CDI Glow Cycle Length - PeachParts Mercedes-Benz Forum

shertex · #1 01-17-2016, 08:42 PM

I know that, because of the design, the glow cycle on an OM648 is much shorter than on an IDI. But, so far, down to maybe 25F, I've never scene the glow light stay on longer than a second. What happens at, say, -10F? Does it say on significantly longer? Or is it always really fast irrespective of ambient temperature?

DDAY · #2 01-17-2016, 10:02 PM

I have a 2008 Bluetec. I can not observe any difference in the length of time the light stays on at 20 below zero. I have "read" that the glow plugs stay on all the time the engine is running but I don't know if this is true.

shertex · #3 01-17-2016, 10:35 PM

Got an email from the PO....he said that one time it had sat for several days, ambient was -35F, and still light was on only for a second.

clacker · #4 01-18-2016, 08:47 AM

On my 2008 R320, the light comes on a couple seconds when its really cold, never more then 5. I cycle it several times. The glow plugs might start running when the door is opened or when the key is sensed, but direct injection does start easier then idi when cold. I find cycling the key makes it start smoother and offers a high idle right away vs a potential misfire upon starting and normal idle (which is not very smooth on a cold start at -30).

Shortsguy1 · #5 01-18-2016, 09:50 AM

Is there something to suggest that the length the GP light is on is correlated to the length the GPs are on? In my ancient 1987, the GPs stay on long after the dash light is off, and I assume MB would do the same on much newer cars.

sloride · #6 01-18-2016, 10:29 AM

If really interested, it may take two people but find the relay and observe it on a really cold day then again when things warm up. On my 85 it does not take two people to tell when it shuts off, I can tell sitting behind the wheel, and I am damn near deaf in one ear.

jay_bob · #7 01-18-2016, 02:15 PM

I can tell you that the glow system on a CDI has no semblance to an IDI system.

The glow light seems to be more of a failure annunciation than a starting prompt. We have always noted a glow time of 1 to maybe 3 seconds irrespective of the engine or ambient temperatures.

Our plug started failing intermittently by just popping on the GP light while driving. Eventually it stayed on all the time and set a CEL for #5 failed. It really started running crappy once that plug failed 100%.

I know they use the plugs to help with the DPF regen cycle.

When they replaced the plugs they had to replace all 6 and a new relay because they redesigned the plugs and the new ones run on a different voltage. It's not 12 V, it's something like 5 or 7 V. That leads me to believe that the plugs are controlled by a solid state device using PWM, and not a relay. The old relay had a massive heat sink on the back which leads me to believe they have big semiconductors in there to run the plugs.

PWM is pulse width modulation, same thing they use on the later model heater blowers and aux fans. Altering the on/off over time rapidly.

Operating the plugs on PWM allows them to operate at a range of temperatures, not just full on or full off. I suspect they are operated at many times during the operation cycle of the engine.

barry12345 · #8 01-18-2016, 04:25 PM

I might want the factory description of how the cdi glow plug system works. The early non Mercedes but possibly manufactured under licence by them cdi engine we had started easily at temperatures many gas engines would not start.

The engine noises where so bad I felt that a block heater was needed. One was as not available from the factory for that model. Some people put stick on oil pan heaters I suspect for the same reason.

We just never used the car at really low temperatures as we had others. This issue was only a few early mornings each winter at most. . Minus thirty degrees F and lower. Strange but we never have temperatures that low anymore locally . Not even close. This winter will set records locally for the low amount of heating energy used probably.

Skid Row Joe · #9 01-18-2016, 06:47 PM

Glow indicator in instrument cluster illuminates for 1 - 2 seconds, tops on my CDI.

Glow indicator stays (ed) lit when indicating one glow plug failure.

That's all I know about mine, and it's performance. I, too, have wondered if the glow stays, after the indicator light turns off. I've also wondered if cycling will allow heat build up before cranking? I guess so, according to the R 320 owner's testimonial given on-thread here.

Wodnek · #10 01-18-2016, 08:14 PM

It has been as low as minus 8 this morning. It starts the same as when it's 70 degrees. You certainly hear it more, and its a little rough for the first 15 seconds. The big difference in the cold is mileage in the 20s. and the fact that the engine runs cold. when it is around zero, the heat gauge never registers, and you become pretty dependent on the heated seats.
Same thing with my now 70,000 mile 06, and 220,000 mile 05.

