how to test glow plug relay? 1987 300TDT - PeachParts Mercedes-Benz Forum

odie · #1 03-09-2010, 11:34 AM

is there any scematic or such to help bench test a glow plug timer or relay with a multi meter for continuity and such?

my glow plug light stopped coming on a few days ago and the immediate result is the car went from super easy cold start to damn near impossible to start....

I have the Euro Haynes book and all it says is replace glow plugs, temp sensor or relay.

the temp guage works fine so I'm guessing that is the same temp sensor that sends a signal to the relay to emergize glow plugs if it's too cold so that is likely not the problem.

I'm guessing it's the relay or power to the relay...

If the glow plug light came on and it won't start that would indicate several or all glow plugs are bad.

There is no fuse that I have found that feeds the relay...but i'm sure there is one...it's just not listed in the repair manual and not labeled on the fuse box...

any help or suggestions to further trouble shot the relay or find a fuse before I spend $100+ on a new relay I may not need?

BoiseBenz · #2 03-09-2010, 11:51 AM

The temperature sender for the later relays (like yours) is IN the relay.

The fuse is IN the relay. Pop off the cover (fingers into open end where wiring comes out, pull straight up) and look for the aluminum strip held on by a pair of phillips screws. A visual inspection is not enough to prove "good." If it's not obviously bad, remove the fuse strip. It will obviously be one or two pieces.

odie · #3 03-09-2010, 01:55 PM

I found an aluminum or metal "strip" type fuse under a small plastic cover right next to the driver side shock tower.

I opened the cover and the metal strip was sagging down a lot. I barely touched the strip with my finger and it just broke in half.

Could this be the fuse for the glow plug relay?

JimmyL · #4 03-09-2010, 01:59 PM

I do believe that that is your blower fuse....

Please re-read post #2.

odie · #5 03-09-2010, 02:28 PM

yep, pulled the cover and there is a 30 amp metal fuse. Looks to be good but I'll have to use the multi meter on it later.....should the fuse itself always be hot or just when the ignition is on?

An yep, the AC blower don't seem to work anymore.

compress ignite · #6 03-09-2010, 07:08 PM

1.30 Amp fuse is the HVAC Blower.
('Will be HOT in RUN Key Position)
2.80Amp fuse is the Pre-Glow Relay.[This Fuse is screwed onto the end of the Pre-Glow Relay.]
('Will be HOT AT ALL TIMES ,when Relay operates the GROUND side of the fuse
is Earthed through the Glow Plugs themselves)

Testing Procedure for Pre-Glow Relay:
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/15-4000.pdf
(The A. section is yours [No Afterglow] )

Complete HVAC Blower post:'87 300D blower motor?

GSXR's instructions from an earlier poster:

odie · #7 03-10-2010, 11:01 AM

cool link for the glow plug stuff. many thanks !!!!!!

i kinda fingers the blower fuse ends together and it blows again, lol...new fuses on order...

it rain it pours....stuff happens in threes....my shock tower strut mount blew out...i got 6 inches of strut sticking thru my fender...oh well...500,000 miles and something has gotta break...lol

compress ignite · #8 03-10-2010, 04:38 PM

Odie,

The Blower Fuse "Blowing" is the Signal that your Blower Motor is most likely due
for Replacement.

(As they get older the lack of Lubrication increases friction and thereby ,Drag
which contributes to the need for more amps...which raises the resistance...
which increases heat ...ETC,ETC. They have "Bushings" instead of Bearings.
Some, I know of, on the W124 chassis have been successful in squirting a
little ATF into the Bushings to get a little more life out of the Motor.)

odie · #9 03-11-2010, 10:44 AM

interesting...i'll see about the ATF trick...is the motor easy to access or a PITA?

compress ignite · #10 03-11-2010, 04:07 PM

First You'll want to do an "Advanced Search" for all things Blower related
(There are all sorts of tricks...Like How NOT to damage the Squirrel Cage Fan.)

1.
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD2/Program/Chassis/82-0680.pdf

2.
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD2/Program/Chassis/82-0760.pdf

3.
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD2/Program/Chassis/82-0765.pdf

4.
http://www.ps2cho.net/downloads/MB%20CD/W124/Index/MiscFiles/83-140.pdf

5.
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-556.pdf

AND just in case:
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-670.pdf

Here's the whole manual:
http://www.ps2cho.net/downloads/MB%20CD/W124/Main.html

tr1cky · #11 03-11-2010, 04:37 PM

Quote:

Originally Posted by odie

is there any scematic or such to help bench test a glow plug timer or relay with a multi meter for continuity and such?

my glow plug light stopped coming on a few days ago and the immediate result is the car went from super easy cold start to damn near impossible to start....

I have the Euro Haynes book and all it says is replace glow plugs, temp sensor or relay.

the temp guage works fine so I'm guessing that is the same temp sensor that sends a signal to the relay to emergize glow plugs if it's too cold so that is likely not the problem.

I'm guessing it's the relay or power to the relay...

If the glow plug light came on and it won't start that would indicate several or all glow plugs are bad.

There is no fuse that I have found that feeds the relay...but i'm sure there is one...it's just not listed in the repair manual and not labeled on the fuse box...

any help or suggestions to further trouble shot the relay or find a fuse before I spend $100+ on a new relay I may not need?

There are new relays available for about $35 bucks if you look around.