Glow Plug Relay Defective? - PeachParts Mercedes-Benz Forum

mechanicalman · #1 12-05-2008, 06:41 PM

Hey guys, i searched around, but couldn't find a thread that spoke exactly to my problem.

i was having intermittent (the worst kind) glow plug problems for about six months and now the circuit never comes on. i checked the lamp by applying 12v directly to the lead at the glow plug relay and it illuminated. i checked both ends of the glow plug fuse for 12v and that was okay too. I checked the glow plugs for proper resistance at the glow plug relay harness and they all tested out at 1 ohm cold.

next i checked to make sure the ignition switch was providing 12v to the relay when the key was turned, by clipping a test lead to the pin on the relay. the ignition switch seems to be doing it's job.

finally i tested for 12v at the glow plugs when the ignition was switched on and the result was negative. the glow plugs are not receiving voltage.

SO, have i ruled out everything except the glow plug relay, or is there more to check before i shell out for a new one?

Oracle12345 · #2 12-05-2008, 06:46 PM

did you check circuitry and for corrison?

mechanicalman · #3 12-05-2008, 06:53 PM

which circuitry are you referring to? the actual board inside the relay?
and what they hey is "corrison?"

i checked the pins in the harness itself and the seemed fine a far as corrosion and build up

tangofox007 · #4 12-05-2008, 07:05 PM

If you have battery voltage at both ends of the strip fuse and you have battery voltage on the red/black wire when the ignition switch is on, you have what you need. If the relay doesn't actuate, it sounds like it's time for a new timer.

If you can hear the relay working, you could try cleaning the relay contacts. Remove the four corner screws under the cover after disconnecting the battery.

mechanicalman · #5 12-05-2008, 07:08 PM

the relay is quiet as a mouse. not a click to be heard.

thanks guys, i can order the relay some confidence now.

i'll post again when the thing shows up.

vwnate1 · #6 12-05-2008, 08:05 PM

Well ;

Often the fuse strip inside the top of the relay has loose screws....

Also , the one in SWMBO's pristine '82 240D stopped working , turns out the alternator was going bad and didn't charge enough (although the car always started and the red light never came on) so the new relay " failed " after two days and I had to do some more testing before I found out I had 12 V @ rest but less than that when I cycled the glow plugs... or tried to .

mechanicalman · #7 12-05-2008, 10:34 PM

geez, where were you 30 minutes earlier?

i checked the voltage and i'm getting 12.6v at rest, but on the (rare) occasion that the glow plug relay does decide to come on the voltage at the relay fuse only registers 10.7v.

should i start looking at the alternator?

vwnate1 · #8 12-06-2008, 08:16 PM

First thing I'd get a low amperage battery charger (under 15 amps) and let the battery charge for 12 hours then see if the relay begins working....

Check the battery's fluid level and top up with _distilled_ water if possible or needed before charging .

If it does , check the voltage when the GP's are heating , it should remain above 11 VDC , you shouldn't go down to 10.5 volts until the starter is cranking the engine .

As I've said before , the weeny 55 Amp. Bosch alternator is barely up to the job on these cars and every time it gets cold , GP and no start complaints begin pouring in .

Often simply leaving it on an overnight charge gets you going again .

DO NOT use the ' boost ' feature of any battery charger as that just beats the hell out of the battery .

The longer you charge it at a lower rate , the better and deeper the battery's charge is . try to not run the engine for less than 20 minutes when the weather turns cold and if you have to leave the car running , turn off all the lights if possible , at night just use the parklamps so the alternator has a small chance to actually charge as the headlights suck a HUGE amount of power , even seal beams in U.S.A. cars .

30 minutes before you read my last post , i was reading other posts , sorry .

At least I'm one of the very few honest Mechanics you'll ever meet .

Jeremy5848 · #9 12-06-2008, 09:14 PM

Quote:

Originally Posted by mechanicalman

. . . i checked the voltage and i'm getting 12.6v at rest, but on the (rare) occasion that the glow plug relay does decide to come on the voltage at the relay fuse only registers 10.7v. . . .

That is reasonable, since the glow plugs draw a lot of current, they pull the voltage down. The only other thing to check is the glow plug relay's 80 Amp fuse. A dirty connection will not pass enough current to operate the glow plugs although it will still register 12 Volts on your meter.

Remove the fuse, make sure it and the screws are clean and shiny. maybe put just a tiny bit of anti-corrosion compound on the fuse where the screws contact it. (This stuff is sold in the electrical department of hardware stores – electricians use it when they join copper and aluminum wires.)

If that doesn't help then you probably have a bad relay.

Jeremy

mechanicalman · #10 12-18-2008, 02:31 AM

allright, after upgrading the alternator to a 65 amp model (because i wanted to anyway) it didn't end up solving the glow-plug problem. two days later: brand new relay comes in the mail. bolted it on, plugged it in, problem solved.

it was a bad glow plug relay all along.

/glad i got it in before the weather got too cold
//didn't realize how cold it'd get

alphadeltaromeo · #11 12-18-2008, 09:12 AM

Glad to hear that you were able to fix the problem. I just finished reading the other glow plug issue post and found the link that UriahT gave on dieselgiant to be an excellent read:

http://www.dieselgiant.com/glowplugrepair.htm

oldsinner111 · #12 12-18-2008, 09:39 AM

Also check wire to temp. sender (near thermostat on 617 turbos).Theres one on the head for gauge,and one for glow control.If wire is broken or get loose from pin,GP relay thinks engine is hot.Been there fixed that.

tangofox007 · #13 12-18-2008, 09:47 AM

Quote:

Originally Posted by oldsinner111

Also check wire to temp. sender (near thermostat on 617 turbos).

The electric temp switch on the thermostat housing of 617 turbo engines is for the climate control system (prevents blower fan operation in heat mode when engine is cold) and is unrelated to the glow plug system. On those models, the glow plug timer references ambient air temp to determine cycle duration.

kerry · #14 12-18-2008, 09:57 AM

Some 617's have the temperature sensor which controls length of glow light (?) in the head and some have it in the relay itself. Don't know when the switch was made. My 79 SD has the temperature sensor in the head. You can tell because there are two senders on the left side of the head between the glow plugs--one for the gauge and one for the glow plugs.

oldsinner111 · #15 12-18-2008, 10:56 AM

My 617.951 had that problem,and I fixed it.Believe what you want to.