OK, I have a nice little electrical system puzzler for you all. Please read carefully and propose your theory as to what is not right...

Back-story: I had a wily battery-draining problem last year that I finally traced to a combination of a sticky passenger window switch and some sort of electrical fault that was allowing the stuck window switch to drain the battery after the engine was shut off. That is to say, the passenger window switch would get stuck in the “up” position, I would park the shark and shut the engine off, but the circuit for the window switch would not shut off, allowing the window motor to try to keep closing the already-closed window, thus draining the battery. When I would return to the car after a couple of days, the battery would be dead. I eventually realized that the sticky switch was the cause, so I began to check that it wasn't stuck in the "up" position when shutting off the engine, and I haven’t drained my battery since. I ordered a new switch, but I haven’t installed it yet (I’m waiting until I have a laundry list of center console fixes so I only go in there once). I thought this was an odd way to wire up a car, but having two windshield-washer systems seemed far odder, so I didn’t think too much of it at that point – I was just happy to be able to start my car again!

Related detail of the back-story: When using my radar detector, I was annoyed to find that the cigarette lighter did not shut off with the ignition, either. I had to unplug it each time I shut off the car and plug it in again when I got back under way. I’ve known other German cars that did that sort of thing so I assumed the 928 was intentionally wired up this way.

Tangentially-related detail of the back-story: my passenger side door courtesy light (the 3-position one on the door) never worked correctly – when I could get it to do anything, it would usually turn on when the door was closed and off when the door was opened.

OK, some of you are probably bouncing up and down in your seats peeing yourself at this point, thinking “I know what it is! I know what it is! It’s the relay!!” Well, it’s not so simple, so wipe up, take a breath, and keep reading. If you are not in this overly-excited group, then here’s what they know and what I just learned – the operation of the windows and lighter are all tied into a relay that is supposed to stay energized after the ignition is turned off, then de-energize once a door is opened. (I am not sure which relay it is, but I’m guessing it’s the one in socket VII?).

[Here’s a stumper on-the-side for the gurus (i.e., Alan)... which door or doors, when opened, is supposed to trigger this relay to shut off?]

Well, not understanding the finer points of how the electrical system is supposed to function, I brought it to my local shop (Nino’s Performance in Bethany, CT) to have them rewire the ciggy lighter and windows to operate from "switched" power instead of what I mistakenly thought to be "unswitched" power. My mechanic Joe, being far wiser in the these matters, realized that those components are operating through this delayed-shut-off relay, so he naturally assumed it was sticking in the energized position and replaced it with a new relay (that is, the same part number – see discussion below).

However, the new relay also stayed energized after the ignition was switched off and the doors were opened. He then turned his attention to the courtesy lights to get them straightened out, suspecting that their miswiring was preventing the relay from shutting off. He fixed my passenger-door light wiring and replaced the hatch light (which had cracked and was shorting out its circuit), but the new relay still stayed energized after the doors were opened (yes, he verified that the relay was getting the door-closed signal).

Although I checked over all my relays last year and verified that I had all the right ones by p/n in the right sockets, I realize now that I cannot be 100% confident that the relay in question was correct, since I just learned that there is a 1989-only version as well as non-1989 versions of that relay, and a PO might have replaced it with a non-1989 relay. I don’t recall what information I used to verify the relay part numbers last year, so I cannot be sure it was correct for a 1989 GT.

Having learned all of this, my mechanic was not able to get one of these “other” relays from his supplier that day (meaning, the other spec’d relay, whether it was the 1989 version or a non-1989 version... whichever I didn't have installed in my car), so he put in a generic Bosch 30A relay with no IC/timer/trigger mechanisms in it, which got the windows and lighter to shut off with the ignition, so that is fine with me for the time being. Of course, I’d like to sort this matter out and have the system function as designed. But before I can do that, I need to figure out what is not working correctly in the wiring to cause the relay to stay energized, so the root questions go out to the forum...

What could be wrong in my wiring that would cause the brand-new relay (with corrected door and lighting wiring) to stay energized after the doors are opened?

Or is it more likely that I had a non-MY1989 relay in there courtesy of a PO (considering that I found two adjacent relays in each others’ sockets last year, it’s not so unlikely) and the solution will be to find a MY1989 relay. If so, what should be right relay p/n for a 1989 GT?

