Is my glow relay dying?
 

I'm having some issues on cold morning starts. Engine cranks and cranks and eventually starts, but it used to start easier. Valves were last adjusted 7000 miles ago, last January by an experienced MB mechanic.

I checked the glow plugs, all show around an ohm or less. The fuse strip in the relay box seems to test good. When I measure its voltage vs ground with ignition off, it shows 12.4V. As a buddy turned the ignition to the "run" position, the box clicks and voltage on the strip drops to 10V for 2 seconds, then returns to 12V. The glow dash light never comes on, hasn't for several years. When we did this test, the car had been sitting out all day, about 55F.

Seems to me I should get a bigger voltage drop if I want these plugs to get nice and hot, and for longer than 2 seconds at that. Is my relay toast?

Thanks,

You should be able to hear the relay open. Is that occurring after two seconds?

Well I don't hear a click after 2 seconds when the volts return to 12.4 - the rise just kind of seems to happen. The initial relay click when the ignition is turned to run is quite noticable.

The Relay has the ability to say on for a total time about 30 seconds if you hold the Key in the Preglow Position before it times out and shuts off.

If the timer is shutting the Relay off a lot sooner when you do the above you have some issue with the Relay.
A good Glow Plug should get hottest at Tip and the Tip should remain the Hottest part. If they are getting hottest in the middle the Glow Plug is no good (this happend to me with one Glow Plug).

If the Relay is OK it is one of those instances where the Glow Plugs could be removed and tested on a Battery.
While the Glow Plugs are out Ream or otherwise clean the Carbon out of the Glow Plug Holes.



Have you ever checked the Glow Plug Light Bulb to see if it is burnt out?

The opening click is usually equally noticable. Measuring the elapsed time from click to click might be worthwhile.

Charlie,
You could try redoing your test except from the strip fuse to the GP. This will show if the voltage drop is in the GP or the relay/wiring.
If you get a drop of more than a couple of volts from the strip fuse to the GP, I would be cleaning the contacts in the relay & all other connections.

Thanks to all for the suggestions. I listened closely this morning - put key in "on" position, heard relay click closed - 2 seconds later, there was a fainter click and the dome light came back on full brightness. So it does click when it opens back up, but it definitely only stays closed for 2 seconds even at 40F. Sorry for the previous statement that it doesn't click open.

Will check for voltage drop bt fuse strip and GP first. Question - even if glow plugs themselves are getting flaky, shouldn't the relay stay closed for more than 2 seconds? Second question - if I find a large voltage drop, how do I open up the relay for cleaning?

Thanks,

Do you know any other older model Mercedes owners in your area that might let you swap in a known good relay?

I don't know if the Mercedes relay is servicable or not. My F250's relay was shot and when I destroyed it tearing it apart (non-servicable unit) I found the contact disk fully toasted. The new relay made a world of difference.

If the relay is only closing for two seconds, there has been a failure in the solid-state portion of the timer. Time for a new unit.

Other than the relay itself. I do not know how it senses if the engine is hot or not. If the relay senses by whatever method the 1985 uses that the engine is already hot the relay will drop out quickly. Find out the system it uses and check it out it should be easy to do.

A salvage yard may turn out as a good source of a replacement relay as well if required. It does sound other than the external sensor circuit possibility it is the relay. Some of our guys that advertise used parts on the parts section might have a spare one to sell as well.

It doesn't.

X2
Even if there are problems with the GP's, the bad timer is your primary problem.
No point prizing the cover off the GP controller as you cant fix it easy.
Once replaced and all good, you can prize it off & see how it works.

I had another customer with similar problems, his was for 6 seconds... swapped in a used good relay, and he's back to 32 seconds, and instant starts.

FOLLOW UP: Installed new relay, car starts much easier. The real test will be when the temps drop below freezing - should do that tomorrow night.

I suspected the glow relay, but before blowing $120 on a new one, I wanted some confirmation. Thanks to all who replied, I'll keep the other tips in mind in case of any future glow issues.

I didn't go to the boneyard because I've had bad luck with parts there - most anything electrical, or made of plastic or rubber, has been pretty much shot by the time I get it, so it seems.

thanks,

Hi guys,
Sorry to butt in, but this seems like the best spot to insert a related question. I've been struggling with lousy starting in my 83 300d for about a month now, and it's now fully unable to start without a jump from another car. Accompanying this is a loud whine that sounds like it's coming from the glow plug relay, and no movement at all from the starter. Has anyone come across something like this before? (It appears there is some sort of parasitic draw on the battery that easily drains it overnight) Could either or both of these be caused by a defective Neutral safety switch? or is this a number of separate problems working together?

I've tested the NSS/ignition bypass in front of the battery with a jumper wire, which will crank, albeit with insufficient power from the tired battery. This seems to suggest a faulty NSS, non?

poor starting/inability to start, and relay whine.

would low voltage at the glow plug relay (about 7-8 volts at the main fuse) from the drained battery cause these symptoms? or am I thinking about this the wrong way? I should mention the battery is under a month old. What could cause that kind of voltage drop?

The glowplugs themselves, as well as the associated wiring, appear to be fine. I tested them using Diesel Giant's walk-through

Sorry about idea vomit. Your help is greatly appreciated :)