Confirmation on GP relay issue, please - PeachParts Mercedes-Benz Forum

firemediceric · #1 01-04-2010, 09:36 AM

Through searching the forum I believe I have my issue narrowed down to a bad GP relay on my 1981 300 SD. My supposition I would like confirmation on is that the relay is supposed to receive power anytime the key is on, both during the glow cycle and while the engine is actually running?

To put it another way, the relay is receiving power at all times the car is running. It is the relay that opens preventing power from going to the glow plugs after the glow cycle. Right?

The answer/confirmation of this will confirm that I am not getting power to the relay when I shouldn't.

Thanks

BTW, I appreciate the new spell check feature. Thanks Hunter

micalk · #2 01-04-2010, 10:42 AM

Depends on how you define receiving power.

12V is available virtually directly from the battery to the single large terminal in the glow plug relay, even with the key off. This is routed to the Normally Open contacts of the relay and will go to the glow plugs when the relay is energized. When the key is turned to the RUN position, the relay is energized. Once the key is moved to (returned from?) the START position, the relay is de-energized.

mobetta · #3 01-04-2010, 10:48 AM

I had a ground wire failure cause a bad relay symptom once- it is grounded thru the washer pump. the connection there, w/ 2 brn wires, was bad.
the large term should always be hot.
brown is grnd.
violet gets key signal.
and the last wire as I recall gets engine temp?

Diesel911 · #4 01-04-2010, 01:35 PM

On my 84 300D relay always has Battery Voltage at the Strip Fuse and the Flat wire that supplies the Battery Voltage.
However, the Glow Plugs only get power when the Key is in the Glow Position and when your are Cranking the Enigine.
Ounce the Engine is running the Glow Plugs get no power.

Some of the Older Glow Plug Setu-ups have a Coolant Temp Sensor that is connected to the Glow Plugs. I am not sure if this keeps the Glow Plugs on until the Coolant Temp is up or not.

The several years newer models have something called Afterglow that indeed keeps the Glow Plugs on for several more minutes; but eventually the all turn off.

firemediceric · #5 01-04-2010, 03:38 PM

Thanks for the replies. I guess now what I need is a definitive answer on what specifically triggers the relay to close and send power onto the glow plugs?

I am trying to make sure that the relay itself is bad as opposed to whatever may trigger the relay to close. As of now, for all I know the relay may be good and the problem may be that it's getting a signal to close when it shouldn't be getting such a signal. Would I be correct that if the violet wire is still supplying voltage once the engine has been running for a few minutes, this is bad? What specifically powers the violet wire and what opens to stop the voltage from flowing through the violet wire?

My issue is that the GPs and relay work as they should at initial start up, but after turning off, they re energize. I have confirmed this through my after market voltage gage that I have on the dash. When I go to the glow cycle voltage drops and stays down for about 40 seconds (even though the dash light only stays on a short fraction of that time), the relay opens with a "click" after the 40 seconds and the voltage goes back up. Car starts beautifully, but after a few seconds the relay closes again. What first tipped me off to this was the GP dash light illuminating while I drive. At first it would flicker on once in a while. Now it flicker off once in a while. Yesterday when I got home following a 30 minute highway drive the GP dash light was still on. I popped the hood and checked with a volt meter confirming that the GPs themselves were indeed all getting power.

tangofox007 · #6 01-04-2010, 04:09 PM

Quote:

Originally Posted by firemediceric

I guess now what I need is a definitive answer on what specifically triggers the relay to close and send power onto the glow plugs?

The red/brown wire is powered when the ignition switch is placed in the "run" or "start"position. That starts the timer sequence. It also prevents the timer from sequencing back on unless power is removed and subsequently restored.

Quote:

Originally Posted by firemediceric

Would I be correct that if the violet wire is still supplying voltage once the engine has been running for a few minutes, this is bad?

The violet wire should be powered only when the ignition switch is in the "start" position. The cycle ends when power is removed from the start circuit.

It sounds like you have an interruption in the "run" circuit (red/black wire), such that the timer "thinks" that the ignition switch has been cycled off and back on again, thereby restarting the timer sequence.

firemediceric · #7 01-04-2010, 05:11 PM

Quote:

Originally Posted by tangofox007

The red/brown wire is powered when the ignition switch is placed in the "run" or "start"position. That starts the timer sequence. It also prevents the timer from sequencing back on unless power is removed and subsequently restored.

The violet wire should be powered only when the ignition switch is in the "start" position. The cycle ends when power is removed from the start circuit.

It sounds like you have an interruption in the "run" circuit (red/black wire), such that the timer "thinks" that the ignition switch has been cycled off and back on again, thereby restarting the timer sequence.

When you say the "start" position, you mean the glow position, right? This would be just before turning the key further clockwise to crank the engine? Once the engine is cranked and running the violet wire should no longer be receiving power?

Where is this "timer"? Is it part of the ignition key switch?

tangofox007 · #8 01-04-2010, 07:22 PM

Quote:

Originally Posted by firemediceric

When you say the "start" position, you mean the glow position, right?

The glow plugs operate in the "run" and "start" position. The "start" position would be the one that operates the starter.

Quote:

Originally Posted by firemediceric

Once the engine is cranked and running the violet wire should no longer be receiving power?

Assuming that the ignition switch is not being held in the "start" position, yes.

Quote:

Originally Posted by firemediceric

Where is this "timer"?

It is contained in the solid state portion of the "relay" box.

firemediceric · #9 01-04-2010, 08:01 PM

Gotcha! So if the violet wire is continuing to get power once the car has started and the key is released allowing it to return to the "Run" position, the issue likely lies in the ignition switch. If the violet wire is no longer energized, that points to the relay, correct?

Any other tips on this issue to keep me going in the right direction?

Thanks for sticking with me on this.

tangofox007 · #10 01-04-2010, 08:09 PM

Quote:

Originally Posted by firemediceric

If the violet wire is no longer energized, that points to the relay, correct?

I would focus on the red/brown wire. A loose connection or short there could cause the symptoms that you report. If that connection is lost itermittently, the timer cycle will start over and run until it times out, every time power is restored.

firemediceric · #11 01-21-2010, 04:32 PM

I received my new relay and installed it. The problem is now fixed, but this relay works differently than the old in that this one must have the After Glow feature I remember reading about in some other threads.

The GPs light up for 30 seconds then shut off. Once I cycle the key to Start, the GPs come on again and stay on for another 3 minutes, at which point they shut off and stay off. I guess this is the way this particulat Bosch relay is supposed to work, but my old relay (when it was working correctly) never stayed on once the car was started.

I feel I got a good deal on the relay (Sorry Phil, but I had to go with the best deal) Bosch relay and 4 Bosch glow plugs for $98.43 at ******** AZ part # 0250201953 And free shipping