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  #1  
Old 06-01-2014, 06:15 PM
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OM606 in W124 Glow System Questions

I recently came into a non-running 1995 E300. Fuel delivery seems ok from a cursory check but all I can see of the engine harness is bare wire, so that's an obvious sign it needs to be replaced. The preglow light does not seem to come on at all.

First question, will a bad harness prevent the engine from starting?

Second, can the preglow relay module be bench tested to make sure the bad harness hasn't caused it to fail?

Last, is there much merit in trying to build a replacement harness? I imagine just the replacement connectors would be somewhat expensive.


Last edited by vespoid; 06-01-2014 at 06:32 PM.
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Old 06-01-2014, 07:38 PM
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Replace the harness with a store bought one. Will the engine turn over at all?
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Old 06-01-2014, 07:50 PM
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if you are seeing bare wire then best you replace it, if you are savvy with building entire looms then you can go ahead and repair this one - its not that hard as its a basic analog system with big connectors, MB connectors are soldered pins as compared to crimped. In short you need to salvage all the pins, the big connectors are easy to take apart - the one problematic connector is the 4 pin coolant temperature sensor one which will require you to cut it and then reglue it back.
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Old 06-01-2014, 07:56 PM
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Originally Posted by engatwork View Post
Replace the harness with a store bought one. Will the engine turn over at all?
Thanks, I've been leaning towards just ordering the harness. The engine cranks but doesn't fire.

Last edited by vespoid; 06-01-2014 at 08:28 PM. Reason: Inserted quote
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Old 06-01-2014, 08:12 PM
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Originally Posted by Zulfiqar View Post
if you are seeing bare wire then best you replace it, if you are savvy with building entire looms then you can go ahead and repair this one - its not that hard as its a basic analog system with big connectors, MB connectors are soldered pins as compared to crimped. In short you need to salvage all the pins, the big connectors are easy to take apart - the one problematic connector is the 4 pin coolant temperature sensor one which will require you to cut it and then reglue it back.
Solder and I don't get along well. I guess that means I'll be ordering the harness.
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Old 06-02-2014, 12:59 PM
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Originally Posted by vespoid View Post
The engine cranks but doesn't fire.
On this model, to prime the pump, you need to keep the starter engaged for at least 30 seconds or more. Glow the plugs for 15 seconds, even though you said you don't see the glow light, turn the key, hold it turned, and listen to it go "nee nee nee nee nee nee nee nee nee nee nee nee." Hopefuly the air will purge and it will start for you.
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Old 06-02-2014, 03:44 PM
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Originally Posted by connerm View Post
On this model, to prime the pump, you need to keep the starter engaged for at least 30 seconds or more. Glow the plugs for 15 seconds, even though you said you don't see the glow light, turn the key, hold it turned, and listen to it go "nee nee nee nee nee nee nee nee nee nee nee nee." Hopefuly the air will purge and it will start for you.
I've got plenty of fuel. There's fuel at the pump output, No. 1 injector, and I can see white fuel-scented mist from the exhaust. I changed the fuel filter to be sure, waited a full 30 seconds for the glow cycle, and cranked for 60.

My big question now is the harness. If any of the sensors running through it become shorted or disconnected, will it prevent the engine from firing? I know how much some engine computers rely on the cam position and coolant temp sensors.
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Old 06-02-2014, 04:53 PM
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Something don't sound right to me. How many miles are on the car?
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Old 06-02-2014, 05:06 PM
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Take a look at the rest of the harness as it may only be the ends or close to the end.

Mine has broken down outside of the black harness cover but is looks okay internally.

It sounds like the glow plug circuit is not active. Try listening for the relay 'click' with the bonnet (hood) up.
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Old 06-02-2014, 06:09 PM
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He has the infamous biodegradable wire harness.

If the glow plug control harness is bad the relay won't pull in.
If the glow plug output harness is bad no voltage gets to the GPs.
Or, is the strip fuse blown due to shorted GP output wires?

Check voltage at the GP terminals - should have 12 V dc at each GP terminal.

