Electrical Problem U1350L - Thoughts? - Mercedes-Benz Forum
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post #1 of 15 (permalink) Old 01-21-2017, 04:24 PM Thread Starter
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Electrical Problem U1350L - Thoughts?

Hi Folks; my 1998 U1350L has been sitting for 6 weeks as I like to keep it off of the salt covered roads. I thought it deserved an exercise after a few days of fine weather so what better destination than a run to the beer store. Turned off the battery disconnect, turned the key, and noted no idiot lights on the dash. But, I continued with the key and it fired right up. No tach, no guages operate (other than air pressure), no signal lights, no blower motor, no wipers, no horn, no high beams. Head and taillights, dash lights, and interior map light work. So, before checking the fuses I turned off the key and the engine..... continued on. Praise the Gods for this site as I found a few ideas from previous posts on how to shut down the fuel pump, and was I relieved when one worked.

Fuses on top of the dash in front of the driver A and B are all good. It appears that most of the items not working are configured on fuse panel A. I double checked the batteries and connections and all good.

There has been no hint of electrical gremlins in the past, and now a whole bunch of stuff is non-operational. There is no rust on the truck and the grounds are clean.

I am on the beg for ideas. Any suggestions, or has anybody on the forum encountered a similar issue. My initial thoughts are the ignition switch or a relay, but which relay? Nothing is more foreign to me than electrical puzzles.

Thoughts?

Thanks!
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post #2 of 15 (permalink) Old 01-21-2017, 08:50 PM
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Looks like you have covered the basics - only suggestion I have would be to check the supply wires as they transit the firewall - my connector box has 3 battery connections that go through there - I have had one in the past so corroded that it was not transferring any juice ! Look for the big thick green wires - and disconnect/reconnect them on both sides (inside cab and inside engine bay) sometimes when spade connections sit like that, they can stop being conductive and a bit of movement will bring them back to life.

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post #3 of 15 (permalink) Old 01-21-2017, 09:25 PM
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Sounds like the earth wire under the dash is no longer working, worth a check and ensure the cab itself is properly earthed to the chassis and the engine.

The other thing it might be is the common power supply for one half the fuse box, check to see if you have power to the fuses, if not, if may be the ignition switch itself, all the power run through it, and that could be a lot of current. Best to add a relay ( 70-100a Solid State relay) and take the load off the switch.

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post #4 of 15 (permalink) Old 01-22-2017, 11:11 AM Thread Starter
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Thanks Guys for the input.

Krietpiel; I checked all connections at the firewall and they are good. I also moved everything around gently but no change. I checked the ground at the engine and cab and in fine condition. I cannot locate the ground under the dash.

All I am getting for power is what would typically work with no key; hazards and low beam lights. I cannot recall if the horn worked without the key, but it doesn't now. There is power to the ignition switch at the left side where the wires attach as per a multi-meter test, and when I turn the key to position 1 I get a distinct 'click' at the switch, but no dash lights. If I disconnect the lead / plug from the right top side of the ignition switch the key locks in the switch, just as it does if you put the key in and try to start the Mog without reversing the battery disconnect. When I plug the lead back in the key is released, so there is no question power is going to the switch.

Ian; any pointers on approximately where to find the ground connection under the dash? It's a spaghetti factory of wires there. As you mentioned it's looking more like the ignition switch may be the culprit, unless all of the 'out' power from the switch toward the main fuse panels goes through one relay that could be boogered.

There's a small button at the bottom of the middle point of the dash above the steering console with the number '20' market on it. Any thoughts on what this is for? Pushing it in does nothing, and there is a large black 'something' on the other side that I cannot get a decent look at unless I begin unhooking the dash connections.

I uploaded photos of the switch and the '20' button.

I will look into ordering a new switch from MB tomorrow.
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post #5 of 15 (permalink) Old 01-22-2017, 12:24 PM
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Originally Posted by Daytonacoupe66 View Post
Thanks Guys for the input.

..................................

There's a small button at the bottom of the middle point of the dash above the steering console with the number '20' market on it. Any thoughts on what this is for? Pushing it in does nothing, and there is a large black 'something' on the other side that I cannot get a decent look at unless I begin unhooking the dash connections.

.....................................
Probably the accessories ...unfortunately, the electronic copy of the wsm I have of the 435 is missing the circuit diagram, so cannot say for sure. This is an overcurrent (thermal) protection switch and they pop out when tripped - pushing it in should reset it. If you put your multimeter accross the terminals on the back with the ignition turned on, you should get 0V - if you get voltage, it is faulty.
Your next step would be to get a circuit diagram and systematically trace out the system using your multimeter untill you come across the problem.

Life is like a sewer. What you get out of it, depends on what you put into it! - Tom Lehrer
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post #6 of 15 (permalink) Old 01-22-2017, 01:11 PM
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If you want to check the ground connection behind the dash, I believe it to be on the right hand side behind the headlight switch. IIRC you may have to move the side window defrost hose back to locate it. Look for a series of brown color coded (ground) wires all leading to a common location in that area.
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post #7 of 15 (permalink) Old 01-22-2017, 03:57 PM Thread Starter
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Thanks for the pointers. I'll dig a bit deeper.
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post #8 of 15 (permalink) Old 01-23-2017, 04:29 AM
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Your U1350 has a lot more electrics than my U1250 used to have, mine didn't have any fancy key locking switch and my engine turns off manually, not with a key. Mine didn't have a battery disconnect, and I don't really understand what you mean by "reversing the battery disconnect", surely it is just a simple on-off switch, one position is on, the other is off? The battery disconnect switch, it either isolates the main positive lead or the - /earth lead.

The earth connection is on the same frame as the fuses and relays, only right on the bottom. On mine it was a bolt welded to the dash. You need to take the instrument panel off to see it, alternatively you could pop out the speedometer by putting you hands behind the dash.

Car electrics are actually very simple, all you need is a multi-meter to see what is going on. Brown wires are earth wires in MB land, and there always needs to be one positive wire, and one earth wire. The circuits as always simple, at one end is the battery +, the other the -. The chassis, engine, body etc is always -. So all you need to do is sort out how the positive voltage is getting to the chassis. If you have power 12/24V going to the ignition switch, but none of the leads coming out of the switch are showing power when the switch is in turn one position, then the switch is dead. Like all thing MB, you can take them apart and clean them, just watch out for the tiny springs and balls, as they will try and escape when you open the switch. It looks like you have a solenoid on the switch, which stops you from taking the key out. Does it also lock the steering? My OEM switch was a 0FF-ON-Start switch on the dash, simple and very cheap to replace. I added a 100A solid state relay to take the load off the switch, as I run a lot of extra electrical circuits

Electrical Problem U1350L - Thoughts?-dsc05942.jpg

Electrical Problem U1350L - Thoughts?-dsc06860.jpg
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post #9 of 15 (permalink) Old 01-23-2017, 04:40 AM
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is the- we call it "Nato Knochen" - set on?
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post #10 of 15 (permalink) Old 01-23-2017, 06:15 AM Thread Starter
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Hi Magmog; yes, the battery disconnect is at the configuration where it does not interrupt the ground. I have tested that this is not the problem.

Ian; poor choice of language for me on the item that Magmog mentions. The battery disconnect is as configured in the above sentence.

The steering locks when the key is in position 0 or not in place.

I am going to spend some time with the multi-meter and begin tracing the electrics under/behind the dash.

I will report back on progress.

Really appreciate the comments; thanks!
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