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Old 04-29-2003, 11:07 AM
Posts: n/a
Testing GP Relay?

How do I test the GP relay. I have a multimeter. The Glow Plugs test as good, but I am getting a rough start after pre-glow. Once engine warms, all is great. Some blue smoke if revved when cold. Also, some pretty strong knocking when cold. I will be adjusting valves with this weekend's oil change. And I would like to get a bead on the source of the poor starts. Thanks.

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Old 04-29-2003, 03:10 PM
Mark Tamburrino
Posts: n/a
I remember somebody saying that a
glow plug could pass the resistance test
(My new ones were ~.6 ohm) but fail
under load. Also you should check the
resistance from the connection that plugs
into the relay to elimate faulty wires.
If you have a high current capacity
multimeter I'm guessing you could look
for 20 (12v/.6 ohm) amps when the key
is turned. I was also wondering about
mine, with the new plugs it starts fine but
the glow plug light comes on even when the
engine is hot, does this mean something is
not sensing temperature and wearing the
plugs more than necessary? I hope I'm not
hyjacking the thread, seems like the two
problems are related to relay trouble.
Mark 82' 300sd 280k
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Old 04-29-2003, 03:18 PM
Posts: n/a
I believe that pre-glow occurs whether or not engine is warm. In the cars that I have owned and driven, the pre-glow light always comes on at start-up regardless of whether or not engine is warmed up. I tested the GP's at the relay plug and they test out strong.

I'll retest the plugs and test the relay this weekend and see where that leads me.
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Old 04-29-2003, 03:45 PM
Mark Tamburrino
Posts: n/a
Thanks for the info. My only other diesel was an 81'
Rabbit (good car) and the glow plug light only came
on when it was not hot. I think it would result in
a shorter glow plug life if they were energized every
turn of the key rather than just the initial cold start.
Anyway, I was looking for an old thread and came
upon another on page 4:"Strange plugs which are
interspersed with glow plugs." and according to
one guy there is a temp sensor for the glow plugs.
The car in question was an 81' 240d, mabee this is
not on all models. Good luck with the plugs/relay.
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Old 04-29-2003, 03:45 PM
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Join Date: Jul 2000
Location: Krum, TX
Posts: 30
The temperature determines the length of time the GP relay provides current to the glow plugs. On my 240D when cold the relay stays on about 20 seconds.

If the glow plugs are .6 ohms and the voltage is 12 volts that equals 20 amps per plug. I think in reality it ends up being a little less as the fuse in the relay is rated for 80 amps. If all four plugs drew 20 amps the fuse would blow.

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Old 04-29-2003, 04:29 PM
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The Haynes Manual only says that there are a variety of equipment configurations when it comes to Glow Plugs and affiliated equipment. I don't know what the deal is with temp. sensor / GP relay interplay. If anyone knows, please chime in.
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Old 04-29-2003, 06:32 PM
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Join Date: Jan 2003
Location: USA
Posts: 699
Dear Johnvee:

I think I have a good explanation for the existence of the metal strip fuse in glow plug relays:

For a parallel circuit (e.g. 4 glow plugs, each with R1/2/3/4=0.6 Ohm)

The total equivalent resistance of this whole parallel circuit (consistent of 4 glow plugs) would be:

R = 1 / (1/R1 + 1/R2 + 1/R3 + 1/R4)

If we take R1=R2=R3=R4=0.6 Ohm, the total equivalent resistance is:

R= 1/6.67 = 0.15 Ohm

Since parallel glow plugs are designed to operate at a voltage of
11 V, the total amperage going through the whole parallel circuit
(all 4 glow plugs) is:

I = V / R = 11 / 0.15 = 73.33 A

(We may say that each glow plug gets about 73.33/4 = 18.33 A.)

Now we can clearly see why parallel pencil-type glow plugs are designed for 11 V, because if they are 12 V, the total amperage
will be I = 12 / 0.15 = 80 A. The glow plug relay's 80A metal strip fuse will blow up right away :-)

That's the reason why the metal strip fuse in the glow plug relay is rated at 80 A (about 6 A more than the maximum total amperage that will go through all 4 glow plugs at the VERY beginning of the pre-glow period, when the resistance of each individual glow plug is lowest.)

When the glow plugs become hotter and hotter, the resistance of the heating element in each glow plug will increase, i.e. R1 ~ R2 ~ R3 ~ R4 => higher, and the total equivalent resistance will become bigger (because of the above formula). Since V stays constant, the total amperage I will drop because R increases.

In many posts, people talked about the need to have an ammeter
that can handle 15-20 A. This means that the ammeter must be connected in series with EACH glow plug in the paralle circuit (where the amperage is about maximum 18.33 A assuming that
the COLD resistance of each individual glow plug is 0.6 Ohm, which is the value I used in my above calculations).

