This (measuring resistance of glowplugs) makes sense providing that it is properly done. This is a project that I would like to do one of these days. I have made some crude measurements and have noticed that the resistance changes very quickly once the engine is turned off, as others have noted.

A thermistor is, of course, just a resistor deliberately made to have a large temperature coefficient. Better quality (more expensive) thermistors have a coefficient that is more repeatable from one unit to another and they are less likely to drift with age. Glow plugs, like any resistive load, can be used as thermistors but they cannot be trusted to be repeatable so they must be "calibrated" by measuring their resistance brand new (cold) and from time to time during their life.

An ideal situation would be a friend who happens to sell glow plugs (e.g., Phil) and who will allow you to sit in his stockroom with a carton of GPs and a good ohmmeter, cherry picking until you have the best set (closest match of resistance at room temperature) you can find. Next, you take them home, put them in an oven, and repeat the measurements at various temperatures. All of the GPs are at the same temperature and hopefully they all behave the same. That gives you a calibration curve for each one. You then install them in your car and make hot/cold measurements as often as you have time until the first one burns out, thus breaking the set. That data I would like to see.

There are three other things necessary for good data:

First, because even a hot glow plug in an operating engine measures only a few ohms, you need a meter that can repeatably measure resistance to two or three decimal places. The absolute accuracy of the measurement is not as critical as the repeatability – you are looking for a difference between hot and cold, between a "good" glow plug and a bad one, and between "now" and "later."

Second, you need to use heavy wire leads with good copper connectors that have almost no resistance themselves and you must always use the same set of leads and connect to the same points so that your measurements can be compared one to another.

Third, you must decide whether to connect directly to the tip of the glow plug (easier in some engines than in others, especially with the engine running and hot) or to the plug that connects the glow plugs to the relay. The wire harness will itself add to the total resistance and technically should be measured separately so it can be subtracted out (but maybe I'm just being anal). If you can live with that error, the plug is a great place to connect because it's easy to access and the (female) pins are exactly the right size to accept a banana plug, which makes for easy connecting.

BTW, I have given the "thermocouple theory" a little thought and would like to solicit opinions from others. A thermocouple generates a voltage between two dissimilar elements. This voltage changes with temperature. Any voltage generated in a glow plug circuit has to be because of the copper wires and the steel or other metals in the wires, connectors, and terminals. If there is a thermoelectric voltage generated in the glow plug itself, it is likely very small and swamped by the voltage generated in the other components (wires and terminals). Therefore, I would recommend against using this method.

Jeremy

__________________

"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

Personally I don't think you are having an out of ordinary experience starting your 606 or any other diesel at Zero or negative degrees. It may have stumbled....but it did start! And after a few minutes it ran fine. You need to figure in windchill temp if parked outside......and plug in the block heater when parked outside in the wind at thoughs temps. The longest injection line is on the injector for cylinder 6 near the fire wall and that is probably the one that is effected the most by the slowed delivery of gelled fuel.
I travel and ski the northern mountains of New England with my 98 E300 with temps dropping below zero at night with windchills to -20 or so. If I forget to plug it in overnight it will always start on the first try but not necessarily on all cylinders and it takes a few minutes to get all the juices flowing. If I pull away before it has a chance for the engine to fire on all cylinders it is in limp mode. The glow plugs function fine with no problems and there would be no CEL for gelled fuel and a super cooled engine.

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

Windchill means nothing to a car. A car parked in -5F in 100 mile per hour winds will never get below -5F, it just might get there faster due to convective losses than it would with no wind. It's the temperature that matters and I can personally attest to starting mine at -5F with not a single stumble and not using the block heater so I can tell you that based upon my experiences these cars should not stumble when cold if everything is working as designed.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Jeremy, I think you're way overthinking this. I personally don't care what the resistance of a hot glow plug is. As long as it's much higher than when it's cold, you know the plug is working. It's like a light bulb. It either works or it doesn't. FYI when I say hot I mean right after the plug was glowed and not just hot from engine heat which is not hot enough.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Yup on the wind chill (don't have much experience with starting in super-cold weather). All it affects is how quickly the engine will cool, but it will never get below the actual temperature of the outside air.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

MartyD......"convective losses" and the length of time that they occur is exactly what I was refering to. "wind chill" was probably a poor choice of terms but the fast rate of cooling/ convective losses as assisted by the wind effects all fluids (diesel fuel), semi solids and solids differently than a slower temperature change.

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

I was talking about leaving a car outside, overnight, in -5F. Regardless of the wind it would be the same temp the next morning in my opinion because that's enough time for the entire engine to get down to ambient temperature regardless of whether the wind is blowing or not.

And I'd disagree with the statement "the fast rate of cooling/ convective losses as assisted by the wind effects all fluids (diesel fuel), semi solids and solids differently than a slower temperature change".

Getting them to ambient temps, fast or slow shouldn't make any difference.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz