what did i do to my glow plug relay? - Page 2 - PeachParts Mercedes-Benz Forum

geoffraynak · #16 10-22-2012, 05:37 PM

okay ...
thanks for all the checking.

looks like i can add 'change glow plugs' to my order of operations...

taking them out and testing them (they are of unknown age and pedigree) is more expensive than the $53.75 the set will cost here.

i will also jumper the two wires to confirm the bulb is okay.

is there a good way to test if the relay is faulty?

jay_bob · #17 10-22-2012, 05:51 PM

With the 4 pin plugged in and the 6 pin unplugged, turn the key to pos II. Probe the 5 posts for voltage to ground. Relays can go bad but not as often as the plugs so let's try the plugs first. Also inspect your leads and make sure they are not cracked or frayed and the end terminals are securely crimped.

One more thing, be really delicate with the terminal nut on the new glow plugs. It is very easy to over torque that nut and break the heater inside the plug. Just finger tight and a gentle tweak with a box wrench or 1/4 drive ratchet is enough. There is a thread on this where it shows a glow plug sawed in half, you can see what is inside. I think whunter posted it.

geoffraynak · #18 10-22-2012, 06:00 PM

Quote:

Originally Posted by jay_bob

With the 4 pin plugged in and the 6 pin unplugged, turn the key to pos II. Probe the 5 posts for voltage to ground. Relays can go bad but not as often as the plugs so let's try the plugs first. Also inspect your leads and make sure they are not cracked or frayed and the end terminals are securely crimped.

thanks, jay_bob.

i basically did this test with the 6-pin connected, i think i did that test
'pos II' is the same as what i was calling 'glow' correct?
basically the farthest position that will not engage the starter.

when i did that test, i got 10.9V at the nut-connection of each glow plug before the relay shut off (maybe 30 seconds)

i will inspect the wiring harness when i have them off to replace the plugs

joy

Diesel911 · #19 10-22-2012, 10:44 PM

When the Plugs are out the Manual calls for the Glowplug Holes to be reamed or otherwise cleand out. There is some alternative metnods: DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum

PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

geoffraynak · #20 10-23-2012, 10:07 AM

Quote:

Originally Posted by jay_bob

You can do a quick check by jumping pin 1 to 3 on the body side of the control harness. With the key in II you should have a glow light.

So i went out in the pre-dawn here and did the test listed above. For all those who may search for something like this in the future. The glow plug light at your dash is connected to the relay harness via pins 1 and 3 bulb light. The schematic says red/black (?) and white/blue. On my 300SD these were on the same 'side' of the connector...above one another. You can identify them via the relay-side of the connector by pin 15 (12V input) and via a pin labeled 'LA'-something (my headlamp light was too harsh to see it all. But by deduction, i could see the other pin call outs (31 = cruise; 50=cranking) ... so that left 'LA...' which made sense as 'lamp'.

ANYWAY, in the dark this morning I was greeted with the warm dash light of a glowing glow plug light.
the lamp is fine ...
so i can cross that off my mystery list.

looks like some new plugs are in my future as another way to decode the mystery.
look at reamers ... that will be another thread all together ...

geoff

toomany MBZ · #21 10-23-2012, 11:09 AM

Glow plugs fail, it's a working one time, not the next affair.

As mentioned, pull each one and apply 12 volts, only the tip should glow.

Do this after taking ohm readings again.

charmalu · #22 10-23-2012, 11:53 AM

I picked up a GP Reamer off E-bay a few years ago for $34.

Charlie

geoffraynak · #23 10-23-2012, 02:41 PM

i can find a 7mm reamer on Mcmaster for ~30$
but that has to have some fab associated with it
Kent has a fabricated one for ~35$

i mostly pledged to myself to buy as much from Pelican as possible (because this form is so amazing) ... but the reamer here is 68$

am i missing something on why the reamers are such different prices?

qwerty · #24 10-23-2012, 03:23 PM

For the occasional DIY reaming, a drill bit (approx diameter of GP tip) works reasonably well.

Zacharias · #25 10-23-2012, 03:55 PM

Prices are all over the place. The one I saw on eBay from a reputable tool seller (who usually has decent prices) was $97.

Prices likely vary according to the same criteria as other tools: professional/mechanic quality vs. medium duty... made in Europa vs. made in Asia, etc.

geoffraynak · #26 10-24-2012, 12:47 AM

Quote:

Originally Posted by geoffraynak

ANYWAY, in the dark this morning I was greeted with the warm dash light of a glowing glow plug light.
the lamp is fine ...

okay ... it would be kinda weird to quote myself ... but because it is an election year, i'm making an exception.

