Question about 1983 Mercedes-Benz 300-Class

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Could a faulty coolant temp sensor make the glow plugs run all the time on a 1983 Mercedes 300CD?

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No the glow plugs circuit is separate. They actually are on a timed delay shut off. Check the glow plug relay module behind the driver side headlamp assembly. If the light on the cluster is staying on all the time it's simply letting you know there is a fault in the glow plug circuit. Doesn't actually mean that the glow plugs are staying on all the time. You might just have a bad glow plug which is breaking the circuit therefor the glow plug light stays on. To check this use a test light and have someone turn on and off the ignition to see which plug doesn't light up.

Posted on Oct 11, 2010

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3 Answers

Mercedes 300td wont start


It's likely a bad glow plug relay found on the left fender well. You may have bad glow plugs as well. You should hear a faint click when you turn the key to glow psn. . Open the hood and have someone glow it. The click should be abvious and the wires will run from the relay to the glow plugs. also obvious.

Oct 06, 2014 | 1998 Mercedes-Benz E-Class

1 Answer

Car won't start when cold . . . i need to use starter fluid to start it . . . any suggetions?


1. Do not use starter fluid on a glow plug diesel engine. (explosive)
2. Most likely 1 or more of your glow plugs is out. They are not too expensive and are fairly easy to change. Facing the engine, look on the right side. There is a set of "plugs" with hoses, and a set of "plugs" with wires. You want to change the set of plugs with wires.

Jan 22, 2014 | 1983 Mercedes-Benz 300-Class

1 Answer

Hi expert boss..i have a mercedes benz vito 108 and i cant start it..i have a edc light on the screem ..ca you hello me?


EDC LIGHT SOUND LIKE GLOW PLUGS NOT WORKING, COULD HAVE FAULTY GLOW PLUG RELAY OR ECM PROBLEMS.COULD HAVE FAULTY COOLANT TEMPERATURE SENSOR.BEST TO LET GARAGE CODE SCAN FIRST DONT BUY NEW PARTS UNTIL SYSTEM CHECK OUT FIRST.

Sep 17, 2012 | Cars & Trucks

1 Answer

Glowplug light comes on and goes off quikly, engine hard to start. Changed all 4 glow plugs and coolant sensor. Light stays on for a good 10 seconds when water temp. sensor is unpluged.


The glow plugs work in direct relation to the engine block temp. The relay for the glow plugs is obviously reading the wrong temp, that's why they turn back off. It's most likely that the temp sensor is bad, but trace the wires back and make sure there are no breaks in the insulation and they are well connected at the plug-ins. I have seen a lot of wires at the connector look good but fall off as soon as you touch them.

If the temp sensor is the same on for your your dash, then borrow a heat sensing gun and double check it against the guage. I have seen them read wrong by as much as up to 20 degrees. If the sensor reads rught and feeds both the guage and the glow plugs, then the problem would be either the wire harness or the glow plug relay. Hope this helps.

Oct 26, 2011 | 2003 Volkswagen Jetta TDI

1 Answer

Glow plug light comes on & gos off Location of Glow Plug Relay


Glowplug relay 180; located in Position 10 in the relay panel. If you unplug the coolant temp sensor, you can trick the computer into thinking it's really cold out, and it will run the glow plugs.

Mar 10, 2011 | 2003 Volkswagen Beetle TDI

1 Answer

The diagram in my haynes manual does not show some parts that are in my 2001 jetta tdi 1.9 turbo. i ran a diognostic and it says glow plug curcuit a malfunction, i changed the plugs and tested the coolant...


In addition to the 4 primary glow plugs, the cooling system also has 3 Coolant Glow Plugs to help provide extra heat to the cooling system in order to warm-up the vehicle's interior more quickly, and enhance overall engine warm-up time. (Example - Some Mercedes diesel engines use electrical heaters in the cabin because their engine is so thermally efficient.)
Likewise, this auxiliary source of heat from the 3 Coolant Glow Plugs is necessary because of the VW 1.9 TDI engines outstanding efficiency, which wastes very little heat.


