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I'm going through the diagnosis of an issue with the glow plug system on my 2004 320 CDI.
The problem is the warning for the glow plug system is always showing on the dashboard.
Apart from the light being on the car runs pretty well with no difficulty starting.
I've plugged it into a laptop with Xentry DAS diagnostics and come back with the following:
"2537-002 Check component N14/2 (Glow output stage). Short circuit to ground". the fault is current (i.e. always present, hence light on dash).
Xentry recommends testing one of the wires and that the result should be somewhere between 11-14v. The two wires that it could possibly referring to are the main power wire which is testing good at battery voltage and the other smaller green/white wire which is on pin-2 of the connector that controls the relay. That smaller wire is only showing 1.25 volts.
I've found a table showing what all the wires are supposed to do and it seems like both the input wire for the main power and the smaller wire should both be 12v (i.e. 11-to-14 volts specified by Xentry)
It says at the top of this table "N14/3" but Xentry says test component N14/2 - I don't understand the difference? I'm new to Mercedes diagnostics system so not up to speed with how they label things.
I've traced the wire from the connector back to the ECU. It terminates at Plug F on the ECU pin -43. With the connector in the ECU I still get 1.25v at the back of the ECU connector so there's no fault with the wire going to the relay... it is 1.25v coming out of the ECU.
So that's where I'm up to.
Question is... should that pin be only 1.25v or should it be 12v? If it should be 12v what are the possible reasons it is reading so low? Is that voltage variable depending on temperature or anything weird like that or should it simply enliven or cut off the relay in the glow plug control module by providing either 12v or 0v
I had the same thing and it was a glow plug. Turns out that they have something like 10 ohms when new and fail when they get to 50 ohms. If it is really high, ther system thinks it is a shorted or busted wire.
My glow plug failed so far it was 5 mega ohms and set that fault.
Test your plug with an ohm meter (multi meter function). If it is mega ohms or zero ohms, the glow plug is the fault and it will set that fault code.
As you've already resolved this by testing your glow plugs what exact process did you go through to verify each of the glow plugs? i.e. what multi-meter setting did you use etc. I've seen a few different ways of testing them but I am much more interested in hearing from someone who: 1. had the issue I've got. 2. did a particular test on each glow plug to prove one (or more) was faulty. 3. solved problem.
My E 320 has 240,000+ miles and I’ve been through about 6 or 8 plugs. I set my meter on ohms and it reads to mega ohms (not so common ability). They are like a spark plug so you just measure from the center electrode to ground. I recall good ones are like 10 ohms but that is just my recollection. As they wear out, the resistance usually climbs to something like 50 or 60 ohms and it will set a fault code. A million ohms is read like a broken wire and will set a different code.
I checked all the glow plugs and they all seemed okay. I was toying with the idea of simply replacing them all as a way of ruling them out as a possible cause. But then the problem resolved itself so for the meantime I'm going to ignore the problem as the glow system works exactly as it should.
I had a bit of an issue with another similar car that I've recently bought where the high level brake light wasn't working. It's an Audi A6 2005. Testing the output from the brake light connector showed a very similar 1.25v to what I had seen with the glow circuit on the Merc. I am starting to wonder whether or not that 1.25v is a standby voltage that you see on CAN bus circuits (or whatever the correct terminology is) when whatever accessory has tested faulty at the initial start up.
When I plugged in the new replacement high level brake light for the Audi it didn't work until I turned off the car and restarted it with the new light unit plugged in. It's almost as if the system does an initial test of all components on the car. If it finds something faulty it greatly reduces the power to that circuit until it tests good on the next start up. Hence why so many running issues with cars of this era can be cured by pulling over to the side of the road and doing a restart.
Can't seem to perform that test on the glow plugs. Either directly or through the glow plug loom from the connector to the glow plug relay. When I test I get connection to ground = 0 resistance. It makes sense to me since the body of the plug is connected to the cylinder head so the plug should have continuity / zero resistance. I guess a failed test would show infinite resistance or perhaps a higher reading such as the 50ohms described earlier.
I've decided to take a gamble on a replacement ECU as the glow plug relay connects to this and I've only been able to see 1.25v from the ECU with the wire itself eliminated as a possible cause of low voltage.
Does anybody know how to code an ECU to the car? I've got Xentry.
It seems unlikely that it is zero ohms on every glow plug. I'd recommend reading it again with a different multimeter. Unplug the connector and read tip of plug to ground resistance. Should be more like 50 ohms.
This problem has somehow fixed itself. I'm not sure if temporary or fully fixed. It is possible I didn't fully seat all the plugs on the relay. If that's the case then it was a failed glow plug as I've replaced all of them a couple of days ago to eliminate them as a potential cause.
After starting up yesterday it didn't fix it. Maybe I didn't fully seat the connector and didn't realise.
Since then the only other thing I did was remove the ECU from the car to get the numbers off it so I could buy a replacement from eBay. Maybe it reset the ECU. Maybe the problem will come back. I've cancelled the replacement ECU purchase.
Aaaahhh... the light is back on again. It stayed off for a few minutes.
It's not the end of the world though.
I think this behaviour is telling me that this is an ECU fault. Going by the logic that if there was some physical disconnection of a wire or a faulty piece of dumb hardware downstream of the ECU then the problem would be permanent. The fact the light stayed off for a few minutes suggests to me this is more like an ECU problem and less like a failed glow plug or a dodgy connection.
The glow plugs have a finite life and don't last forever. Replacing them wasn't a total waste of time and money even if it didn't solve the problem.
Maybe a detailed scanner would help narrow it down. I got a Foxwell scanner and it told me about a hundred small failures I wasn't even aware of (e.g., Failed right side mirror heater, ha).
I've got the MB system. You know, the dealership stuff.
I hope when I get the replacement ECU that I'll be able to marry it with the car using the diagnostics system. If not then there is a technique which involves moving the immobiliser chip from the old ECU to the new one which looks doable.
I got the replacement ECU today and I plugged it in. When you turn the ignition to position II the glow plug system activates as expected, then after a short period the light turns off... this is how it is supposed to work!
Sadly, as expected the car will not start with the replacement ECU as the immobiliser will not even allow it to crank.
There is a function in the 'Activations' section of Xentry for replacing the ECU but it will not complete the job if the replacement ECU is from another car. Only if the ECU is a brand new one!
The message says "The test counter of control module CDI 3 has elapsed, it is consequently already activated."
So what I'll have to do is de-solder the immobiliser chip from my original ECU and solder it into the replacement one.
I've swapped the ECUs back and forth a couple of times and the behaviour is entirely predictable. With the old ECU the glow plug system does not function. With the new ECU the glow plug system works!
I'll have a crack at moving the immobiliser chip tomorrow.
I don't know how you would solve this if you didn't have these skills, I guess you could pay someone else to do it. I have seen a YouTube of someone replacing all the linked control units to resolve this problem. I think there are 3 in total. To do the change you have to disassemble the dashboard, change the instrument cluster and buy the three units and get the associated keys.... it's a much bigger job but really is just unscrewing things so you can access and replace all the control units.
I managed to move the immobiliser chip from my old ECU to my new one. The car now starts with the replacement ECU and the glow plug light is off! Problem solved.
I'll create a separate thread for the write up on how to change the immobiliser chip as it's not documented anywhere as far as I have been able to find. I worked it out by researching in a few different areas and managed to figure it out. So I'll write it down in one spot for anyone else who has to do an ECU replacement.
01-02-2023, 10:29 AM
