what did i do to my glow plug relay?
 

so ... in my weekend of over-repairing (i love the compulsion thread, btw) ... i decided to dust off my multimeter and decided to look at my glow plugs ... AKA how diesel giant did them.

my dash glowplug light was fine ... everything worked great ... (i think), but was compelled to learn a bit more while i had an hour in the day

battery voltage: 12.5
running: 13.2

turned everything off, and checked each plug from the relay connector
everything was fine.
less than 1Ω on all 5.

plug the only plug back in ... nice and snug
go back into the house to deal with other things.

come back about 4 hours later to give it a cold-start ....
and the glow plug light on the dash no longer illuminates
i double checked the tightness of the connector
try again
nothing ....
i have not had time to look at anything else since then.
and I'm not even really sure where to look

did i fry something?
what testing should i do?

one solution finds 3 more problems.

geoff

Pull the cover off the GP Relay and check the 80 amp strip fuse.
It may look good, but could be cracked.

I keep a spare taped to the inside of the Glove Box door.


Charlie

could i have cracked it (or been the final straw) by opening the cover and pulling the connector?

i'll give that a look at lunch.

thanks
g

The odds are severely stacked in favor of the connector being the culprit.

are you saying the connector from where the GP wires come into the relay?
does that need to get R&R?

I'd pull the cover, and turn on the key to the glow position, and quickly measure voltage to ground on both screws in the fuse. if 12v on both, it's not the fuse.

Take the connector out again and look for green or whitish corrosion on the contacts. Use 600 grit wet dry paper rolled up lubed in soapy water to clean the contacts, rinse, blow dry, coat it with some Nolux (or any grease will do in a pinch) and see it that fixes it.

I got that voltage on 2 screws without turning the key...

.

As you should. The glow plug timer is "hot at all times."

Did you plug it back properly?
On the E class the connector need a good push, its a tight fit.

the drama continues...
 

Hi Everyone:
here are my findings from lunch...

• pulled the relay cover and measured the 80A fuse
  • no short, near 0Ω across the two screws
  • switched to voltage: 12.5V on each side of the fuse
• looked at the fusebox for fun ... all was good there
• took the glow plug harness connector out of the relay, cleaned each of the five female connections with sand paper ... they were not bad, but now they are shiny
• checked continuity from harness connector to glow plug in engine on all 5: all had good connections near 0Ω
• firmly re-connected the harness connector to the relay
• turned the key to 'glow' ... the buzz is still buzzing, but no glow light
• turned off the key, went back out and just looked at everything
• turned the key to 'glow' again, and went to the relay. stared at it for about 20-seconds and then heard it click.
• key off, then 'glow' again: then ran out and measured voltage from battery ground to the 'head' of each glow plug: ~10.9V on all 5 plugs ... then click and near-zero voltage at all plugs
• decided to try and start it.
• local temp says about 41°. feels warmer, but not much
• anyway, it was horrible. really horrible. lots of cranking and sputtering. on the third try, i got it to idle, barely. went inside and had some lunch while i left it running.
• came out about 10-mins later ... and there was more smoke than idles in the past. wtf?
• then turned off the car, turned key to 'glow' again ... still no light
• started up and it started quite easily.
• turned it off, put all my junk away in the garage and blasted back to 'work'

so ... there is my recap of my lunch.
is there anything i can rule out?
any more diagnostics?


why the heck did i open the thing in the first place ... it was working fine!


sorry for that scream ...

as always, any help/ideas are appreciated


geoff

If you were getting 10.5 volts at the Glow Plugs the Relay is working. You were also hearing the Relay Click off.

Could it be that the Bulb in the Glow plug Light is no good?

Also does you 81 300SD have the Glow Plug Relay that is connected to a Temp Sensor?

pre-glow system...
 

here is a diagram of the 300SD pre-glow
there appears to be a temperature sensor...
the diagram says 'as temperature increases, resistance decreases'

i also find the coincidence of the light going out at the exact same time as my messing-about to be tooooo coincidental.


anything else to check?

On your car assuming that is the correct schematic, the thermal sensor is inside the relay body.

The only external connections are the 4-pin control plug and the 5-pin (6 position body with 5 holes used) connector for the glow plugs. The control plug on your car does not have any thermal sensing wires. Later cars had external sensors so they had more external wires.

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.

The glow relay does 2 things with the light.
First is the light turns on to help you know how long to pre-glow based on ambient. Note that the relay continues to supply current to the plugs after you crank, that is known as "after glow". That is why there are 2 power wires in the control plug. One is hot in II and III, that triggers the pre-glow, and the other hot in III only, that is the after glow trigger. The light does not illuminate during the after glow interval.

The second function is that the relay has a current sensor built in. It compares the current draw from plug #1 to the sum of the current draws from the other 4 plugs. The approximate mathematical operation it is doing with the sensing coils is (plug 1 current * approximately 3.5) - (sum of currents in plugs 2 - 5) = 0. If the result is not zero then there is a plug not drawing current. If the summing circuit is not satisfied then the glow light will not illuminate.

Unfortunately the design of this circuit had enough room for error that it is possible for 1 bad plug to get through.

Also there are documented cases on this forum of members whose glow plugs passed the ohms test but did not glow hot enough to do their job as verified by bench testing with jumper cables. (Danger- extremely high temperatures - almost 3k *F - and risk of short circuit - battery explosion likely if you short the cables - so proceed with extreme care on this if you decide to try this test.) If you had a plug that was on the way out maybe it just failed while you were testing.

If your bulb passes the jumper test I would go ahead and change out the plugs.

Be sure to use only Bosch or Beru plugs. Auto lite is bad juju. And use anti seize paste on the threads, you will thank yourself next time.

There is no "after glow" capability for the OE timer on a 1981 300SD.