Thanks in advance for your help!

 

Well the story is a bit confused...

It doesn't work the way you think: there are 3 relays here you are discussing:

The X-Bus repeater relay, the window regulator relay and the interior light relay.

1) There is no relay that controls the cigar lighter it is driven directly from the ignition switch accessory position (X-Bus) - so maybe your switch is screwed - or its been rewired.

The X-repeater relay (In position IV) is a regular SPST ('56) relay and just repeats the X-Bus - which does not stay on after the ignition switch goes to off position (this seems to be a well established myth that is just wrong). It has no impact on anything here...

2) The windows are controlled by the window regulator relay - its a special type
and goes on with the ignition and off when a front door is opened (same switches that control the interior lights). If your lights don't come on - there is also a good chance your window relay doesn't ever switch off. Even without operating a window this is about 200-300mA continual current draw - enough to kill your battery in << a week. Easy to check open a door and test it (incidentally on a euro the windows are designed to be active whenever a door is opened - so on a euro open & close the door to see if the windows are really off).

3) The interior light relay comes on with a door opening and turns off a delay after a door is closed.

Alan

 

Gaz,
Are you sure that both interior door lights are wired up the same way? I had some really quirky things going on with my interior lights and other electrical gadgets in the car. I am, by no means, an electrically oriented person, but what I found was that one of my door lights was wired with a power wire where a ground should have been. This caused there to be power to things when there shouldn't have been, etc. It's an easy check and YMMV. Good Luck

 

Practical things to do/test:

Fix the interior lights first - if they work correctly - you can rules those issues out

Test the direct ignition X-Bus at the top of fuse 2, 4 or 5 ('89)

Test the repeated X-Bus at the top of fuse 3 & 6 ('89)

Test the window regulator output at Central Electric Plug/Pin G24

Alan

 

Thanks for the tips!

Alan, yes the story is a bit confused - I tried to relay the history of the problem (pun intended) as I saw it happen. I now see that I have to check on the door lights to make sure they are indeed fixed, which I will do tonight (although I didn't actually drag the interior lighting relay into the story, as I knew it was not related to my problem). I realize that my mechanic didn't give me back the original window relay, so I am not 100% sure of which relay he was toying with. And I was assuming the lighter is now switching off with the ignition, but I didn't actually check it.

Nevertheless, my mechanic categorically told me that he corrected the passenger door courtesy light wiring and replaced the shorting hatch light, and the new window relay still did not shut off once the door was opened. Yes, he might be wrong, but if that was truly the case, is the x-bus swithc the only thing that be causing this problem?

With all of that in mind, here are my follow-up questions, Alan:

Is the window relay supposed to shut off when either front door is opened, or only when the driver's door is opened?

How exactly do I test the window regulator output at Central Electric Plug/Pin G24? (I assume for the others I should be checking that power to these fuse positions should be coming on and off with the ignition, correct?)

If the lighter is unrelated to my relay issues, is it supposed to switch off with the ignition? If it doesn't, what would be the likely cause, considering the window issues.

 

The X-Bus (direct from switch) is what turns it on - so if the X-Bus stays on even the doors opening won't turn it off.
Either door should shut it off
- Yes - and you can just probe into the metal tab in the indent at the top of the fuse (no need to remove it).. For the Plug/Pin G24 - same thing - except you need to probe into the back of the plug while its still connected. Plug G the 4th pin up on the right hand side. - measure voltage to ground - just like on the fuses.
Yes on all model year 928's it is supposed to go off with the ignition (on in position "Accessory" & "Ignition" only). It seems several folks have claimed theirs didn't - its either creative rewiring or their ignition switch is broken as I suspect yours is... It doesn't actually cause a lot of horrible symptoms.. it will allow equipment to run - but usually doesn't cause more drain than the 200-300mA - so if you drive it every few days its probably OK - maybe there are many of these out there.

if your ignition switch Accessory (X-Bus) output is always on - your cigar lighter will always be active.

Does your blower motor run with the key out? do your foglights work with the key out - good indications of this issue...

Alan

 

Alan, your knowledge of the 928’s electrical system is amazing. Thank you so much for helping me sort out this problem!!

I was able to double-check my interior lights last night after the PCA meeting, and they are all working properly – all four of them (2 doors, hatch, and cabin lights) came on when I opened my door and shut off after a 20-30 second delay.

