PeachParts Mercedes-Benz Forum

PeachParts Mercedes-Benz Forum (http://www.peachparts.com/shopforum/)

- Diesel Discussion (http://www.peachparts.com/shopforum/diesel-discussion/)

- - Glow plug issues (http://www.peachparts.com/shopforum/diesel-discussion/331237-glow-plug-issues.html)

Glow plug issues

My glow plugs are not working. I pulled glow plug 1 and even though it is not lighting at the tip it is lighting. When I unplug the wires that go to the glow plugs at the relay the glow dash light comes on when I turn on the key. I also have power on the pins when the harness is unplugged. Do I need a new relay? Is there a short in the harness? Is the relay bad?

Need year and model. What are your symptoms which makes you conclude the glow plugs aren't working? If it's not lighting at the tip it's bad. You need to do more diagnostics before condemning the relay. Check to see if the plugs are getting 12 volts with the key in the glow position.

What is the year and model? Is it safe to assume that it is a Mercedes? Are the pin type or loop?:confused:

Our mind reading skills are quite limited here.:)

It is an 84 300 D. The dash light does not turn on when I position the key to ligh the glow plugs, and the motor certainly will not start. When I remove the wiring harness attached to relay and glow plugs, the dash light comes on when I turn the key to the proper position. I also have 12 volts at the pins when the harness is detached and the key is turned to position. When the harness is attached I do not get 12 volts at the glow plug with the harness attached.

I'm rusty on this, but I will try to help.

The number one glow plug is measured for resistance by the relay. You can think of that plug as a temp sensor. When the plug heats up, resistance goes down and the relay turns off the light. I mention this so it will hopefully help you in your troubleshooting process. For grins, why don't you disconnect number one glow plug and see if it effects the light in any way.

Quote:

Originally Posted by xyz12383318 (Post 3057305)

I also have 12 volts at the pins when the harness is detached and the key is turned to position. When the harness is attached I do not get 12 volts at the glow plug with the harness attached.

It's perfectly normal to read less than 12 volts with the glow plugs in operation.

Quote:

Originally Posted by xyz12383318 (Post 3057263)

When I unplug the wires that go to the glow plugs at the relay the glow dash light comes on when I turn on the key.

As it should. With the harness disconnected, there is no current imbalance to generate a fault indication.

Quote:

Originally Posted by Air&Road (Post 3057310)

I'm rusty on this, but I will try to help.

The number one glow plug is measured for resistance by the relay. You can think of that plug as a temp sensor.

If that is true, what is the purpose of the temp sensor in the timer unit?

Quote:

Originally Posted by Air&Road (Post 3057310)

For grins, why don't you disconnect number one glow plug and see if it effects the light in any way.

The timer unit looks for a current imbalance by comparing the #1 plug to the sum of the remaining plugs. If the light does illuminate with the #1 plug disconnected, the fault detection circuit is definitely faulty.

Quote:

Originally Posted by xyz12383318 (Post 3057305)

position. When the harness is attached I do not get 12 volts at the glow plug with the harness attached.

With the key in the glow position?

Quote:

Originally Posted by qwerty (Post 3057315)

It's perfectly normal to read less than 12 volts with the glow plugs in operation.

As it should. With the harness disconnected, there is no current imbalance to generate a fault indication.

If that is true, what is the purpose of the temp sensor in the timer unit?

The timer unit looks for a current imbalance by comparing the #1 plug to the sum of the remaining plugs. If the light does illuminate with the #1 plug disconnected, the fault detection circuit is definitely faulty.

The temp sensor in the timer unit can not measure temperature in the glow plug. It is measuring ambient for some sort of comparison or to change the glow time based on ambient.

You could go to the Diesel Giant Website where there is a pictorial on testing the Glow pLugs.