W123 Glow Time
 

Happy Tax Day. My glow plugs burn for a good 2 seconds when first starting out on a regular 65° morning. It starts fine and I consider that normal. But when it's cold outside, the plugs glow for about .5 second and I have to cycle them 2 or 3 times to make sure it's warm.

Is this a relay issue? Surely somebody else must have experienced this too.

There is 2 things going on when you pre-glow. One is the Glow Plug Light comes on and that stays on related to the temperature. In general people use that light as a prompt to turn the key to crank.

At the same time inside of the relay is a timer. If you ignore the glow plug light and don't turn the key to crank when the glow plug light goes out on a nice crisp Southern California morning the glow plugs remain on for about a total of 27 or so seconds till the timer runs out.

In addition to those 2 things when you are cranking the starter the glow Plugs are again connected during they time you crank.

That is what happens on mine. The coldest I have ever started at was about 42 degrees F.

I also had my Glow Plug Light go out but I could tell the glow plugs were still working so I never bothered checking the Bulb. About a year later the glow plug light stated working all by itself.

On a cold day simply ignore the glow plug light and keep the key in the pre-glow position as long as you like or till the timer shuts off the glow plugs and attempt to start.
If it was extremely cold you can turn the key off and start over with another timed cycle of the glow plug relay.

On your year and model there is a Temp Sensor inside of the Glow Plug Relay.

Am I wrong in assuming the plugs glow for as long as the rest of the bulbs are dimmed by some apparent draw?

That would depend on whether you start cranking before or after the timer runs its course.

No.

Then they are glowing for 2 seconds warm and half second cold. Shouldn't it be at least the other way around?

FYI: Tax Day this year is May 15.

Sure about that?

That doesn't describe normal operation under any condition. It might be time for a new timer. In fact, I would bet on it.

Glow Plug Timing
 

I'd check the actual glow plug time with a test light .

My bet is your relay is nearing end of life .

You can use the seat belt lamp as a full glow cycle timer, I find that helps me to get *instant* starts in temps below 40* F .

Remember : the glow plugs are only one part of the stating system ~ you need to have the valves adjusted at least one every year plus new filters and no crud in the clear plastic fuel intake screen.

If you're handy with a soldering iron, replace the capacitors in your glow plug relay. I went through stupid glow plug timer behavior with my SDL a few years ago and replacing the caps fixed it. See linked thread:
http://www.peachparts.com/shopforum/diesel-discussion/383193-glow-plug-relay-troubleshooting.html

For your apparent fault condition the above suggestion is well worth an attempt. Chances of it solving the problem are good. A time constant type circuit is very dependent of the capacitor in it.

It's unlikely that the capacitors themselves are used as timing devices, but if they're used as filters or decoupling capacitors, their failure will result in erratic operation of the timer circuit they're attached to, essentially causing rapid reset of the digital circuitry or faking the chip into being "overclocked", thus the abbreviation of the time limits.

*May 17

Are you judging by the dash lamp or by the audible click when the glow-plug relay turns on and off? I can hear it in the cabin with the windows up, but open the door and hood if needed. The dash lamp is just a prediction "should be able to crank now", based upon a temperature sensor inside the relay. The relay should stay on for about 25 sec regardless. If the dash lamp doesn't light, that indicates that at least 3 glow plugs are bad (or bulb burned out). If your relay doesn't stay on for >20 sec (should hear a loud clunk turning off), it is defective. You can also connect a voltmeter to any glow-plug and measure from BATT- to see when "on". I never fooled around inside the relay, but there are posts w/ photos. I have a bag of maybe 3 spares and never had an issue with one.