The 3 Auxiliary Coolant Glow Plugs are located at the end of the aluminum cylinder head (in a protruding flange) directly below the vacuum (brake booster) pump, which also connects to a coolant hose.

tdisline_301.jpg
The 3 Auxiliary Coolant Glow Plugs use two (underhood) relays which are monitored and controlled by the ECU. During a cold start, the vehicle's interior temperature selector switch helps the ECU choose one of three modes depending on the amount of heat required to warm-up the coolant.

For example, Coolant Glow Plug #1 can be selected to operate all by itself, or #2 and #3 can operate together as a two-some, or lastly, all three glow plugs can form a triple source of heat for maximum enhancement of engine warm-up time.

After the engine is completely warmed up and the thermostat is open, the Coolant Glow Plugs normally shut-off and remain off unless the (thermally efficent) engine cools down sufficiently to have the computer signal them to come back on to keep the engine and cabin air warm. Extensive idling times at stop lights or in the driveway are scenarios where these glow plugs are most likely to recycle on and off.

In conclusion, the period of Preglow and Afterglow is determined by the engine's coolant temperature (during a cold or hot start), which helps the ECU select a predetermined time-cycle for the combustion chamber and / or Auxiliary Coolant Glow Plugs to follow.

Finally, one myth that needs to be immediately debunked, is the belief that the TDI's advanced Glow Plug System is linked to the opening of the driver's side door, which is false! It is also noteworthy to mention that some earlier VW diesel glow plug systems were operated in this manner, but not so with the advanced VW Jetta 1.9 TDI engine.

Feb 23, 2011 | Volkswagen Jetta Cars & Trucks

1 Answer

1997 Jetta: glow plugs not working. Glow plugs tested good, ne...


Before you invest in a long drawn out Glow Plug replacement etc. A little known issue occurs on the ALH TDI engines. The temperature sensor sending unit. A small blue module that plugs into the coolant hose assembly will cause a P error on the glow plugs that is nothing to do with the glow plugs at all. The part fails and sends a temp error to the engine computer. I tested all the coolant glow plugs, engine glow plugs and glow plug harness and all worked fine. One of the test procedures for the glow plugs is to undo this sensor that will force the glow plugs to come on no matter what temperature. This confirmed my GP system was functional and the sensor was bad. I purchased the part from local VW dealer for $33 US and installed and it is fine. Dependent on your model it can be square or round. The link to this part for your car is http://www.partsgeek.com/catalog/1997/volkswagen/jetta/body_electrical/water_temperature_sender.html

Be sure to have VW Pentosin coolant on hand to refill coolant system as you will lose quite a bit when removing this part. It is located slightly behind the coolant glow plug assembly.. This is the round ones not the ones on the engine block at top.

Be sure that when you replace it you put in a new rubber gasket seal.. its a round rubber seal that should come with your new part. Dont reuse the old one or you will have a leak.

There is a c-clip that holds this in the hole you slide that clip off and wiggle the sensor out. Coolant will run out of the hole, so do this outside. The sensor is easily pulled out. You will need to insert the new part in teh same hole with new rubber gasket and replace c clip. Then plug in wiring harness and you should be good to go.

May 17, 2010 | 1997 Volkswagen Jetta

1 Answer

2006 crv 2002 willnot start with turning key willstart with tow or short pushallparts of fuel (diesei)line cleaned pump&inj.test ok engine will turn when key is used key test ok test show lack of fuel...


hi if when engine starts and warms up it then runs ok ? then starts ok rest of day ? considering vehicles age/milage ? have you checked heater glow plugs ? you only need one glow plug to fail/ leak to earth on cyl head and this will knock out/ leak power to earth of all other g/plugs also consider the possibility that the engine coolant sensor ( not the temp sender) has gon faulty this sensor gives ecu signal to provide excess fuel for cold starting on cold starts and auto cuts off as coolant heats up? rather like using choke control on carbs years ago ? also diesels used to have excess fuel device on pumps on trucks in days gone by ? whick assisted in cold starting hope this info helps?

Feb 03, 2010 | 2006 Honda CR-V

1 Answer

1983 300td hard to start when cold.


The glow plugs can be worn also.Make sure the glow indicater is nice and red.

Aug 30, 2009 | 1983 Mercedes-Benz 300-Class

2 Answers

The glow plug light flashes while the engine is


this means you most likely have a burnt out glow plug or glow plug timer control or both

Mar 18, 2009 | 2001 Volkswagen Jetta TDI

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