I was also able to double-check the cigarette lighter, and it still stays on after the engine is shut off and the key is pulled.

Although I haven’t actually checked it since reading your post, I am quite confident that my HVAC blower shuts off with the engine since I know the A/C function shuts off, as does the radio, too. I am not sure about the operation of the foglights, though, so I will check those (turn low beams on without the key in the ignition and try to switch fogs on and off, right?).

I have not been able to check the x-bus signal at the fuses yet, but I will. It certainly sounds like that’s the problem, though. I do not have a high current draw with everything shut down (it was tested repeatedly last year at 18-22 mAmp), as long as the window switch is not stuck, of course.

That said, while I understand the concept of what’s going on, I do not understand which pieces of hardware are involved. When you say “x-bus” that’s just a switch output, not an actual electrical component, right? If so, then which part supplies this output? More to the point, which part should I replace? Is it one of these:

Ignition Control Unit, 928 602 706 01
Ignition Electrical Switch, 964 613 012 00
Ignition Relay, 928 618 175 00

or something else entirely?

Thanks again!!

 

It is just a major wiring bus in the car fed from the ignition switch:

Bus - TermID - Source - Function - On w/ Switch (of 0-3)
30-Bus - 30 - Battery+/Alternator+ - Const. System Voltage - 0-3 & Out
15-Bus - 15 - From ignition Switch - Eng. Run Mode Power - 2-3
X-Bus - X - From Ignition Switch - Accessory Mode Power - 1-2
R-Bus - R - From Ignition Switch - Radio Switched Power - 1-2/3
50-Bus - 50 - From Ignition Switch - Starter Switched Power - 3

Ignition Electrical Switch, 964 613 012 00 - But test the X-Bus first...!

Alan

 

Yay!! Only $55 new!!! It must therefore be a real PITA to replace, huh?

I want to double-check this point with you, Alan... is it consistent with our ignition switch theory that the HVAC blower shuts off with the engine? Or would this fact refute the theory?

And out of curiosity, what else operates off the x-bus besides the windows, foglights, and the lighter?

 

X-Bus Equipment All S4 plus years (direct from ignition except as noted)
============================================
Headlights High Beam
Headlights Low Beam
Driving Lamps
Brake Lamps
Cruise Control
Cigar Lighter/Power Outlet
Windshield Wipers Front
Fan Control Unit
HVAC Unit ’87 - ‘89 Via X-Bus Repeater relay (in slot IV)
Fog Light ’87 - ‘89 Via X-Bus Repeater relay (in slot IV)

If none of this equipment works it would suggest its a different issue - if only the cigar lighter works with ignition off - maybe its been rewired

Alan

 

I am inclined to think that the problem is more than a rewired cigarette lighter, since the window relay was not shutting off either. Is there any reasonable explanation if some of these x-bus-activated accessories shut off properly and others do not?

The next thing for me to do is to check the rest of the accessories on that list, as well as the fuses, and proceed from tat set of facts and observations.

 

Alan,

I finally got around to checking everything. You were right all along - the ciggy lighter has apparently been rewired, because it still works with its fuse pulled out and never switches off. When I dive into the center console, I'll see if I can't find its original switched power supply.

I checked the x-bus and x-bus repeaters at the top of fuses 5 and 6, respectively, and they both go on and off with the ignition switch - with no delay after turning off. You were dead on about that as well.

As for the window relay, it was replaced by a different one by my mechanic, and he did not keep the original, so I can't do anything about that now. I'm happy with my windows not working after the ignition is switched off. I am guessing that the window relay I had in there just wasn't working right, or that it wasn't getting the open-door signal.

Thanks for your expert help with this!!

 

the cig lighter is easy to get to, just remove the 2 screws on the right side of the console cover . Then you can also replace the pass window switch, push up on the switch from under the console to remove it the connection point is just in front of the radio.. As always disconnect the battery before you work on the elex system

 

Actually you can just unscrew the ashtray unit as a whole from the console - including the power outlet... (a screw at each side) but there isn't a lot of extra wire so removing the console side may help also - but you should be able to see what you have got going on...

Alan

 

My cig lighter stays powered at all times...which I like and thought was normal...all other electrics in the car seem to work fine.