Glo w plug follies
 

I had a glow plug fault recently which had the usual symptoms: glow plug indicator light stays on after start up, and CEL.

I identified GP #6, by the firewall as being completely open and replaced it. My bad: I picked an NGK GP instead of the usual Bosch.

I'm getting 7 ohms on all plugs and this one gives me 12 ohms, and my GP light will still not turn off. Or I should say, with some cranks it will turn off and with most it will not.

Is this a predictable outcome, or would a 12 ohm resistance cause this?

Thanks,

Chuck

My guess would be yes. A difference in the Ohm is a red flag. Hope someone else chimes in as I am going by what I have read on the forum.

There is no way to guess this. First of all your meter sounds out of calibration or something like the ground for the meter you used was poor. If you used the negative terminal for the battery as ground especially. Your ohm readings on cold plugs are far too high.

Find out what is going on first. Normally the plugs should read two ohms or less. The replacement then will read just a little more I suspect. Seem to have some fixed resistance other than the glow plugs in place when reading them. Use the engine block as ground for the meter. Getting the same resistance values would prove the meter is off calabration.

Or short the probes. You should read zero ohms.

The resistance of all the plugs has to be nearly the same, or the light will stay on.

There are several different types of glow plugs that will mechanically fit these cars, but they must be matched to the application. The part number is more important than the brand, as they all look and fit the same. The two main types of plugs are "afterglow" and "no afterglow", in Beru's terminology, GN and GV plugs. The "no afterglow" plugs have simple resistance elements. The "afterglow" plugs are designed to remain on for a while after the car is started. They include a control coil which increases in resistance as it heats. This regulates the ultimate temperature of the plug.

Your best bet is to replace all six plugs with the correct plugs.

If your glow plugs are indeed all 7 ohms and one is 12 ohms, then they are all bad. I suspect your ohm meter is not calibrated or has a very weak battery or .....

I re thought this as he may have measured their resistance on a hot engine as well. Then I would expect a higher ohm reading.

Is this a Digital Meter that has an LED Display or an Analog Meter. The instructions on This Forum are for a Digital Meter.

To check if your Ohm Meter is working properly set it on 200 ohms. Put the 2 probes together and hold the firmly together. If you have a high quality Meter you will get nothing or infinity.
If you have a cheaper Meter the resistance of the Probes and the Probe wires will show up in the display. That is not a problem. You just subtrace that amount from the reading you get when you check the Glow Plugs.

Hey guys,

I think I misspoke. My multimeter is set for 200 ohms and the readings are all .7 ohms except for the #6 GP, which reads 1.2 ohms. For some reason I was thinking that I should be moving a decimal place over for this low of a resistance. Still that's the difference is that the non NGK plugs are all the same reading (.7) and the one next to the firewall (NGK) is 1.2. I've ordered a Bosch with the hopes it will straighten all this out.

Thanks for the tips!

-Chuck

That would be a broken meter ready to go into recycling trash.

Edit: reading your statement again, I guess it depends on what "nothing" is. Note that a "zero" reading is not "nothing", nor is zero the same as infinity.

The resistance measurements should not vary that much between brands for the same part number glow plug. It is more likely that low resistance measurements are known to be inaccurate using a plain old digital ohm meter. There are special meters called a milli-ohm meters that will give accurate low resistance measurements but they are not common nor cheap.

'cmon guys. No matter how bad the meter, these readings should be consistent. It's the inconsistency that is causing his CEL. The safe bet here is six new correct plugs.

Maybe you have a better meter than most. Every digital meter I have (I own about 5) never gives repeatable exact same readings down to 1 decimal places on very low ohm readings.

Take your meter to where you buy glow plugs. Measure the resistance at the counter. Decimal six to decimal eight ohms cold resistance is what I would want to see. Buy the german brand plug and install it with anti seize compound. By and large there is no known good substitute so far.

This should put the issue of if it is the ngk plug causing the issue or not to bed. The other brands are inferior in general usually. I do not know the status of the ngk plugs as to durability non swelling etc.

Not really wise to be test pilot on your particular engine as physical problems with glow plugs and the six cylinder diesels are well documented. Even the german label plugs can cause issues and do.