A few days ago I changed out the radiator hoses and a bad glow plug on my '81 300 SD.

I had trouble shot the glow plugs via Diesel Giant's site. Just the #4 showed up bad. Every thing else, including the reley, checked out fine.

Since doing this work the other day I have strated the car about half a dozen times and have driven over 300 miles. Everything was good until this morning.

I started the car just fine, pulled out of the driveway and the glow plug light started flashing. It flashed about 6 times with several seconds between each flash. A few minutes later it did it again. It then continued to do it intermittantly over the course of a 30 minute drive. It seldom did it on the highway, but it did repeat when I exited the highway. Sometimes it flashed just once or twice.

Maybe related or maybe not, my temp. guage also read higher than usual. It had never been rising much above 80*. This morning it shot up to 90* I cycled the heat on and off while driving and by the end of the commute it was back to holding at just over 80*

Just a freak occurance

some air trapped in the cooling system maybe causing the temp problems.

The glow plugs I can't even hazard a guess or suggestion.

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87 300D Turbo (still wondering about #14 head)
98 Buick Regal GS (one quick grocery gettr)
07 Honda CBR1000RR
04 Dodge Ram Laramie 4x4 CTD (heaven)
95 Dodge Ram 3500 CTD. Need snow plowed?

Flashing GP light usually means one or more of the plugs it out. You might try checking them all again.

My 300TD is usually at 90C when the car is warmed up. From what I have read and seen between 80 and 90C is normal for these cars. I wouldn't worry about that too much. But keep an eye on it and make sure it doesn't do anything odd.

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After the car is cranked the glow plugs should be off. If your GP light flashes I would be inclined to think that someohow the GP's are being turned on, either a bad GP relay or bad ignition switch.

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Good thoughts. Regarding the temperature, I will keep an eye on the coolant level. I was just surprised that after a 300 mile drive with the temp never getting just above 80*, the system may still have air in it, as I guess is indicated by the temperature fluctuation.

The glow plug light really has me stumped. I rechecked every thing after installing the new plug. Every thing checked out fine, including the #4 plug I had just installed, the reley shut off after about 20 seconds, and the car had been started several times afterwards without incident.

If the dash light is correctly indicating that the GPs are coming on while driving I guess it's not hurting anything, but it's not right and I don't like it to be that way. I saw some posts about a loose ground causing electrical problems, but I don't think it would be the case with these symptoms.

It's highly unlikely that the light is indicative of the plugs cycling.

The issue is almost assuredly a bad glow plug.

What are the resistance values from the five plugs as measured with a reliable ohmmeter that can work down near 1 ohm?

Please confirm the full scale reading of the ohmmeter to ensure that your test equipment is proper.

I'll try to do some more diagnostics when I get home tomorrow, but I can state that the GP light did not behave this way when the #4 plug was known to be bad. It did only light for a second or two with the bad plug in place.

The ohm meter is not top of the line, but when the probes were crossed it did zero out and the glow plugs and wires also zeroed out when tested according to the Diesel Giant web site, except for the #4 which I knew was bad. Once #4 was replaced with a Bosch it also zeroed out.

I must misunderstand how the GP light works to indicate a bad glow plug, as I thought it only blinked when the circuit was cycled. This GP light is flashing anywhere from 2 minutes to 30 minutes after the car is moving down the road and very intermittantly

Such a tool can detect an open condition on a glow plug, but cannot determine if one, or more than one, have excessive resistance. You'll need a proper test meter for that.

When the light starts misbehaving, it's almost always the glow plugs presenting a resistance issue or an open condition to the relay. Otherwise, it's the light circuit in the relay itself.

With a proper test of the glow plugs, you'll have additonal data for a diagnosis.

Perhaps I am misunderstanding how to diagnose, so bear with me as I walk you through what I did.

