Last evening I was putting the car away to get ready to drive it to work today and I figured I had better make sure the lights worked (hasn't been driven in a while). I noticed my bulb out light on the dash came on, both park lights on the passenger side were out. I checked and #3 fuse was blown. I tried replacing the fuse and it blew again. I put the car away and looked into it more this evening.

I first pulled the radio (the last place I worked) thinking I had left something amiss. Everything seemed fine. I used the bulb holder for the ashtray to check for a short. The wires at that point were grounded. I unplugged both front and rear lights hoping that would isolate it and no dice. Still grounded. I went to the next easiest thing that I could find on the schematic. The N7 bulb monitor unit in the back of the fusebox. I pulled that and the ground went away at the ashtray. I checked all the pins on the connectors under the N7 and nothing was grounded. Shouldn't I read the bulb elements at that point? Everything there was open to ground. I then plugged the N7 monitor unit back in place and the ground was back. Is that the possible problem? Seems to me it is or should I be looking at something else?

Any thoughts?

Yes those monitor relays can fail. Mine did, an open circuit to the right front parking light.

What was most ironic is that the open in the relay did not trip the bulb out warning. The relay caused the problem and did not indicate that it happened.

That relay should be easy to find in the junkyard, every 124 and 2nd gen 126 (86-91) came with one of those. One of those rare parts under the hood that does not care about whether the car is gas or diesel.

Also remember that an incandescent bulb filament when it is cold presents a dead short on your meter. It has a positive temperature coefficient with respect to resistance. In other words the resistance goes up when it heats up.

Try this trick instead: get a bulb socket with leads from a car in the junk yard. You want a socket that has both the shell and the base of the bulb on wires. Put alligator clips on the wires and clip it into the fuse box in place of fuse #3. If the bulb glows bright you have a true short. If it glows dim then that is because it is in series with the other bulbs in the circuit. This prevents damage from repeatedly replacing fuses when you have a short.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks jay bob!
I am just confused by the relay being the source of the short when I couldn't find a path to ground hooked to it. Not seeing the bulb elements confuses me too. I was expecting to see very low resistance. Maybe my long workday yesterday had my eyes crossed. I like the thought of the bulb to test the circuit rather than throwing fuses at it. I went through several old fuses last evening that I kept when I changed over to brass ones. I think I actually have a bulb with wires on it in my garage. It should work.


Sent from my iPhone using Tapatalk

Those monitor relays are prone to cold solder joints. I've seen a few come past my workbench with a few cold solder joints on the pins.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Just checked with a bulb as jay Bob described above. It is a dead short.

Now the question, can the monitor relay/module fail to a dead short or do I need to start searching for a pinched or bare wire someplace? The only other thing that I could see in the circuit was the instrument cluster but by the schematic that should be in the circuit with the monitor pulled.


Sent from my iPhone using Tapatalk

Myt 91 had some cold solder joints as well. Good news is the only hard part is getting the monitor unit out of the fuse box!

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The bulb-fail circuit consists of a very low resistance (usually an aluminum strip) through which the lamp current passes, thus generating a small voltage drop. An amplifier monitors this voltage drop and lights the warning lamp if it goes away (because a bulb has failed). If someone puts the wrong-size bulb in a socket (brighter headlamps is #1 cause), so much current can pass through the dropping resistor that it gets hot and sags or melts. This can cause a short-circuit in the module. I found one in a junkyard car that had been so treated—someone had used oversize license plate lamps, of all things! You can open up the module and inspect it.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Thanks Jeremy. That makes sense to me now. I probably should check all the bulbs to make sure they are correct before I replace the module. I am hoping I can find a list on a bulb manufacturers site. I have not changed any since I bought the car but I do not doubt there are incorrect bulbs installed.


Sent from my iPhone using Tapatalk

Almost impossible to draw too much current with LED lights

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Question, I am going through all the bulbs on my 300d just to make sure they are correct. I went by the chart on the sylvania website and they recommend bulb number 7506 for the brake, turn signal and back up lights in the rear. These are 25w bulbs. Everywhere else I look recommends 21w bulbs. What I removed from the car was 21w bulbs. Which is correct?

LEDs may draw too little current, thus making the monitor electronics think the bulb has failed. Note that I have no data and would love to hear from someone who has tried, for example, LED taillights.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

21W sounds right to me; 25W is a little high but may still be within the car's ability to handle it and not go up in smoke. Why don't you try it and let us know if your car burns up or not.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The module was the problem. Found one on eBay problem solved. All lights work now and the #3 fuse holds.

The new 7506 (brake, reverse, and rear turn signals) bulbs from sylvania are 25w bulbs. Found a probably 10 or 15 year old package of 7506 bulbs in my garage and they were 21w. So I replaced the 25w ones.




Sent from my iPhone using Tapatalk