Which came first? Bad plug or bad harness?

TravisG

Active member
Joined
Nov 9, 2000
Location
Salt Lake City, UT, USA
I replaced all 4 plugs about a month ago. A few days ago the CEL came back on. I went to the dealer and picked up a harness. While preparing to change the harness, I thought I would go ahead and test the plugs again. Suprisingly one ready 5.6 instead of .8 like the others. I removed that plug and found it to be covered in soot. While replacing my glow plugs a month ago, I noted that the bad plugs had a significant amount of soot on them. I didn't have a replacement plug with me, so I cleaned it off and replaced it. After I replaced it the reading changed from 5.6 to .8!

Is it possible this plug went bad in 3-4 weeks? Or is it the harness? Could a bad harness connection lead to the plug sooting and subsequently providing a high reading? Do I have a faulty 4 week old plug or a harness that is causing my plugs to fail?
 

Borborygmi

Veteran Member
Joined
Nov 13, 2002
Location
Cedar Park, TX
TDI
2003 Jetta Wagon
It's possible for any manufactured item to experience infant mortality. It's virtually impossible for the harness to cause the glow plugs to fail.

I kind of doubt your glow plug was defective, it's easy to get a false reading with an ohmeter when decerning low resistances. It's possible cleaning it up gave you a better contact. As for the soot, you didn't say if you pulled one of the presumed good plugs to observe the soot on that one for comparison.

The glow plug relay has two sets of contacts, each of which switches a pair of glow plugs. There are two very small resistors in the relay in series with each glow plug circuit for current sensing. The ECU can determine if there is a fault, which is almost always less than expected current in one of the circuits.

Excellent contact is essential in a high current circuit. While the system was designed to discern a faulty glow plug, cheap wiring and oxidative processes cause many or most of the realized faults.

It appears that some board members have had success by carefully cleaning the glow plug harness and glow plug connectors, and applying dielectric grease to retard oxidation.
 

Growler

Got Soot Vendor
Joined
Nov 24, 2003
Location
Millersport, Ohio
TDI
Schmutz, 2015 Golf Sportwagen DSG & Schnurren, 2001 Golf GL 2 door 5M
when you replace the harness..

it only has 2 connections, and the old wiring had 4 wires... which wires connect together?

or are there 2 different harnesses?
 

ymz

Top Post Dawg
Joined
May 12, 2003
Location
Between Toronto & Montreal
TDI
2003 Jetta TDI Wagon, 2003 Jetta TDI Wagon
There are 2 different harnesses for the ALH motors:

The older one (through 2001 ??) had only 2 wires... it's a bit cheaper to replace than the newer model which has 4 wires...

If you don't care about getting informed of problems with the glow plug system, you can just short out the wires inside the harness (they're all "hot") and you won't receive any GP warnings - real or imagined...

Yuri.
 
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