The meter I used at home reads "I" for infinity when the probes are apart. It reads ".000" when the probes are together. On the bad plug I replaced it read "4.0", IIRC. The meter was set to "200K"

Tonight I used a co-workers meter. It's a brand name Fluke meter. On the ohms setting it is labled "1K" When the leads are apart it reads "OL" when the leads are pressed together it reads "3." As I test the glow plugs from the connector on the fender, as Diesel Giant describes, the meter reads "3" at each port on the female plug. To me, this indicates all of the plugs as being good. In additon, I have battery volts at both sides of the fuse. I also have battery volts at each of the pins on the male half of the plug when the key is turned, until the reley kicks it off about 20 seconds later.

It sure tests like every thing is fine

Yes, that was my suspicion. Wrong testing equipment and/or operator error.

You were using a scale for 200,000 ohms in the first instance and 1,000 ohms in the second instance. In both cases, you cannot accurately read a resistance of 1 ohm. Your readings are meaningless.

You must have a meter that can read 10 ohms full scale to effectively test the glow plugs.

Brian, do you happen to know how many amps a single glow plug draws? If the strip fuse is 80 amps, the glow plugs must draw quite a bit of current.

Just curious.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

When dead cold, each plug draws about 15 amps (.8 ohms). But, that current draw drops down quickly as they heat and the resistance climbs.

G'mornin Folks,

A Fluke Multimeter can be a PTA if one is not very familiar with electrical troubleshooting. There are many models of Fluke, mine is an older Fluke 87.
My Fluke has an "autorange" and I find it way too sensitive, so I generally set it to the range necessary to read the value required for a specific test.

Depending on model of Fluke.....there should be several buttons just under the display, above the selector switch.....one marked range.....turn the Fluke on.....set to resistance...the little omega....then press your range button....each time it is pressed it cycles to the next value....watch the decimal point in the OL (Over Load).....
after pressing range, mine initially goes to Mega Ohms...further cycling of range button...
00.xx with nS to the right..is either micro or milli Ohms, I rarely use this scale and don't remember what the "nS" stands for
OL. with the little "omega" to the right, will be Ohms and will read 000.0-999.9 Ohm
.OL with K "omega" will be Kilo (Thousand) Ohms and will read 0.000-9.999 K Ohm
O.L with K "omega" will also be Kilo Ohms but will be 00.00-99.99 Kilo Ohm
OL. with K "omega" will also be K Ohm but will be 000.0-999.9 Kilo Ohm
.OL with M "omega" will be Mega (Million) Ohms, generally used to measure insulation resistance, and it cycles thru as does the Kilo Ohm scales do....

link to Fluke Meters, find your model and then go to manuals.....you can find all sorts of instructional material there... www.us.fluke.com/usen/products/default.htm

I think that you will need the scale with the "Omega"...with will give you a tenth of an Ohm....someone here will correctly direct you

Flukes are generally pretty bullet proof...if you try to measure your glow plugs with voltage applied and with meter set to resistance, you may hear an audible alarm.

I apologize, I did not attempt to use the "character map" to express the little "Omega"...don't know if it will work on this forum....but I think y'all can get the idea.....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

No, not the meter , the actual problem. It seems to have gone away as it did not repeat itself this morning. The GP light functioned as would be expected, the temp guage held steady just over 80* and the car ran flawlessly. I guess the problem resolved on it's own, for now anyway .

My co-worker is an electrician and it was his meter. He assisted by holding one probe to the battery negative as I tested the GPs, but he didn't have any input on how I was testing the system.

When I got home this morning I looked at my A.W Sperry meter from Home Depot. On the Ohms section I can turn the dial to 2M, 200K, 20K, 2K, 200, and a resistor symbol (I think that's what it is.) Anyway, on each of those settings all GPs read "000."

I went to Lowe's and asked for a meter that has a "10" on it and would read just 1 ohm. I was looked a cross eyed and told they never heard of such a thing. Oh well. As long as the problem doesn't re-occur I guess it's fine. Just a "Fluke?"