ANYWAY...
i feel like i've got a ton of questions, and i am currently wearing my 'know just enough to be dangerous' hat. if anyone has answers to these questions, i'd love to know them:

what circuitry or electric logic is going on that is preventing the glow lamp from illuminating on my dash?
is there only one branch of the logic tree that explains my situation (80A fuse fine, conductivity to glow plugs fine, GP resistance fine, 10.9V to plugs during 'pre-glow', relay clicks off at about 20-30s, dash lamp does not light, starting is hard)?
did the glow plugs suspiciously tip over to the 'bad' side to make the lamp not light?
what does the circuit describe as bad at the glow plugs?
if my plugs are good, is the relay bad?
if i replace the plugs, and the light is still not on, is the relay bad?
is the answer simply black-magic in the relay box?

i fully respect electronics ... but hate it at the same time.
i almost became an electrical engineer, but could never come to terms with not 'seeing' what was going on. i fried some $4K linear induction motors in grad school because i wired something backward with the 240V supply, and not understanding the actual information scares me to this day.

i plan on replacing the plugs this weekend, more out of diagnostic curiosity than causal certainty...

any answers to my questions are warmly welcome.

thanks
geoff

funola · #27 10-24-2012, 08:18 AM

go buy a Radio Shack 22-172 if you want to see where and how many electrons are going to your glow plugs.

www.peachparts.com/shopforum/diesel-discussion/288970-best-easiest-way-test-glow-plugs-2.html

qwerty · #28 10-24-2012, 09:49 AM

If the timer unit is operating properly, the reason for "no light" condition is a current imbalance between the glow plugs. The current monitor in the timer unit compares the current in the #1 GP circuit to the total current in the remaining 4 GP circuits. (4a=b+c+d+e). If the current monitor isn't happy with the current balance, it doesn't shine the light. The light is a totally artificial indication and has no direct affect on the actual operation of the GP's. Given that you have voltage to the GP's, there is no reason to suspect the relay. (Relay being defined as one component of the timer unit.) The "hard start" condition, coupled with "no light," would suggest a problem with one or more GP's or their associated wiring/connections.

funola · #29 10-24-2012, 10:22 AM

Quote:

Originally Posted by qwerty

If the timer unit is operating properly, the reason for "no light" condition is a current imbalance between the glow plugs. The current monitor in the timer unit compares the current in the #1 GP circuit to the total current in the remaining 4 GP circuits. (4a=b+c+d+e). If the current monitor isn't happy with the current balance, it doesn't shine the light. The light is a totally artificial indication and has no direct affect on the actual operation of the GP's. Given that you have voltage to the GP's, there is no reason to suspect the relay. (Relay being defined as one component of the timer unit.) The "hard start" condition, coupled with "no light," would suggest a problem with one or more GP's or their associated wiring/connections.

+1.

I do not care for how the glow plug light operates and have modded my glow plug relay to manual control via a separate switch such that the glow plug light is on as long as the glow plugs are on. I only activate the glow plugs when the engine is dead cold (overnight start). On warm engine starts, they do not get used, extending glow plug life and reducing battery drain.

geoffraynak · #30 10-24-2012, 10:35 AM

Quote:

Originally Posted by qwerty

If the timer unit is operating properly, the reason for "no light" condition is a current imbalance between the glow plugs. The current monitor in the timer unit compares the current in the #1 GP circuit to the total current in the remaining 4 GP circuits. (4a=b+c+d+e). If the current monitor isn't happy with the current balance, it doesn't shine the light. The light is a totally artificial indication and has no direct affect on the actual operation of the GP's. Given that you have voltage to the GP's, there is no reason to suspect the relay. (Relay being defined as one component of the timer unit.) The "hard start" condition, coupled with "no light," would suggest a problem with one or more GP's or their associated wiring/connections.

i believe the timer unit is functional ... relay is working, voltage is going to plugs, sends electrons to the dash lamp with a jumper.

i also tested the resistance at each leg of the connector/wire chain, and i seem to have good continuity.

thank you for the explaining the theory of the unit. now it makes sense to me, i think. the lamp is only an indication of glow plug health relative to GP1.

when i replace all the GPs (for good measure since the vehicle had no good maintenance history ), i will test each old one and keep any good ones as back-ups.

one solution creates 3 more problems!

thanks
geoff