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Old 09-15-2013, 11:40 AM
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Join Date: Jul 2003
Location: Northern Virginia
Posts: 1,074
2 bad glow plug relays in a row?

85 300td. I removed the bosch gp relay because it was not turning off which cost me an alternator. After I r&rd the alternator, I installed a used gp relay from p&p.

The p&p relay tested fine according to the diesel giant test page. GPs tested fine too. Cold engine started really rough and smoked like one or two glow plugs weren't getting juice.

I tried a second p&p gp relay which also tested fine. Cold engine smoked and bucked like one or two gps weren't working.

Just for fun, I reinstalled my original relay. Cold engine started smooth as glass with hardly any smoke, but the relay stays open, which is no good.

What the heck is going on?

2009 E320 Bluetec 117,000
1995 E300D 306,000 sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000
2016 Ford Fusion 24,900
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Old 09-15-2013, 11:46 AM
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Join Date: Jan 2002
Posts: 18,355
Check to see if all plugs are getting 12 volts from the p&p relay.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 09-15-2013, 02:32 PM
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Join Date: Jul 2008
Posts: 3,740
Buy a new one! They are all still going to be 30 year old electronic items....they go bad, sorry Mercedes didn't build things with magic pixie dust from the fountain of youth....
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Old 09-15-2013, 11:54 PM
Zacharias's Avatar
Not so amused
Join Date: Apr 2010
Location: West Quebec
Posts: 4,025
Given that your alternator was killed by the relay, it may have compromised your battery as well. Check it's holding a charge.

Are all the relays the same part number? There are two possibles and based on my experience, one works much better than the other with an iffy battery.

This is just to add to what Kerry advised.


2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22
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