What engages glow plug relay? The key?

[jessie_eaton]

My car (1997 E300D) will not start sometimes. The starter engages properly but the glow plug light does not come on. I lose the voltage that engages the glow plug relay anytime that the glow plug light does not come on.

I can turn the key a few times and then the glow plug light will come on, I get voltage to engage the glow plug relay, and the car starts fine.

What would cause this intermittant failure of the glow plug light and simulantaneous loss of voltage required to pull the glow plug relay in. Is the ignition switch the only possible culprit?

thank you for ANY thoughts you may have.

[nhdoc]

Perhaps it is the GP relay itself...that is where I would begin.

[jessie_eaton]

Good suggestion.

I checked the glow plug relay. It operates as expected and the relay seems fine. The relay has a constant 12-V (big wire) supply and all six contacts that go to the six glow plugs get 12-volts anytime the relay is engaged. The relay must be engaged from voltage supplied by a smaller wire.

The problem is that I don't always get the voltage (small wire) required to engage the relay. I need to determine why I sometimes don't get this voltage when I turn the key.

Could this be only a problem at the ignition switch or could it be something else (the computer or something)?

[Jim H]

Originally Posted by jessie_eaton

Good suggestion.

I checked the glow plug relay. It operates as expected and the relay seems fine. The relay has a constant 12-V (big wire) supply and all six contacts that go to the six glow plugs get 12-volts anytime the relay is engaged. The relay must be engaged from voltage supplied by a smaller wire.

The problem is that I don't always get the voltage (small wire) required to engage the relay. I need to determine why I sometimes don't get this voltage when I turn the key.

Could this be only a problem at the ignition switch or could it be something else (the computer or something)?

It's either the ignition switch directly, or a relay. A wiring diagram is needed.

Best Regards,
Jim

[pberku]

Another likely culprit is the glow-plug strip fuse. It is notorious for developing hairline cracks, and/or making intermittent contact with its mounting screws. Change fuse and try again.

Phil

[jessie_eaton]

Jim: I agree - I would love to have a wiring diagram but so far I can not locate one. I apologize if the post on the diesel discussion board was also mine that you responded to. I'm not sure if it was. I posted my question in the "Tech Help" AND the "Diesel Discussion". Please let me know if that's taboo and I will not do this again. I was obviously looking for maximum exposure. Thank you for your insight on my glow plug problem.

Phil: Where is this fuse that you speak of? What amperage rating is this fuse? I have seen no fuse. Thank you also for your input.

If I don't respond for the next few days it's because I just found out I live in a mandatory evacuation area for Hurricane Dennis. Tomorrow will be busy boarding up the house and who knows when I'll get back to the computer or have electricity.

Thanks again guys.

[jessie_eaton]

FYI: I closed the thread on the diesel discussion board to eliminate confusion and duplicated efforts.

[Jim H]

Originally Posted by jessie_eaton

FYI: I closed the thread on the diesel discussion board to eliminate confusion and duplicated efforts.

Please accept my appolgy, it was rude of me to yell. (I have removed my rude remarks.)

I do not think that the 80A strip fuse will be a culprit in this case, since you state that the relay sometimes activates, and other times does not. The fuse is a flat strip of metal bridging two phillips screws on top of the relay.



A melted fuse strip does not affect the relay; the coil still energizes and the contacts close, but cannot apply 12V to the GPs since the strip is open.

It may be the ignition switch contact, it may be a connector.

I hope somebody here has a wiring schematic and can help further.

Best Regards,
Jim

[Jim H]

Originally Posted by jessie_eaton

FYI: I closed the thread on the diesel discussion board to eliminate confusion and duplicated efforts.

My appologies for yelling. Double-posting is not taboo, and will get maximum exposure, but it can also be counter-productive, like trying to listen to two simultaneous conversations.

You said that your relay is not receiving power at times. That said, I don't think it will be your fuse strip, since if the fuse melts the relay will still continue to function, the coil will energize and the contacts will close, there just will be no 12V applied to the GPs since the fuse is open.

