Re: [MBZ] Trailer Wiring Confusion - ML550 - Bucket of Prop Wash/Snipe Hunt

dan penoff.com via Mercedes Sat, 13 Aug 2022 15:17:30 -0700

The real irony here is that the only place I’ve been able to find some sort of 
adapter that converts from the 7 pin to 4 pin connector while also 
accommodating LED trailer lights is at UHaul:

https://www.uhaul.com/MovingSupplies/Wiring-Lights/Adapters/LED-Light-Trailer-Module/?id=4089#

Even more ironic is that I found a broken link to this in the Banz Whirled W164
forum. I’m swinging by my UHaul place tomorrow to get the one they have in
stock so I’ve got it on hand for the next time.

-D

On Aug 13, 2022, at 5:54 PM, Buggered Benzmail via Mercedes
<mercedes@okiebenz.com<mailto:mercedes@okiebenz.com>> wrote:

Good work! You learned something with all that sweat and effort!

I think my CL has some LED replacement bulbs in the headlights. They work fine
but I get a whole bunch of warning messages on the instrument cluster. Might be
some in the taillights as well. I haven’t chased them down yet since everything
works but the nannies are Not Happy.

I guess newer cars have LEDs so the new replacements will work straightaway but
I wonder why there are no adaptations from the bulb manufacturers for backwards
compatibility.

--FT
Sent from iFōn

On Aug 13, 2022, at 4:57 PM, dan penoff.com<http://penoff.com> via Mercedes
<mercedes@okiebenz.com<mailto:mercedes@okiebenz.com>> wrote:

Kudos to Jim for confirming the pull up voltage theory!

I believe there has been a comedy of errors that led to this situation:

1.) The voltage at the 7 pin socket is a “pull up” voltage from the SAM and
won’t carry any current to speak of.

2.) This voltage, while minimal, was enough to light the LEDs in the adapter
socket, which would explain why they were lit for the turn signals when the
signals weren’t on.

3.) The UHaul trailer, despite what the lot guy told me, almost certainly had
LED lights.

4.) The circuitry in the SAM has “lights out” functionality much like it does
for any other light on the body. This means it wouldn’t have played nice with
LEDs in the circuit unless there was a load resistor somewhere.

How did I confirm?

A tried and true and quite antiquated approach - an 1157 bulb as a test light.
When connected to the appropriate terminal on the 7 pin socket, would light up
and perform as expected for turn signal, brake light or running lights.

So I burned maybe 90 minutes and five pounds of sweat chasing this down, but
now I have a better understanding of the system and discovered the location of
a number of different components on the car. And I got a full diagnostic scan
on it as well, showing almost 100% check marks, which is always nice.

I finished my time up with a good Martha’ing on the interior with the shop vac,
and even found a little bit of cruft in the cowl vents I had missed that has
now been vacuumed out.

Lesson learned. I need to find an adapter that will allow the use of LEDs in
the trailer harness. Time to play inside for the rest of the day and enjoy the
AC.

-D

On Aug 13, 2022, at 4:10 PM, Kaleb Striplin via Mercedes
<mercedes@okiebenz.com<mailto:mercedes@okiebenz.com>> wrote:

The u haul trailer we rented a while back was all LED. Everyone I have seen for
a long time has been. Even with that it worked fine with our GL. I’m wondering
if someone installed the resistors or whatever is needed on mine to make it
work.

Sent from my iPhone

On Aug 13, 2022, at 2:40 PM, dan penoff.com<http://penoff.com> via Mercedes
<mercedes@okiebenz.com<mailto:mercedes@okiebenz.com>> wrote:

Thanks for the insight, Jim, I was wondering this as well since the SAM drives
it directly. However, when it was plugged into a “real” trailer we still got
nothing.

I’ve got an 1157 lamp with some leads soldered to it - I’m going to try that on
each terminal and see what happens.

Everything else has checked out. Now I’m wondering if the UHaul trailer *did*
have LED lamps, as these do not work with this system for the same reason as
above. I asked the UHaul guys if the trailer lamps were LEDs and they said no.
Hmm. That would also explain why the diagnostic LEDs were lit on the adapter -
it would only take a a few mA to drive them.

More as I know it.

-D

On Aug 13, 2022, at 3:07 PM, Jim Cathey
<jim.cathey...@gmail.com<mailto:jim.cathey...@gmail.com>> wrote:

And there is still power to all terminals at the 7 pin socket (except for
trailer brake and ground.)

Power? Or +12V measured with a multi-megohm input impedance meter?
(Zero current, essentially.) Chances are it's got a pullup system, looking for
the load of bulbs to draw signals to ground so that the car can do lamp-out
checking, and trailer-plugged-in checking, etc.

-- Jim

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