I have a 1986 300E with 294k miles on it that I have been maintaining over 24 years. I have a problem with the left stop lamp triggering the bulb check unit light and I have run out of ideas. So far, I have:
1. Replaced the bulbs (Sylvania 7506L) on both sides.
2. Disassembled the rear taillight assemblies, cleaned contacts, resoldered connections, checked grounds. Checked all with an ohmmeter; looks okay.
3. Today I pulled the bulb check unit and checked the resistance back through the wiring to the right and left stop lamps. The resistance was a little over 1 ohm in both cases. Infinity with the bulb removed. Looked okay to me.
4. Replaced the bulb check unit with a used one.
5. The only change with all the above is that the check light used to come on instantly when I stepped on the brake. Now I can step on the brake for a second or so and the light does not come on. As soon as I step on the brake and hold it, the check light comes on in about three seconds.

Any thoughts? I'm out of ideas.

Thanks.

Brent Smith
Snellville, GA
1986 300E 294000 miles
2003 530i
2003 Boxster S

After thinking about this some more, I wonder if a worn out or damaged brake stop light switch could do this. Since the bulb check unit is looking for some level of current through to the bulbs, would a faulty switch (additional resistance somewhere) make this happen? Has anyone had this happen before?

Thanks.
Brent Smith

Brent,
I am a little confused about your problem. You say that you have a problem with the left brake light. But you don't really define what your problem is. Does it not work at all sometimes, is it dim sometimes, does it stay on sometimes? You say that the lamp warning light comes on because of the left brake light.
So, we can help you more if we know exactly what your problem is.
If the right brake light works all the time, then the brake light switch is good. There is only one circuit in the switch for brake lights and it serves the same signal to both lights.
Another question which may seem unrelated, does the cruise control work?

Together, we can fix this problem.

PaulM

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84 500 SEL (307,xxx miles)

Thanks for the comments. Last week, before I did the resoldering and put in new lamps, the left lamp would go out. Without my wife here yesterday to observe, I just assumed the left lamp was still out when the bulb check light came on. But not so. Using the garage door as a reflector, I just checked and both stop lights come on okay with the key on. With the motor running, both lights come on okay, but the bulb check unit light comes on when I step on the brake (no other time). Brake lights now work fine it seems.

BTW, I have a complete set of the original M-B manuals, including the electrical troubleshooting book.

Thanks again.
Brent Smith

The cruise control occasionally works, but normally it is erratic or 0does not work at all. Since the 300E is no longer used for trips, but just around town, it doesn't get much use unless I just try it out. I've cleaned and lubed the actuator and replaced the amplifier (used), and for a while it worked.

Brent

The reason I ask about the cruise there is a clutch (or that is what we will call it) in the cruise actuator that gets its power from the cruise control unit and it's ground through the brake light bulbs. If the resistance through the brake lights is to high, the clutch won't enguage and the cruise dosen't work. When you hit the brakes, then the actuator doesn't see a ground anymore, it sees power and the cruise goes off.
Now back to our problem. I believe there is a bad ground somewhere associated with the right tail lamp assembly. The way to prove this is have someone turn the ign on and press the brake pedle. Look very carefully at the right and left sides and compare the brightness of the bulbs. If the right is dimmer then there is a ground problem. Now turn on the parking lights with the brakes lights still on. Check again at the brightness of the right and left, this time also looking at the brake lights and the parking lights. If something is different then there is probably a bad ground somewhere. Continue with these test using the turn signals and backup lights. With the answers to the above test answered, we can narrow down the problem.
It takes much longer to discribe the test than it does to do them.
The two most common problems we would see was either the ground between the bulb and the bulb socket or a bad connection somewhere in the tail light assemble. The above test would lead us to the problem 95% of the time. To actually pinpoint the problem we used a $10 dollar test light, not a high dollar or a high tech piece of equipment.

PaulM

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84 500 SEL (307,xxx miles)

I've already checked the ground by measuring the resistance at the bulb check unit socket to battery ground for each stop lamp wire (bulb unit out-open circuit). In both cases, the resistance was solidly one ohm with the bulbs in place and infinity with the bulbs removed.

Since the current for the stop lamps flows through the brake switch to the bulb check unit on it's way to both bulbs, a bad spot on the switch adding a llttle resistance could lower the current and trip the bulb check light. It would look like a poor ground or bad connection. At least that's still my theory. I'm going to pull the switch and check.

Thanks for the comments.

Brent Smith

If the 'Check' light comes on when you step on the brake then it probably is not the switch or cruise control. It has to be a brake light.

Have you checked the center brake light? I think the Brake fluid reservoir low also trigger the light.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I hate to admit it, but the bulb in the center brake lamp (like the brake switch) is the original one and I have not paid the least attention to it. That's probably the problem. I'll check this afternoon and report back.

Brent Smith

Well, gents, it was the center bulb burned out. I'm still red-faced over spending all this time and your time on this. It started out will a left stop lamp problem, but apparently the center brake lamp was out too and neither my wife nor I noticed.

All is well.

Thanks for the help!

Brent Smith
Snellville, GA

Brent, check for flat spots on the contact surfaces of the brake lamps. The flat spots can cause problems with the "Lamp Out" indicating system. Solution is to replace any lamps with flat spots on the base.

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Fred Hoelzle

Brent, where is the bulb check unit and how is it accessed/removed? Mine has acted flaky from time to time. I also have one rear running light which is driving me crazy. Bulb is fine, I measure 12V at the socket, but bulb does not light.... Then it will work fine for a month or two. Two separate issues.

Rgds,
Chris W.
'95 E300D, 429K

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Objects in closer are mirror than they appear.

Chris, my bulb check unit is located in the relay box behind the main fuse panel (left side by firewall). You remove the fuse panel lid and loosen six screws and the relay lid comes off with the fuse panel top frame. The bulb check unit is the long relay on the left. If you have a good wiring diagram for your model, you can remove the relay and check the wiring continuity back to the lamps.

However, most problems are going to be poor connections in the rear lamp unit itself, e.g., poor ground connection, lamp socket connections, cold solder joints, deteriorating plastic, etc.

Brent Smith
Snellville, GA
1986 300E 294,000 miles

I am sorry, but he wouldn't have to ask for help, if he knew exactly what the problem is.

I admit, I didn't read all replies, so someone might have already sugested this one.
Does your vehicle have a third brake light?
It probably does, check all the light bulbs for correct wattage.
Sylvania bulbs have partnumbers or model numbers on it, but no wattage, I believe.
All stop lights are 21W light bulbs. If one of them is less than 21 watt, you may get intermittend "Check Light" dashboard triggers.
It could also be, simply a bad connection, sometimes wire crimps look good but are broken inside the wire insulation. Way to find out, just pull on it, if it stretches, the wire is broken inside.

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

I'm sorry, you are right. I see that you are a BMW tech, but I have never worked on those (I refused to) so I need a little more information than you. I just can't seem to diagnose problems until I know exactly what the problem is.

PaulM

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84 500 SEL (307,xxx miles)