howdy folks, i have a 1997 e300. 606 engine non-turbo. i suspect a bad glow plug relay, but want to confirm before dropping $130 on a new one.

symptoms: no gp light when turning key to position 2. engine cranks and cranks but never fires.

here's what i've done so far:

1. from the gp relay, tested voltage at the large gauge wire. got 12.83 volts.
2. from the gp relay, tested resistance on the 6 pin connector. exactly 0.7 ohms on all 6 pins between the connector and a ground point in the engine bay.
3. last year when i had the intake manifold off, i replaced the coolant temperature sensor at the back of the cylinder head. i heard the gp relay uses this sensor. so it's less than a year old, genuine mercedes part.

my question for the group is two fold:

1. from the gp relay, there is a two-wire connector, very small gauge wire, one white one brown. what does this do? does this go to that cylinder head temp sensor?
2. can i test the cylinder head temp sensor and associated wiring from this two-wire connector at the gp relay? if so, what is the test procedure? measure resistance across the two wires??

thanks for any ideas

Heres the wiring diagram for the GP relay.
http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1997/E300/SYSTEM%20WIRING%20DIAGRAMS/6187.pdf

When I had identical symptoms on my 97 the problem was the small 3 pin plug that connects to the glow plug relay. Make sure that plug is connected securely. Inspect it closely for any cracks or loose pins. Have someone hold down that plug with their hand while you turn over the key. The plug is very fragile. The solution was to bodge together a new plug to connect to the relay. The relay itself was fine.

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I have the same problem now on my '97 E 300 D, on an intermittent basis. I have just ordered a relay module (some call it a K 40 relay) as others on the web have found it solved the "cranks but no glow light and no start" problem. I did not consider the glow plug relay because glow plugs are not really needed for starting in summer, however, perhaps it does something else in addition to energizing the glow plugs. My car will start cold, but will crank, with no glow light and no start after it has been driven enough to heat things up a bit (4 or more miles). I should get the module tomorrow and will get back when I determine if it fixes the problem.

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'97 E 300 D

that's a nice diagram, thanks. my 3 pin connector only has two wires. the 3rd position is vacant. i've got brown and white, that's it.

sounds like the brown one is ground (no surprise there) so i should test continuity to ground from that wire. and the white one goes to the starter?? i had the starter off a few years ago, i don't remember seeing any small gauge white wires. i wonder if i supplied +12v to the white wire and grounded the brown if that would fool the relay into closing?

i did a preventative k40 relay change ~4 years ago with a new genuine mercedes one, so i doubt that's my problem. yes my issue was intermittent... sometimes it would do the no-glow-no-start thing, and other times it would work fine. it became more frequent in recent weeks until now it's permanently in no-glow-no-start mode.

interesting that yours starts without glow plugs. mine has never been able to start even in summer time w/o glow. 85F outside today and it just cranks and cranks, no start.

yes please do report back if your k40 fixes your issue, or not. thanks

No codes and no GP light? Probably not the GP relay.....

Guessing K40 relay.

Open hood, roll drivers window down, turn key to pos 2 and listen for the click of the SOV on the side of IP. No Click, no start. Possibly no GP light, and possibly no a lot of other things too. Cycle key again, and don't crank until you hear the click.

Rare occasions its a bad SOV.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

i don't know if there's no codes... there's no CEL. but i didn't pull codes. i'll do that tomorrow and see what i find.... thanks

No CEL, and no start usually points to the K40. Cuts power so is impossible for codes, if present, to register.

Is a reasonable system, if codes did set it would be a jumble of data. With no power to engine controls, things like the SOV, GP's, CPS, etc would all throw codes and you wouldn't know where to begin.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

I installed the new relay module and ran through the sequence that seemed to cause trouble with the no glow no start problem. So far it has worked perfectly with quite a few starts and quite a few miles this evening. I think it is fixed, but my bets on it would be small until I get more time on it. It is interesting that everyone calls this relay module a k 40 relay, but it doesn't seem to show up with that name on any listing of parts for this car that I have seen. The part's box calls it a relay unit. I think most cars have a 40 amp fuse on the relay box, but our mechanical injected cars do not. Perhaps that is where the name came from. When my indi called the dealer to order the part the dealer said there were only 6 in the world, or was it in the US. I'm not sure, but now there are 5. Of course, I don't really believe anything I hear from a dealer. The part number for the Mercedes part is A 210 540 03 72 for a '97 E 300 D.

Incidently, before I broke down and ordered this part I disassembled the old unit and carefully examined the circuit board and cleaned the relay contacts. I could not find anything suspicious. I put it back together and it still acted the same, so I was skeptical that the new one would work. Sometimes it is nice to be wrong.

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'97 E 300 D

here's the latest update-

- when key is inserted and turned to position 2, the k40 makes an audible click. the sov also clicks, and the intake resonance flaps do too. the electric radiator fan also spins up. gp relay does not click.

- there are 3 fuses on the k40, all are good. all other fuses on the car are good too, side of dashboard and in the fusebox. all tested with a multimeter.

- i cannot read the obd2 port! i've tried three different scanners, and none are able to connect. i have successfully used these scanners in the past.

- i tested pin 16 (bottom corner pin) of the obd2 connector and i get a good +12v from it.

- when key is turned to position 2, i get all the expected lights illuminating on the dash .... except the gp light. all features and functions of the car (aside from gp's and obd2) seem to work perfectly.

does any of this new data point to an obvious solution? thanks in advance.

I had similar symptoms down to the no GP light and fans switching on. I think when the 3 pin 2 wire plug is loose it sets a code. If memory serves P0380 but I could be wrong. Once that code is set the ECU won't switch on the glow plug relay until you can delete the code. Since your OBD2 code reader isn't working, disconnect the battery for a few minutes then reconnect it. That should delete the code. The previous owner of my car had a zip tie wrapped around the relay to help secure the 3 pin 2 wire plug but I think the best solution is to modify the plug.

There is some evidence to suggest this is a known fault.

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is there any documentation identifying which pin from the ecu supplies the signal to the preglow relay? there's multiple wires that are solid white going into the ecu connector...

thank you

It is pin 33 on the ECU.


N3/7 - IFI control unit (ECU)

N14/2 - Glow output stage

M1 Starter

If the glow plug light doesn't light up but your engine does crank over, that means the electric problem is at the GP relay, not the ECU end since the same pin 33 on the ECU also triggers the starter motor through the white then violet wires. The diagram is misleading because the connection between white and violet wires is is in the wiring harness prior to the GP relay.

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perfect, this info is most helpful!!

yes my engine does indeed crank.

i think my next step is to replace the gp relay.

Since it is summer, I doubt that your glow plugs are needed to start your engine. I think you have another problem. Mine starts right up if I turn the key like you do on a gasser, however it does do a bit of hammering until the glow plugs heat up. The relay unit has three relays in it on your car. My guess is that one of them is not clicking, but try your spare key to be sure it is not a key recognition problem, as if your key is not recognized, it would crank but not fire. This may be unique to the '97 diesels.

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'97 E 300 D

Try disconnecting the battery for a few min first, then secure the 3 pin plug with a zip tie then turn the key. I bet the gp light will switch on.

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