DDAY · #11 01-18-2016, 11:15 PM

Wodnek, My 2008 Bluetec warms up to normal operating engine temperature within less than 3 miles driving and the heater has us toasty warm no matter how cold the weather. You may have a problem with both of your cars.

I recently made a 200 mile trip. 70 mph, temp 7F, 35 mpg

Wodnek · #12 01-19-2016, 05:41 AM

Quote:

Originally Posted by DDAY

Wodnek, My 2008 Bluetec warms up to normal operating engine temperature within less than 3 miles driving and the heater has us toasty warm no matter how cold the weather. You may have a problem with both of your cars.

I recently made a 200 mile trip. 70 mph, temp 7F, 35 mpg

From my understanding the OM642 is improved over the om648 in maintaining a good operating temp. I have heard other 05 and 06 owners with the same complaints. if its in the teens out, the heat is fine. at or near zero... it just sucks, although it defrosts the windows fine.

barry12345 · #13 01-19-2016, 01:57 PM

Quote:

Originally Posted by Skid Row Joe

Glow indicator in instrument cluster illuminates for 1 - 2 seconds, tops on my CDI.

Glow indicator stays (ed) lit when indicating one glow plug failure.

That's all I know about mine, and it's performance. I, too, have wondered if the glow stays, after the indicator light turns off. I've also wondered if cycling will allow heat build up before cranking? I guess so, according to the R 320 owner's testimonial given on-thread here.

The actual mass of the glow plug could not get red hot in one to two seconds. You would almost need battery cables to it if so. And that same glow plug if kept energized at that current much longer would probably melt.

Without a good description of how this glow system functions by design it will remain up in the air to some extent. Knowing more might even be beneficial to owners. Controlled pulse width modulation allows many concepts if that is how they are controlling them.

What I am driving at there might be an optimum time to wait before cranking. It may be useless to try a few cycles. maybe your interior light could give some indication of what is going on for the length of cycle before brightening up again.

This would have to be done in cold temperatures as my old non Mercedes engine had no glow plug function at all above a certain fairly low temperature. I suspected that may have been the reason the glow plugs lasted so long.

Again I know nothing about these cdi mercedes glow plug systems. Still a lot of these cars are going to show up in members hands eventually. So both the actual functioning should be known totally and the common failure issues. For example there may come situations where they still operate but do not glow with enough intensity. So more actual expected current flow etc has to be know under certain temperatures. Plus I wonder what that controller might cost and how even that may be controlled. Depending on how it is built it may even be repairable.

I no longer think it is a relay as soon as I heard the mention of heavy transistors and an obvious heat sink.

God help people if the glow plug controller is heavily linked to the computer system on board with these cdi cars. There has to be a workable description and trouble shooting guide out there somewhere for these systems. Hopefully they are simpler than what I may suspect or extremely reliable.

barry12345 · #14 01-19-2016, 02:29 PM

Quote:

Originally Posted by DDAY

Wodnek, My 2008 Bluetec warms up to normal operating engine temperature within less than 3 miles driving and the heater has us toasty warm no matter how cold the weather. You may have a problem with both of your cars.

I recently made a 200 mile trip. 70 mph, temp 7F, 35 mpg

My early cdi type non Mercedes engine had a piss poor heater. I think they eventually added a couple of glow plugs into the coolant system to increase the fluid temperature through the heater. Talk about extremes. One of Volkswagons intents was to have a great heater and they did. Then they went to direct injection engines with very little cabin heat. One extreme to another totally.

It was so bad they did something. Seven glow plugs on a four cylinder engine is what I thought. I was experiencing this sixteen years ago so it is clouded a little. Only being absolutely sure there was not enough heat when cold out. The car was brand new and my complaint was common.

Their indirect diesels took the heat from the head only rather than the total cooling system and their cars heater could burn your hand if left near a vent too long. Also from cold engine to heat was the shortest time of any car we ever owned. We had more than a few of these cars and they were all the same.

shertex · #15 01-19-2016, 05:23 PM

I've been pleasantly surprised by how quickly I have cabin heat....thanks to the electric heaters the begin working immediately.