Car: 87 300SDL (126.125)

My glowplugs quit working yesterday.

The dash indicator light doesn’t work, either. The engine still starts after an overnight cool-down with only 2 or 3 cranks, a tribute to good pump timing I guess. It runs rough for several seconds and then goes to regular idle speed.

I’ve checked the 80A fuse (replaced last year) and it still looks good. Where do I check next? Please advise.

Tom Kanis
Cincinnati, OH

That fuse may have failed even though it looks OK. Check for power on both sides of the fuse.

If you have power on both sides, you could have one or more plugs faulty. The other thing to check is for power at the plugs when the engine is cold and the ignition is first turned on.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Just for the heck of it, I replaced the fuse. Still no light on the dash. BTW, is the circuit supposed to be "hot" when the key is 'off. Impressive sparks; hard on the screwdriver! How do you check the glowplugs themselves?

Tom Kanis

The power wire has no other fue than the 80A one; meaning its straight battery hot all the time.

Next to the fuse are two connectors on the relay. The larger one will have six wires. One to each glow plug. With an ohm meter check each wire to ground. Use the lowest scale as the value is less than one ohm if good.

Most likely you have a bad relay as its unlikely that all the plugs went at once. As plugs burn out the glow light will flash after starting.

Remember to remove all jewelry such as rings, watches, gold chains, body piercings, etc!

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Deezel
87 300TDT
150,000 miles

Have now replaced fuse and relay. Still no light on dash. Further suggestions?

Tom Kanis
87 300SDL 125K
Cincinnati, OH

Have you tested the glowplugs with an ohmmeter? Disconnect the big plug on the preglow relay and check each glowplug from that point to the valve cover. Should be less than 1.0 ohm per glowplug. Replace the bad one or two. If you have more than a couple bad ones replace them all. You may not be able to return the relay, too bad bad because most of the time it is just defective glowplugs.

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So far, I’ve done the following:

1. Replaced 80A Fuse.
2. Replaced Glow Plug Relay.
3. Checked all power fuses in fuse box. All look good.
4. Replaced Fuse again (just for luck)
5. Bought a meter. Obtained MS in EE by reading book that came with it.
6. Checked for continuity between plug and connector at relay (all OK)
7. Checked resistance for all glow plugs. #2 is at .5 Ohm, rest are at about 1 Ohm.

Questions:

Where does the power to run the control side of the relay come from? I noticed that my outside temperature gage went out after I replaced the relay. Which fuse provides power to it? Just before the glow plugs went out I replaced my Climate Control Module. Is it possible that I have some form of back-feeding problem going on?

Tom Kanis
Glowless in Cincinnati

I have a few thoughts; is the temperature sensor in the block okay, and is it properly connected. There should be a one conductor connector near the relay for this sensor wire, also there is a snap on connector at the sensor itself.

Other thought is the glow plugs themselves. I have in the past gone through all sorts of troubleshooting gyrations, only to find that the system works perfectly after replacing pin type glow plugs that measure okay and don't look too bad.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

The later glow plug system has the temperature sensor buildin to the relay box, not in the block as Larry stated.

The relay box has electronics to compare the electrical current between #1 plug to each other plug. Your #2 plug drawing different current from #1 as your test implied, the relay box would not turn on the indicator light (If my memory is correct, the light flashes rather than is out on older diesel cars).

David


[This message has been edited by be459 (edited 05-16-2000).]

be459,

Great explanation. So, from this it sounds to me like replacing the plugs may very well correct this problem. With pin type glow plugs, I've had to replace them a few times when nothing else seemed to make sense. Replacing the plugs has corrected the problem for me every time I've done it. They're not that expensive.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Thanks, guys. Anything tough about this job? Special tools? Secret handshakes? Other stuff that should be done "while you're in there"? Lemme know.

Tom Kanis

When removing/installing the plugs, be careful with the electrical leads; they're easy to damage if you're not careful. Sometimes when you are removing the nut to get the power wire off, the wire wants to twist as you're turning the nut; if you twist it too far ... *snap*

Other than that, it's a pretty basic job. Standard recommendation is to replace all the glow plugs at once, because they all tend to have about the same lifespan. I don't always do that, though...

- Nathan
'83 240D, 250k miles

You need a tool called Glow Plug Reamer to clean the prechamber opening before installing the new plugs. I do not know why you need to clean it but there must be a reason for doing so. It is possibly a $50 tool. Also, I would disconnect the battery's negative terminal before working on the plug, just to be safe.

Good luck

David

[This message has been edited by be459 (edited 05-17-2000).]