ELECTRICAL GREMLINS '88 300E
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Joined: May 2019
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From: Washington, D.C
1988 300E, 1993 400SEL, 2000 BMW M Roadster; 2001 Toyota 4Runner
ELECTRICAL GREMLINS '88 300E
HI guys, new to the forum and looking forward to participating. Already have an issue. My 1988 300E has strange electrics behavior: if either door is open, overhead reading and other lamp light up. if doors closed, overhead lights (front) do not work at all no matter what position I put the switch in. In addition, if doors closed, 4-way flasher doesn't work (does work if either door open). Also, if flasher switch is left on accidentally (because it doesn't work) the turn signals don't work; I'm thinking the latter would fix itself if the door open/closed issue were resolved. Also, console lights behind window switches are dead.
Recently installed (professionally) new Kenwood head unit which had to be wired with a relay directly to the battery to bypass this on-off,door open/closed issue (since radio installer did not want to try to diagnose the separate issue). When he installed the radio it worked fine in the shop, probably because they never tested it with the doors closed. As soon as the job was completed and he was going to demonstrate the use of the head unit, we closed the doors and radio went off. I don't know if the radio installer did something to cause the problem. Original Becker unit played fine with the doors closed, but installer says it was a 100% straightforward install/replacement with no weird connections or changes (other than the eventual decision to wire it directly to the batter to solve it) . Any ideas?
Recently installed (professionally) new Kenwood head unit which had to be wired with a relay directly to the battery to bypass this on-off,door open/closed issue (since radio installer did not want to try to diagnose the separate issue). When he installed the radio it worked fine in the shop, probably because they never tested it with the doors closed. As soon as the job was completed and he was going to demonstrate the use of the head unit, we closed the doors and radio went off. I don't know if the radio installer did something to cause the problem. Original Becker unit played fine with the doors closed, but installer says it was a 100% straightforward install/replacement with no weird connections or changes (other than the eventual decision to wire it directly to the batter to solve it) . Any ideas?
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From: Massachusetts
2018 GLE350, 2014 G550, 2000 SL500, 1980 TR8, 1995 E320 Cabriolet
Your description doesn't quite fit exactly, but it may be related to the infamous seat diode. But before going there, check your fuses for corrosion, especially fuse 9 and fuse C. Take them out can clean the ends to make sure they are making contact. For some reason, the circuit associated with fuse 9 (flasher, overhead lights, radio) is not getting power unless the door is open. Fuse 9 is also involved with the seat diode.
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Junior Member
Joined: May 2019
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From: Washington, D.C
1988 300E, 1993 400SEL, 2000 BMW M Roadster; 2001 Toyota 4Runner
Thank you Floobydust. I have checked (and cleaned the contacts of) the two fuses you mentioned. It didn't make any difference, though one of them appeared to have a broken contact. I am getting a pack of new fuses to replace them and to have some spares. Fingers crossed. Interesting side note: Though the interior overhead lights work, the switch that controls them (on the headliner) works opposite to what it says in the manual. The setting for "interior and reading lamp switched on continuously" (setting 1) is the setting that actually controls them to go on when the door is opened, and vice-versa. And none of them work when the door is closed. The rear courtesy light rocker switch on the console does work, however, and is unaffected by the door opening/closing.
I have also, after reading some other threads, managed to remove the seat diode on the off chance there was something going on there (even though my electric seats worked fine). It seems that lots of folks have electrical problems connected to the doors being opened/closed and, as you alluded to, many of them involve the seat diode. Well, I managed to drop the diode behind the box where it sits, and it is now unretrievable, as it is way back in the engine compartment and I can't even see it. I have ordered a new one and will hopefully be able to install it without dropping it also. I suppose I could try to retrieve it with a short piece of stiff wire with chewing gum at the end! So now I still have the same electrical issues, plus seats that don't move (temporarily, I hope!) .
I have also, after reading some other threads, managed to remove the seat diode on the off chance there was something going on there (even though my electric seats worked fine). It seems that lots of folks have electrical problems connected to the doors being opened/closed and, as you alluded to, many of them involve the seat diode. Well, I managed to drop the diode behind the box where it sits, and it is now unretrievable, as it is way back in the engine compartment and I can't even see it. I have ordered a new one and will hopefully be able to install it without dropping it also. I suppose I could try to retrieve it with a short piece of stiff wire with chewing gum at the end! So now I still have the same electrical issues, plus seats that don't move (temporarily, I hope!) .
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From: Massachusetts
2018 GLE350, 2014 G550, 2000 SL500, 1980 TR8, 1995 E320 Cabriolet
That's a bummer about dropping the seat diode behind the relay box! I've done things like that before. BTW, the seats should be able to move with the key on even without the seat diode in place. Your problem is perplexing because according the schematics, it can't happen. That makes me suspect that there may be an issue with the fuse box itself, or the +12 battery junction block (near the battery), or perhaps someone has done some "custom" wiring in the past. Please post if the new fuses solve any of your issues.
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Joined: May 2019
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From: Washington, D.C
1988 300E, 1993 400SEL, 2000 BMW M Roadster; 2001 Toyota 4Runner
Thanks Floobydust. I am awaiting the new set of fuses and will report if that changes anything once I get a couple of new ones in there in addition to the new seat diode. Will post when i have something to report!
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