Resistance check won't tell you anything, the harness could be shorted to ground. But you can check the individual wires to the GP terminals, with the terminal pulled off the GP.

In any case, no preglow = no ignition, white smoke out the exhaust is the unburned fuel.

Whatever you do, do not use starting fluid!

The good thing is that the W124 has an actuator and sensor on the IP but that is just for idle regulation. The engine will start and run without it.

I would first attack the GP output harness and make sure you have GP voltage. Once you have that confirmed , you can jumper 12 V dc to pins 1 and 2 of the control connector and pin 4 to ground. That should pull in the preglow relay and get you voltage at the GPs.

I would then disable the entire engine wiring harness by unplugging it from where it comes from. This will hopefully save you from things getting fried due to being shorted.

Then using a remote starter switch, try to crank the engine and see if it fires after giving it about 15 seconds of manual glowing by jumping out the relay. You will need to put the key in II so the vacuum valve allows fuel into the IP.

The 95 is the last year you can pull this stunt, the 96-99 are drive by wire and you aren't getting anywhere without the computer. However they went back to regular wire on those models so at least that's not a problem.

Once you successfully get it to crank, then you will be more confident spending the $$ for a new replacement wiring harness. The nice thing about the replacement harness is that it is plug and play. You just lay the new harness next to the old one and swap the plugs and fastenings 1 by 1.
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Old 06-02-2014, 06:25 PM
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That's handy to know jay, just in case I decide to replace mine.

I haven't explored where it connect to the other end, how far into the main harness does it go?
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  #12  
Old 06-02-2014, 07:30 PM
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Originally Posted by jay_bob View Post
In any case, no preglow = no ignition, white smoke out the exhaust is the unburned fuel.
Thank you so much, especially for this part. My dad almost had me convinced that at the 50-60*F ambient temperature, the compression alone would have it going. I'll check out what I can, and try jumping the glow relay. Only one question: Where does the darn thing plug in, somewhere on the firewall? I'm still waiting for my service manual CD to come in the mail, otherwise I'm sure I'd find the diagram there.
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Old 06-02-2014, 08:01 PM
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It is mounted on the drivers side fender. Look for a black box with a couple cables coming out the top.

Reason I asked about mileage is that these cars will crank with just a couple glow plugs working if the engine is pretty healthy. It would have to be a really healthy engine to start at 50-60dF without glow plugs and I am not certain a healthy engine will do that. If you can rig up a way to glow at least two or three glow plugs you may be able to get it going. Don't be scared to crank it up without the intake manifold installed. Just don't drive it around like that.

Don't use starting fluid.
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  #14  
Old 06-02-2014, 09:45 PM
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Originally Posted by spock505 View Post
That's handy to know jay, just in case I decide to replace mine.

I haven't explored where it connect to the other end, how far into the main harness does it go?
I believe most of it goes behind the battery to the EDS controller/Klima relay/OVP/etc and the rest goes into the fuse/relay box. There might be a leg to catch the connections from the cockpit but there should be a connector on one side of the wall or the other for that. That may be in the base of the fuse box as well.

Best bet is to get on EPC with your specific VIN and look at the engine harness picture.

The entire car was wired with the bio-wire but the remainder of the car is *usually* ok, the high heat in the engine bay cooks the insulation worse than elsewhere.
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The OM 642/722.9 powered family
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)
2008 ML320 CDI (Older sonís DD)
2008 E320 Bluetec (Younger son's DD)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
----------
gone but living on
1983 240D (body to greazzer, engine to t walgamuth)
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  #15  
Old 06-02-2014, 11:52 PM
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Well, the previous owner did comment about it being extremely difficult to start in cold weather so a few of the glows retired well before I got it. I jumpered the glow relay as jay_bob described, held it for a 15 count, then gave it the starter. It felt like I only had one cylinder going with the rest along for the ride but at least it's one cylinder more than I had before. As far as mileage goes, the odometer says it's gone 451 thousand. I guess I should start spraying penetrating oil on the glow plugs now if I'm to have any luck getting them out when the fairly obviously needed replacements come.

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