Diesel guru needed - 606.910 GP relay issue! - PeachParts Mercedes-Benz Forum

WINGAS · #1 06-02-2015, 10:03 AM

We recently picked up a pretty cherry 1995. 232K on the clock. This Sunday I replaced the wiring harness as it was still oem and deteriorating. This because the GP system began exhibiting issues, ie. the glow indicator would come on for a minute after starting , indicating a fault. And then it simply failed. No GP indicator, no preglow. So, I reckon lets get a proper harness in there first before the relay.

The only ? I had during preglow harness install was the 4-pin temp sensor which didnt seem to have an orientation. ??? which may be the issue now....

With new harness on , my son ran the car w/o GP system ( which I pucker on badly) and when the new relay arrived, I unpuckered - not ...so ...fast.....

With the new relay in, the car will not start. Plenty of CCA, spins that 22:1 compression motor quickly, but it simply wont fire.

Disconnected the 5 pin connector on the relay, GP indicator doesn't come on, yet same condition. Turns well - but no start. Check fuel - proper.

Put old relay back in, and it fires to live. Unhappily w/o preglow, but fires.

Suppose a bad relay, but what else could be going on? It is my "?" during preglow harness install on the orientation of that 4 pin temp sensor?

Thank you for any thoughts..... I am stumped..... please help!

Maxbumpo · #2 06-02-2015, 10:53 AM

Start by checking the health of the glow plugs. A "quick & dirty" check: at the glow-plug connection to the pre-glow relay, measure resistance to ground for each pin (which corresponds to each glow plug). They should all fall around 0.7 - 0.8 ohms. You need a decent digital multi-meter to measure that resistance consistently.

Maxbumpo · #3 06-02-2015, 11:06 AM

A better test of the glow plugs is to measure the current flow for each plug, which should initially spike at 15 amps and then fall off to 8 amps.

A clamp-on DC ammeter is the easiest way to measure this, but that meter is expensive and not common. A "good enough" solution would be to build your own in-line ammeter, using a cheapo dash-mount ammeter from FLAPS, which has a scale suitable to ready the 8 - 15 amp per plug range, add and some thick high current wire on each terminal, and then at the same harness plug that that connects the individual plugs to the pre-glow relay, pull off that plug and for each pin use your meter connected to battery voltage.

Jeremy5848 · #4 06-02-2015, 11:10 AM

The late-124 diesel car is not very intelligent—it shouldn't be aware of the preglow relay's presence or absence. Except for idle control the 1996 OM606 engine's fuel injection pump is completely mechanical. Try starting the engine with no glow plug relay at all.

As far as the wire harness goes, I seem to remember reading that replacement harnesses are intended for more than one car and can have an extra connector not used in the 1995 model. Could you please confirm that you were sold the correct harness?

Jeremy

WINGAS · #5 06-02-2015, 11:20 AM

THIS IS A 1995! Different IP setup, at least the throttle inst by wire as in the 96.

Correct harness as it matched the old one precisely. The only ? I had was the 4 pin temp sensor ( near rear of engine on manifold) which didnt appear to have a orientation pin of any kind.

I did check the ohms of all the glow plus as I was waiting for the indian head shellac to setup to glue the intake gasket down ( helps for future intake R&R) and #2 showed no resistance ( assume its shot), all the rest show 1.6 ohms precisely. I wont be changing them out until the fall.

None of this answers why it wont fire with the new relay in, but fires w/o with the old relay. Had no idea the relay would somehow cause a non-start condition, even if #2 GP is shot.

Thank you all.

Maxbumpo · #6 06-02-2015, 12:23 PM

Quote:

Originally Posted by WINGAS

THIS IS A 1995! Different IP setup, at least the throttle inst by wire as in the 96.

Is your car a 124 car or a 210 car? I don't understand your statement quoted above.

I suspect your new relay is bad right out of the box, and is not energizing the plugs at all?

Your old relay is telling you, via the behavior of the pre-glow timer light in the instrument panel, that there is a fault in your pre-glow system (the number 2 glow plug is a short to ground). This is explained in the owner's manual and the factory service manual.

I'm not enough of a guru to know if the old relay will continue to energize the other five plugs if one is a short to ground or not. I would have thought that the short would cause the internal breaker to trip and de-energize all the plugs.

Maxbumpo · #7 06-02-2015, 12:31 PM

According to the 124 factory service manual, there is only one temperature sensor for the pre-glow relay, and it has a two-pin connector, so your 4-pin connector may be for some other system?

WINGAS · #8 06-02-2015, 12:44 PM

Thnx. This is a W124. Someone above was talking W210. ( I've owned both)

You clarified the "omnidirectional" 4 pin. Thank you.

We do have, by resistance, testing a bad GP. They must crap out all of a sudden as the system worked flawlessly up until two weeks ago, other than an occasional GP indicator light on while driving ( which I know indicates an issue) I thought the system would still function.

Also thinking the OVP relay is somehow involved, but again, with old nonfunctional preglow relay, the engine fires up. Would a bad OVP still allow that?

Lucas · #9 06-02-2015, 01:06 PM

The knee bones connected to the, leg bone.

Thats how it always goes.

I always say this and people probably get tired of hearing it. But just put a button on the dash the triggers the relay to glow. If you don't mind cutting up your harness.

Better glow plug life because you will use it lest often.

And less complicated.

mach4 · #10 06-02-2015, 01:46 PM

Quote:

Originally Posted by Lucas

I always say this and people probably get tired of hearing it. But just put a button on the dash the triggers the relay to glow. If you don't mind cutting up your harness.

Better glow plug life because you will use it lest often.

And less complicated.

I was going to say - everytime I read one of these glow plug relay threads it reinforces my decision to go with a manual system. On the 617 at least I didn't even need to modify the harness - just gutted the GP relay so it's just a pass through that the existing harness plugs into. A starter solenoid and switch and you're done.

It's not the solution for everyone (and maybe not for the OM606), but for simplicity and control and longevity and lack of aggravation and periodic troubleshooting, and cost, it's hard to beat. [just my $0.020]

Lucas · #11 06-02-2015, 02:04 PM

I had a timer stick on a 4bd2t and it melted the plugs. I said "ok I am done."

Im drawing a blank on his username, om617yota?, the guy with the 617 in a yota had his starter relay fail quickly. Maybe a bad one, but I wondered if they aren't meant for extended cycles.

I bought a box of 50 5 pin 50 amp relays with pigtails off ebay for less than a buck each. I use one, or two of those. Have yet to have any fail.

But I use them all over. On older vehicles I splice a new fuse box straight from the battery and run the headlights off them. Etc.

Dodge and ford have a few that run current through the switch. And it always breaks.

Ok back to the point. Maybe he doesn't want to rig his nice benz.

Zulfiqar · #12 06-02-2015, 03:29 PM

the glow plug relay temperature sensor is the one at the very rear end of the engine, its a 2 pin item and actually are two sensors referenced to ground from the cylinder head.

One of these pins go to the glow plug relay and one goes to the gauge cluster. Their body connection junction is near the glow plug relay itself in the shape of a square 4 pin connector.

The round omni plug can be fit in any way you want, the two sensors inside them are arranged in a X fashion. Those two sensors are for the ELR and EGR controller cubes behind the battery curtain.

the most front sensor on the engine is for the HVAC push button unit and is one sensor with 2 pins. Its body junction is also in the same square connector I mentioned.

Sounds like you have a dud glow controller.

WINGAS · #13 06-02-2015, 04:22 PM

Sure does sounds like a dud glow controller.