I need to borrow a good ammeter that can handle about 20 A. Most cheap junky multimeters can only handle a maximum 10 A.

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Old 04-30-2003, 01:23 PM
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Join Date: Jan 2000
Location: PA
Posts: 5,440

Although your calculations are correct, I have a better and simpler reason for the fuse in the GP circuit.

Should a GP short, the fuse will open instead of burning wires or destroying the GP relay.

And you didn't explain why the GP are operating at 11 volts. The will GP conduct a current proprotional to the voltage presented to them (within limits). This voltage will be the battery voltage (which is reduced slightly when the GP current flows thru it) minus the IR (voltage) drop in the wires, contacts, connectors ETC in the circuits that connect the GP to the battery.

So the GP may be actually operating at 11 volts because of the battery and circuit voltage drops not because the fuse is rated at 80 amps. A higher ampere rated fuse could have easily been used if necessary.

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Old 07-02-2003, 11:28 AM
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Question for PEH

You posted that if a plug shorts out that this can cause a high draw and blow the fuse. My 240d still has the old style plugs and the little fuse box on the firewall melted the studs that held the fuse that was also blown. So I replaced that part and the car started fine. The GP light came on for about 30 seconds as it always does when the car is cold, just would flash when it gets warm. This morning my daughter was all over the place in "her" car yesterday the fuse was blown. Now this was the fuse that came with the bone yard part so I don't even know what it was rated for, seemed very thin to me to handle anything close to 80amps not much thicker than tin foil so it may not have even been the right one. I got it started by jumping with a piece of 14 guage solid copper wire and it was cool to touch about 2 minutes after starting so I will ASSume that the relay is working.
I need to get out to the dealer and get the correct fuse and I also need to check each plug for a short but if all that comes back ok is it also possible that the relay is bad and allowing power to the plugs without lighting the GP light intermittently while the car is running and thus to much heat is burning the fuse?
I also replaced the wire to the fuse box to the first plug yesterday as the old one was about worn in half. I figured that caused the first problem to start with.
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Old 07-02-2003, 01:49 PM
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Join Date: Jul 2000
Location: Krum, TX
Posts: 30
The fuse in the fuse box is not the 80 amp fuse in the glow plug relay.

Something else is causing the fuse in the fuse box to blow.

I have seen where previous owners have put the wrong fuse (bigger amps) in and that has melted the contacts in the fuse box.

In general it's a good idea to replace all the fuse in the fuse box. It's cheap and solves alot of weird electrical problems.

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Old 07-02-2003, 03:35 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,800
If the glow plugs pass the resistance test but it doesn't start quite right, check the current through each glow plug. The spec for OM603 glow plugs is 8-15amps each. I guess it's higher or lower if something's not right.

If you find a really cheap tester, let me know. I don't want to risk my $30 tester with a 10Adc limit. I was thinking of trial and error testing with a whole bunch of fuses but I got lazy

95 S420
91 300SE
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Old 07-02-2003, 03:59 PM
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This is not a fuse in the fuse box. It's an inline bar type that is in it's own little tiny black box mounted to the firewall. All it's for is the GP's. Seems to me exclusive to only the 77-81 model of 123 240d models. At least when picking the bone yard that is what it looked like. Just got finished running an ohm test across the whole set of 4 and came up 2.4 which is right on target. Each plug was running around .58 to .63 and none tested at zero so no dead short. At this point I suspect either the bar fuse was the wrong size or the relay is faulty and somehow is kicking on and off while the car is running but not lightning up the relay light. Car starts fine with no miss or stumble, also no smoke, well no more than you would expect from the 240d when cold.
One screw was not as tight as I thought it should have been in that fuse block either so it may have been my fault and a weak connection. If it blows the dealer fuse which was much thicker than what was in the bone yard box then I am going to have to suspect the relay.
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Old 07-02-2003, 04:38 PM
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Location: Renfrew, Scotland
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SIXTO makes a good point about not using your expensive multimeter to test glo plug current. A good alternative is an old type ammeter out of a car that used a dynamo. they usually read something like 60 - 0 - 60 amps, so you can connect it up either way round. The idea is that the current should be similar for all plugs, a faulty one will usually read low or zero. note that a zero current reading could be due to a faulty section of the glo relay or wiring. prove this by connecting the ammeter between battery pos & the glo plug terminal. A good plug will give about 20 amps. I used one last weekend on a 1995 OM605 engine & found no 1 plug faulty, when I measured its resistance it was about 500 ohms. Before i replaced it the engine rattled badly for 10/20 secs after start, its now dead smooth. Just goes to prove how effective the post-glow is, even if plugs dont last as long.

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