Sorry, I don't have a wiring diagram for that series automobile.

Best Regards,
Jim

[whunter]

Originally Posted by jessie_eaton

FYI: I closed the thread on the diesel discussion board to eliminate confusion and duplicated efforts.

FYI: Did you know that you can delete threads started by you, Click the check box then the delete button.
I deleted the thread I started in Diesel Discussion, here are the posts.
Thank you Wes and P E H.

whunter
Senior Member Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 3,002

To answer your question:
The most likely cause of your problem is a bad key switch or failing glow plug relay.
You can open the glow plug relay and check the contactor points, if they are badly burnt, it may be possible to clean them with a file.

whunter

#3 Today, 12:04 PM
Wes Bender
Retired User Join Date: Mar 2001
Location: Alpine, AZ / Green Valley, AZ
Posts: 466

I think he closed it intentionally. He left the same thread open on the Tech forum...

(but your advice is right on)

Wes

Wes Bender

#4 Today, 12:29 PM
P.E.Haiges
Registered User Join Date: Jan 2000
Location: PA
Posts: 3,930

There is a timer in the GP relay that keeps the GP energized for about 45 seconds. Possibly that circuit is not working properly and not keeping the GP turned on.

P E H

[jessie_eaton]

Thanks for the replies. Hurricane Dennis has not allowed me to work on my problem. I'll review the posts (and my car) more closely this weekend and report back.

[jessie_eaton]

There are two power supplies (inputs) to the relay and 6 outputs (going to each glow plug). Both power inputs are 12-v but one is a small wire for small current and the other is a larger wire for more current. The small wire is used to "engage" the relay by closing the contract inside the relay. Once the contact is closed, this allows the main power from the large wire to power up the six outputs / glow plugs.

BIG INPUT WIRE:

I always have (high-amp / LARGER WIRE) 12-V supplied to the relay as I should. This 12-v is ALWAYS available regardless of ignition-key postion. This is GOOD.

SMALLER INPUT WIRE:

I do not always have (low-amp / SMALLER WIRE) 12-V supplied to the relay when I should have this voltage. This is bad. In other words, I should get 12-volts to close the relay anytime the key is turned from the "off" to the "on" postion. Perhaps one in 10 turns of the key result in no voltage thru this smaller wire. Therefore the relay does not close and the 12-V from the larger wire is not allowed to power up the glow plugs.

The relay operates as it should. I think the problem must be with the key / ignition switch or a computer problem. But this is a guess as I don't have a wiring diagram. I can't do anything about a computer problem so I will get a new or used ignition switch.

I don't think the strip fuse you mentioned is the problem because the relay operates as expected. I always have pleanty of amperage available anytime the relay is engaged. The problem is the signal to enage the relay is intermittant. This is not good.

As always, thanks for your input. Your input helps me to understand how the system works. It is always best to fully understand a system before you diagnose the system. I doubt I will be able to gather enough information to fully understand the system - but this ain't a perfert world.

[jessie_eaton]

The startup problem that I've described in previous posts was due to a faulty K40 relay. I got lucky and bumped the relay and then my glow plugs energized and diesel-flow was allowed for combustion. A few more failures and a few more knocks to K40 confirmed the diagnosis.

K40 costs about $87 at the local dealership and takes less than 5-minutes to change. K40 is a black box that contains a circuit board, 3-relays, and 3-fuses. I inspected K40 and no problem could be seen (I even used a magnifying glass). K40 is located in another black box at the passenger's side / back of engine compartment (there are a few other items in this black box - I think they're the computer and N35).

From what I understand, K40 can lead to strange symptoms for both diesels and gasoline models.

I could not have located the problem w/out clues from this forum. This is the type of intermittant problem that can cost big money to fix even if you're lucky enough to have a mechanic with honesty and intergrity. Thanks to all on this forum.