses a rare three legged relay under the hood...

If you have loop plugs still, upgrading to bosch 80035 pencil plugs needs to be done.

80 240D and 300SD use the 3 legged Relay also, but from what I have seen, it is only the 80 model year that used the 3 leg relay. Not sure what the 80 300D uses. I do know if you want to buy a 3 leg relay, dig deep for the cash.

Charlie

cooljjay · #7 08-11-2014, 08:04 PM

The three legged relay was only used on 1979 cars, if you remove the cluster of the 80's car and look at the build date. It will have a build date of 79, only cars with the build date of 79 has them. My car is registered as a 79 but the build date of my car is sept of 78 thus it didn't see the three legged relay.

Zacharias · #8 08-11-2014, 08:43 PM

Which glow system your car has will probably influence how you proceed, should you feel you need to do anything.

Very early w123s had a series glow system that is readily identified by the glowplugs being larger with prominent insulators on each plug, and heavy wire wavy thingies running between the plugs. The preglow relay is under the steering column, inside the car. I highly doubt you have this system.

Next generation puts a relatively preglow relay under the hood behind the driver's headlamp on the inner fender (with three attachment points, hence the reference to three legs), but with parallel wiring to the plugs and the later, less expensive "pencil" style glowplugs. My 1980 300td has this setup.

Later cars use a smaller, box-like relay on the inner fender (two attachment points).

The inside and 3-point mount relays are very expensive to buy new and not cheap used. All of these relays are getting old now, speaking of used items, and tend to be problematic. I have gone through three in a year, for example.

If you do have the very early system, I would look into converting it to a parallel setup with a manual controller before spending any money on the OE setup, which is painfully obsolete (IMHO).

cooljjay · #9 08-11-2014, 09:20 PM

As long as the relay is good, why convert to a manual setup, simply install the pencil
updates as I mentioned. If you have a junkyard close by it is also very simple to install the later glow system, with a cheaper relay. Both my cars, have the original relays and are both preforming as intended.....even with the upgraded plugs...

I still can't understand on this forum, why automatically when someone says they have a glow issue.....to run right out and install a manual system...

Zacharias · #10 08-11-2014, 09:37 PM

Quote:

Originally Posted by cooljjay

I still can't understand on this forum, why automatically when someone says they have a glow issue.....to run right out and install a manual system...

Because after 18 years of driving these cars in the extreme weather of my region, I have had to change a relay twice too often outdoors at -29 Deg C in February?

Having gone through three this year alone, as mentioned in my post, one of which locked on and burned out a set of brand new glowplugs and finished off my old alternator, the small amount of automation that the OE relay brings, becomes a very relative benefit.

snorider · #11 08-12-2014, 11:21 AM

My car was built in 5/79. Lots of good information above thanks too all who posted it up. I am seriously considering one of the manual options. Being a denizen of north of 45 degrees I need a reliable glow system to get these things started. Block heaters, hood blankets and glow plugs. Some day I will have a garage again and it will all get easier.

JamesDean · #12 08-12-2014, 11:32 AM

Quote:

Originally Posted by Zacharias

Because after 18 years of driving these cars in the extreme weather of my region, I have had to change a relay twice too often outdoors at -29 Deg C in February?

Having gone through three this year alone, as mentioned in my post, one of which locked on and burned out a set of brand new glowplugs and finished off my old alternator, the small amount of automation that the OE relay brings, becomes a very relative benefit.

Have you considered converting to the 82+ style glow plug relay?

mach4 · #13 08-12-2014, 11:36 AM

Quote:

Originally Posted by cooljjay

I still can't understand on this forum, why automatically when someone says they have a glow issue.....to run right out and install a manual system...

simplicity, reliability, longevity, maintainability, cost, wear, control, audible feedback...

just for starters.

...not for everyone for sure, but there are some compelling reasons for the right mindset - I'd never go back.

JamesDean · #14 08-12-2014, 11:50 AM

Quote:

Originally Posted by mach4

simplicity, reliability, longevity, maintainability, cost, wear, control, audible feedback...

just for starters.

...not for everyone for sure, but there are some compelling reasons for the right mindset - I'd never go back.

Maybe this issue is more prevalent for owners of the older style glow plugs. In the 30 years we've had our diesels the 80+ style relay has never given much problems. Bad glow plugs once, and some issue with dirty contacts.

snorider · #15 08-12-2014, 12:04 PM

Quote:

Originally Posted by JamesDean

Have you considered converting to the 82+ style glow plug relay?

Just did a bit more searching around and I found my car has both relays! The car has a 617.952 engine conversion, it is using pencil style plugs which are wired to a post 82 relay which is in turn attached(lag bolts screwed into the inner fender

) behind the washer bottle on the drivers fender. The original gp relay is still attached up under the kick plate. I might just wire an audible timer to a switchable lead for my wife something like this perhapshttps://www.youtube.com/watch?v=YHv5jgXz9I8

. Glow light time when warmed up is 1-2sec , cold it is less than 10sec, I now wait an extra 10 count but it would be nice to have a reminder as rough starts have got to be bad for the engine.