Glow plug fun....Resistance Values.. 95 Sub

[rameye]

Having fun with a no cold start situation.....plug the block heater in and she fires up in one revolution of the engine.

Thought the PCM might been misbehaving as far as the GP cycles go...did some probing around and found the ground pin in the plug to the controller is NG...open circuit somewhere down the line. Spliced in a new ground, installed new made in USA controller/relay, lite comes on, ....no start. Got crazy and let it crank for about 15-20 seconds and she made her own heat and started. Ran perfect upon starting, no codes, no issues.

Noticed some other anomilies..Voltage really doesnt take a hit like it used to when the GP relay is activated. Ran another heavy gage wire to the controller.... rule out bad wire ...probably not.

Put an amp clamp on the input side...only drawing 50 amps upon relay activation, GMC manual says 100 amp minimum draw for the GP's

Have my multimeter on the GP's ...ground to engine... probe on GP spade..zero resistance on 2 so far on the drivers side...

Question...what is a good resistance value for these quick-heat GP's...?? Ballpark is fine....anything but zero??

Thanks

[DmaxMaverick]

An open circuit plug is obviously bad. Resistance should be near a dead short, less than 1500 ohms, depending on the plug brand, age, and your probe contact, typically less than 800. If you've tested 2 so far, and they are open, you have 2 bad plugs, so far. It could surely explain your low amp draw, and hard start issue. A long time ago, I quit ohm'ing plugs. Open = bad, of course, but a successful continuity test will usually confirm a good plug. If the resistance seems weak, I'll pull it and throw some batt voltage at it and watch as it heats up (go right to the horse's mouth). At 12V, a single plug should be glowing bright red in about 2 seconds, and an AC60G takes about 5.

The engine should start with a couple/few bad plugs. As more fail, starting gets harder, especially in colder weather. It will reach a point when you have too few plugs operating for a good (or reasonable) start. Longer cranking will generate cylinder heat (air friction), and the remaining plugs will help just enough to get it running. Lose one more, and it doesn't happen, no matter how long you crank.

My 1985 6.2 will start spring/fall on 2 good plugs, and no plugs during mid summer. Can't say the same for the 95 6.5, it barely starts on 5-6, in the heat of summer. I see similar results from like year models.

What you are experiencing is rather typical for your era of engine. The glow plug system needs to be reasonably healthy for reliable starts. It becomes more important as the temperature drops.

If your "quick heat" plugs are Kennedy plugs, you may want to contact him about it. He got a bad batch a couple years ago. The plugs failed prematurely, no rhyme or reason (similar to the Ebay plug experience). Otherwise, just put in a fresh set, and don't look back.

[DennisG01]

Hmmm. Trying to remember... I think you should be looking for 1.2 to 1.8 ohms (NOT "K" ohms).

Quick heats? Unfortunately those have been NOTORIOUSLY awful over the years. You never know what you're gonna get - they might last a year or two, they might burn out the first time you use them. I'll never use them again - went through about 12 plugs in less than a year.

[rameye]

Thanks Dennis...

Yep they were the Kennedy quick-heats...put them in maybe 2 years ago...

Do you have a reccomendation for a solid plug??

The ones on the turbo side cant suck more to change...maybe I'll do just 6 and wait till spring for the 2 real fun ones!

[rameye]

One more question....

Wouldnt a dead short/no resistance on continuity mean the glow plug is toast??

I figured if I got a resistance value it was do to the heating coil..

Obviously I'm not an electronics engineer

Thanks again

[DmaxMaverick]

One more question....

Wouldnt a dead short/no resistance on continuity mean the glow plug is toast??

I figured if I got a resistance value it was do to the heating coil..

Obviously I'm not an electronics engineer

Thanks again

This usually indicates a good plug. A bad plug will usually be open, as in, no continuity. If it has continuity, the heat from the battery voltage has to go somewhere. If you have doubts, pull the plug and hit it with some battery voltage and watch it.

[DennisG01]

Thanks Dennis...

Yep they were the Kennedy quick-heats...put them in maybe 2 years ago...

Do you have a reccomendation for a solid plug??

The ones on the turbo side cant suck more to change...maybe I'll do just 6 and wait till spring for the 2 real fun ones!

The 60g's are always a safe bet. Lot's of guys use them and they seem to last a long, long time. I personally have Beru - only because I wanted to try something different and my engine builder (Ron Schoolcraft) uses them. However, he only has them because he got them as a door prize from Penninsular (who sells them), although he's had them for a long time. I've also heard good things about the Bosch Duraterms.

Also, just found my old notes on GP's. Ohm value should be .8 to 1.4 ohms. I think the idea is that it should be more than 0, but as it goes over about 1.5, it starts to lose it's max heat.

[rameye]

Installed new Glow plugs...fired up in 1/2 revolution...problem solved.

I never suspected the plugs as being bad because they only had 15K on them.....guess I got stuck with some from the "bad lot"

I did install a manual glow plug overide however....in case my PCM disagrees with my idea of a good glow cycle.

Thanks

[DennisG01]

Glad you got it figured out!

[More Power]

Wouldnt a dead short/no resistance on continuity mean the glow plug is toast??

I don't recall anyone reporting a confirmed dead short in a glow plug. I suppose it could happen, but if it did it would burn out the fusible link wire for that glow plug. All eight glow plugs are powered through fusible links, which helps to prevent a fire in the wiring due to a gp short.

I prefer to test glow plugs using the 20-A setting on my multimeter. Unplug the glow plug, touch one meter probe onto the positive batt terminal and the other on the gp spade terminal. A good PTC (Positive Temperature Coefficient) plug like the AC-60G should draw about 16 amps when stone cold, which then tapers off to something less than 10 amps after a few seconds.

Jim

[rameye]

Jim...thats me the non-electrical guy...I meant to say that it was a no resistance reading. on the Ohm meter.

Sorry if my lack of terminology piqued your interest.

[DmaxMaverick]

No resistance is an open circuit (no needle or digit movement on the meter). Bad plug.

A dead short should zero the meter. Test this by touching the probes together. This is also how you calibrate the zero on a Simpson type meter (analog, with a sweep needle). Some older digital meters (like early Fluke) have a calibration mode.

I like Jim's method of the amp test. I've never had a problem with the open/closed test, and always got a good diagnosis on the plugs. My meter has a continuity test mode with a tone. If it tones, the plug has always been good. No tone, bad. (works the same as a test light) Depends on how much information you want to toss around in your brain.

[rameye]

On behalf of the electrically impaired...Thank-you for the explanation.

[Robyn]

The only sure way to know what a plug is doing (other than one that shows no movement on the needle of a VOM) is to hook the little creature to 12V and see what it does.

If it glows nice, its good, if it take a long time and only sort of glows in the center area (DULL Red or barely hot) toss the sucker in the brush and replace it.

Your makeing a real tough and technical job out of a simple good or bad issue.

If it glows good screw it back in, if it dont, toss it.


Robyn

[tommac95]

not to belabour this ... but:
The (electric) loads drawn by resistance heating elements and incandesant bulbs is a tad problematic. When connection is initially made (to an apt power supply) , the current soars far beyond 'normal' . Once the element heats up , its resistance increases dramatically , which dynamically limits further current, and actually decreases the current flow to that which only maintains the hot element.

ApplicationSpecificIntegratedCircuits have been designed which purpose is solely to moderate the current inrush while the elements are cold (and resistance low).

[JohnC]

Well, to be semantically correct...

No resistance is the same as a dead short. Infinite resistance is an open circuit.

Most bad glow plugs will open (infinite resistance). Good ones read close to zero (no) resistance.