OM617 glow plugs troubleshooting
 

Hi people!

My old GW has become impossible to start without running the diesel heater.

The engine was swapped from an M110 to a OM617 in the 90s. The glow relay is manually operated using a switch.

While glowing, am I correct you should feel the plugs becoming hot to the touch? Cause they remain ice cold.

From what I know, if one fail, they all stop glowing (being connected in series). I then measured using a voltmeter between ground and each of the glow plugs, and got solid voltage on all of them. Is it possible to read good voltage on a busted glow plug?

Thanks for any insight :)

Yes if the glow plugs are wired as series with the squiggly wires between some of them. For example if number one glow plug has an open element your would read about 12 volts no matter where you read the voltage along the glow plug string. Until you go past the burnt out glow plug.

The potential voltage is present to enable the current to flow through the glow plugs. It would just be the path to enable the current to flow that is open. That being a burnt out glow plug or a really bad connection.

Since you can read voltage your meter should be useful. You will see two heavy conductors to each glow plug. Take both of the probes and place one on one wire of the individual glow plug and the other probe to the other wire. With your manual glow plug circuit on but not working. The glow plug terminals on the glow plug you read about twelve volts on will be the open or burnt out glow plug or source of a bad connection.

From your description I suspect the number one glow plug might be your problem but you have to check.

When you measure voltage on the Loop/Filiment type Glow Plugs you do not measuer them to a ground like the Cylinder Block.

See the pic. The V in the pic is your Volt Meter.

I tried to download the test proceedure from the manual but it would not down load.

A single Glow Plug cannot take the full battery voltage and the loop/filiment/heater element will instantly burn. Apparently the resistance wires reduce the voltage to a managable level.

Unlike a Spark Plug or the later style pencile Glow Plugs the loop/filiment type Glow Plugs are not grounded on the Glow Plug Body. The Electrical Current passes through the Glow Plug, heating Element and back out to one of the wires or resistance wires and the circuit is eventually grounded at the very front by a single ground wire.

As eletrical current passes through each Glow Plug there is a voltage drop; the possible amount of the drop is listed on the testing diagram.

For more testing info look here:Repair Links
Fast navigation http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html

Assuming your pushbutton switch can handle 80 amps you are a really good canidate for the updated pencil type glow Plugs. Mercedes part 0250201044 Bosch number 80035.
Those Glow Plugs get hotter faster and people who have change over to them are seem very satisified with the change. See the Repair Links on how to.
Should be pictures in the below thread.
http://www.peachparts.com/shopforum/diesel-discussion/378463-glow-plug-upgrade.html

Ahh, gotcha. Then I didn't measure it correctly.

I'm actually doing an engine swap to an OM606.910. But I intend to sell the OM617, so I'll need to fix it anyway. And hopefully the switch can handle those 80 amps glowplugs I assume the OM606 has too.

I'm sorry if I'm a bit dense. Electric stuff to me is like magic. Using the voltmeter I measure according to Diesel911's picture below - on the squiggly connectors on each side of respective glow plug. But are you saying 12 volts will distinguish a burnt out glow plug while NOT glowing? I'm not entirely positive, but I don't think it normally has any voltage over it unless I flip the switch? Or is that how they always work?

No electrical skillse needed.
If for instance you had no voltmeter you could simply record the position of the various wires (a photo is nice) and remove the Glow Plugs. Usually you will see the loop/filiment/heater element is burned in two.

I believe a 13/16 wernch is the size for your Glow Plugs.

The last glow plug is attached to the engine block. The first to a wire coming from the fender. Follow the wire from the fender to the first glow plug should have 12 volts before and after the GP. go to the second, should have 12 v before and after the GP. etc. Now, you will need an assistant to pull on the glow plug knob because 12 volts is only applied while the knob is pulled (IF auto preglow system, pre glow is only on for a few seconds (up to 30 for old style) so 12 volts will only be applied during that time period.

AFA electricity's magic, imagine you're in a car called 12 volts. Starting point is the positive battery cable. On the road to Ground (your final destination) there are 5 traffic lights ahead. A green light is a good GP a red light would be a bad GP. the 12 volts stops at the bad glow plug because it can't complete the journey to ground.

You need to measure resistance. A good GP will be under 20 ohms (guesstimate) or so, a bad one could be as high as infinity.

You cannot measure the GP's to ground. you need to measure from one wiggly wire to the next. (one glow plug to the next) In my previous example, all the traffic lights are on one street. That's series. Now imagine 5 streets each with a traffic light, that would be parallel.