Dim lights for the first 3-5 minutes

[jhermit]

My 1982 300SD has dim lights on the dash and headlights for the first 3 to 5 mins after a start even when at operating temperature. It then goes to normal full strength and all is well.



Is this normal?

[psaboic]

Sounds like you have a glow plug relay that has the "afterglow feature". Some glow plug relays turn off a few seconds after the engine is started, but at some point Mercedes installed glow plug relays that keep the glow plugs on for 3-5 minutes after starting to help with cold engine drivability and cold engine idle. Glow plugs draw a lot of current and if you have an older alternator that is marginal it will definitely cause dim lights until the relay de-energizes. So, unless you have other issues, I would say it is normal. My 83 300D has the afterglow relay and has worked well for years.

[jhermit]

Okay, I had not heard of this "afterglow feature". Thank you!

[jay_bob]

Quick check for this, on top of the GP relay, there are 2 connectors, one has 5 heavier leads going to the engine, and the other connector has 4 small wires. Unplug the small connector after starting the engine, this will disable the GP relay and see if your voltage comes back.

I would be looking at the alternator still, but first check your battery and alternator connections, the ground connection to the body at the battery, and the main ground between the engine and body as well (this is under the car).

[Junkman]

None my 84 or 85SD or 85TD have the "after glow feature" so I doubt the 82 would have it unless it was an early design that was deemed unnecessary.

[WDBCB20]

To prep my '83 SD to run on WVO I replaced the stock glow plug relay with the afterglow relay. Did not notice dimming lights which doesn't mean it wasn't happening.

[hercules]

Glow plugs would be long gone if was timer.
Alternator or regulator most likely.

[87tdwagen]

Does it change with rpm? If you try revving the engine when the lights dim, do they then brighten?
If so, I would suspect your voltage regulator carbon brushes are at end of life. Luckily, the voltage regulators are inexpensive, easy to change and worthwhile having one as a spare in the trunk just in case.

[Murkybenz]

If you notice a slight flickering of your interior light at night at idle thats usually a good indicator the voltage regulator brushes are getting short.
I had an old ford years ago that after driving for 30 mins or so at night the headlamps would go dim , turns out the battery was tired and alternator put out about 12.6 volts which was enough to stop the warning light coming on but not supply enough to keep everything happy.
Modern cars are not so forgiving of these kind of traits they just stop dead , this is what I love about these old Mercedes is that you get plenty of warning before things need to be fixed.

[psaboic]

Ok, folks, this is from my personal experience. I have an 83 300D with the afterglow feature on my glow plug relay. I have also had a non-afterglow relay in the same car with the same alternator and the same glow plugs. With the non-afterglow relay the glow plugs would shut off within seconds of the car starting and all accessories worked normal. With the afterglow relay the glowplugs would stay energized for 3-5 minutes after starting. During that time, the headlights would be dim, and the wipers would work slower than normal along with the blower motor. As soon as the relay timed out, everything worked as normal.

I hooked up a digital voltmeter to see what was going on (more out of curiosity than concern) and it would read about 12.0 to 12.2 volts during afterglow with the headlights and blower motor on. Revving the engine would not change the voltage output. Once the relay timed out, the voltage popped back up to 13.4 to 13.6 volts and again, revving the engine did not change this. And no, it did not burn out the glow plugs prematurely, I've had the same set for over 3 years in my daily driver.

Yes, it is possible that the OP has an alternator or regulator issue (a good test with a digital voltmeter should answer that question) however, I believe that if he had an alternator, regulator, or even a battery going bad, the problem would show up more frequently than just 3-5 minutes after starting each time......

[jhermit]

I am going to do the voltmeter test and check the regulator this weekend. It is 15 degrees outside, so not eager to get out there and mess with this right away...

[tangofox007]

Originally Posted by hercules

Glow plugs would be long gone if was timer.

I have had an "afterglow" timer in my '82 300D for over 10 years. No issue with glow plugs. (I suspect that glow plug longevity has more to do with the number of cycles than cycle duration.)

[tangofox007]

Originally Posted by Junkman

None my 84 or 85SD or 85TD have the "after glow feature" so I doubt the 82 would have it unless it was an early design that was deemed unnecessary.

I doubt that there are many '82 models with their original timer still working. (My '82 is on timer #3.)

[vwnate1]

Noe of my W123's have the after glow GP timer, was it perhaps a cold weather area (like Mid West ?) thing ? .

I too would be looking at the alternator's regulator and brush holder as the brushes simply wear out over time and are dead easy to replace albeit not out side in the snow....

FWIW, I experimented with many different aftermarket brands of these and _only_ the BOSCH or HELLA branded ones charged to 14 + VDC .

man claim the German HUCO ones ad others do but they don't ~ I put a meter on everyone who claims this and not a one ever makes the fully charge necessary .


Only a couple dollars more for the right part .

[JHZR2]

These alternators don’t really put out a lot of voltage to begin with, and with cold start/low rpm/weak battery, they may not get the field sufficiently high. You may well have a weak battery and/or some parasitic discharge. The batteries in these cars are pretty large and can handle a decent bit, but if they are slowly depleted, then the alternator will get bogged down trying to pull it back up.

A multimeter with readings when the car sits, cranks, and cold/hot idles would be helpful. They sell usb chargers that go in the cigarette lighter hole and read out voltage. These are helpful when diagnosing things. Recommend you get one and take notes of the readings awhile. Report back.