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  #1  
Old 07-28-2006, 05:12 PM
1980 300CD
 
Join Date: Jul 2006
Location: Los Angeles, CA
Posts: 20
New to me 1980 300CD - working out the kinks.

So I'm driving along- AC blowing on high- radio on. Slowly the fan slows and - the radio stops working and i get no horn- no directionals- no power to any accessory. Pull over- car still running and idling well- lift the hood and 1st thing I notice that the windsheild wiper fluid jug was not put back into the holder by me and was leaning against the glow plug relay (no fluid leaking on it)- I put it back and press on the connectors at the glow plug relay and my wife yells from the car that the radio came back on- she checks the directionals- OK- and turns AC back on- we have full power again. So we're off. Weird. 10 minutes later- same thing. AC starts to blow slowly- radio goes off- no directionals- headlights etc. Pull over again- look at the battery- alternator- glow plug relay- nothing blatently wrong- shut the car off (stupid). Try to restart- dead battery.

I get a jump and bring it to Enrique at MB Motors in Tarzana, CA. I tell him what led up to this and he says it has nothing to do with glow plug relay- tests my battery - alternator - the standard charging system tests. He finds about 12.5 at battery with car at idle all acccessories off. Says my alt. is bad- battery is OK-sends me to his electric guy to upgrade to 80 amp Alt. I go over to his shop and he opens the hood and hooks up a voltmeter while the car is running- not sure what numbers he got- then he unplugs one of the connectors at the glow plug relay- the number on the meter goes up. So he shakes his head and says " You have 2 problems- your alternator is no good- and your glow plug relay is staying on- the combo is draining your battery" So- we install new alternator and I drive away with the connector to the glow plug relay disconnected. This morning I connect glow plug relay- start car- disconnect glow plug relay- drive car with radio and AC- no problems. So I guess it's time the replace the GP Relay. Has anyone had an experience similar- is there anything I can test to make sure it's that and not something else? It's an expensive parts I see prices from 200- 350. So just want to see what the collective experience is with this first. Thanks

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  #2  
Old 07-28-2006, 08:26 PM
Jeremy5848's Avatar
Registered Biodiesel User
 
Join Date: Mar 2006
Location: Sonoma Wine Country
Posts: 8,402
The simplest test is to get in the car, shut the door, wait for the timer on the inside light to time out. Then turn the light on manually using the switch. Now insert your key and turn it to the "preglow" position. The overhead light should dim a little when the key goes on because the current drawn by the preglow system pulls the battery voltage down. You can even monitor the battery with a voltmeter if you want to get fancy. If you listen carefully, you may be able to hear a "click" as the relay closes. One possible reason for not hearing the "click" is that the relay may be stuck in the closed position.

After a few seconds (depending on outside temp), the glow plug light will go out. Don't start the car, just keep watching the overhead light. The light tells you that the engine is warm enough to start but the glow plugs actually stay on for a total of about 30 seconds. When the glow plug relay times out, the overhead light will brighten a bit, the battery voltage will come up a little, and you may be able to hear a "clack" from the relay. Now you can start the engine or turn off the key switch.

If you wait a few minutes and don't get any of that, then the glow plug relay is indeed staying on all the time. If you don't want to replace that expensive part just now, you can always jury-rig a switch that allows you to turn off the pre-glow relay/timer once the engine has started.

Incidentally, the failed pre-glow relay may have caused the demise of your alternator: having to generate that extra current for who knows how long would have put quite a strain on the alternator.

Jeremy
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  #3  
Old 07-28-2006, 08:32 PM
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Join Date: Nov 2005
Location: Iowa City, IA
Posts: 1,647
First, 12.5 at idle is normal for these alternators. I was getting that, thought i had a bad alternator as i expected around 13.00 V. I took it off, put in new not needed voltage regulator, took it to get tested, they said it was fine. Been using it for months. I read this 12.5 at idle fact in the FSM i believe or maybe the Mercedes Technical Companion.

Second, I have a used glow plug relay off of my 1980 300CD parts car I will sell you. 25.00 plus shipping. That is weird that they want so much for that part. I got a new realy for a 1981 300D plus 4 glow plugs for 110.00. Is it so expensive because of the different wiring connections and they dont make as many? To test your relay yourself, a) turn key to glow and get out and listen for a "clunk" at the relay. Should take a max of 45 seconds i think. At that time there should be no volatage at the glow plugs. You can check with a volt meter.
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  #4  
Old 07-28-2006, 09:00 PM
1980 300CD
 
Join Date: Jul 2006
Location: Los Angeles, CA
Posts: 20
Thanks for the info guys- really good- i will do that test soon. I have the whole ignition switch and steering lock mechanism out right now- so when i get her back together i'll certainly do that test. Thanks.
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  #5  
Old 07-29-2006, 04:25 PM
stcbenz83's Avatar
Mercedes Euro 240D
 
Join Date: Mar 2006
Location: Louisville, KY
Posts: 1,056
200 for a glow plug relay??? i spent 100 on the upgraded relay and a new set of plugs
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  #7  
Old 07-29-2006, 07:47 PM
1980 300CD
 
Join Date: Jul 2006
Location: Los Angeles, CA
Posts: 20
the relay on my car seems to be less common.??? 1980 300CD. Any other good places to source part (new) for cheaper??
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  #9  
Old 07-29-2006, 07:50 PM
1980 300CD
 
Join Date: Jul 2006
Location: Los Angeles, CA
Posts: 20
Called local dealer they quoted me 200- ha- when is it ever cheaper to get it right from the dealer??
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  #10  
Old 11-28-2006, 11:23 AM
edward
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Posts: n/a
Do you have an idea of what would be affordable for it? Thank you
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  #11  
Old 11-28-2006, 12:10 PM
Registered User
 
Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
Edward, we need parts for our old cars but this segment of this site is not for selling them I believe. You can private message anyone asking of needing parts of course plus list the car parts in the for sale section. This guy might have a faulty key switch that is feeding a constant on signal to his relay. He has the dash key switch all apart now for some reason. So I have to assume it is perhaps an outside possibility at this point. Or he just loves suffering. Why do you have that apart right now by the way?

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