924s stereo wiring

[nagykurva]

Hello Pokes,
First post asking for assistance , so apologies if things aren’t all correct.
Recently purchased 1987 924s stereo didn’t work.
I pulled it out and the harness unfortunately was all cut and a wing nut connected mess. And what I think is the 12v B+ had a fuse wired in.
Looking at other resources I made some identifications, but I can’t piece together all the wires but 2 are unaccounted for.
The speaker wires I understand.

This is what else I see, (the PO had labels on the wires directly heading to the stereo unit, but since nothing works I can’t assume it’s correct) :

Red white : connected and labeled “B+” I assume the 12V
Red/blue : split into 2 wires to unit one labeled “illumination” , other labeled “ACC”
Brown : “GND” I assume ground !

One branch Off the harness has brown/red and a brown/black they head toward the shifter, I don’t know where they end.

Branch 2 has a brown and a green/white curl around to the left and go towards the sunroof button. Again don’t know their connection pt.

I obviously haven’t done much stereo wiring so any help in figuring this out and even stereo recs to replace this one would be appreciated.

[usury]

Welcome!

Looks like it's an aftermarket head unit spliced into the factory wiring harness, yes?

I've done a decent amount of stereo installs over the years. I'll try to help.

The aftermarket head unit likely needs at least two power inputs. One "constant" (or B+ as in battery positive), meaning "always has power regardless of the vehicle ignition switch position". The other is "switched" or "accessory" (ACC), as in "has power when ignition switch is on" (or in the accessory position without the vehicle running).

Brown it's indeed ground/earth, for many German cars from the 80's era in fact.

You'll need a test light or digital multi meter to confirm which wires provide those power situations. If you know how to use those tools, great. Otherwise, just ask. It's pretty straight forward. The $5 DMM from harbor freight is just fine for this kind of troubleshooting.

Some head units have a third power input that dims the display/button illumination with the rest of the dash lights. Voltage on that wire will change with the dash dimmer knob (and with the headlights on, of course). Or at least with the marker lights/parking lights/running lights on.

Voltage will be present on the illumination wire based only on the headlight switch, *not* the ignition switch. So find the "accessory" wire in the vehicle harness with the headlights off and the ignition switch on. Then identify the "illumination" wire with headlights on and ignition off.

It's always possible to connect the "illumination" wire on the head unit to the "accessory" wire from the vehicle harness, making the head unit display/buttons fully bright regardless of the dash dimmer knob.

Edit to add: it's also good practice to avoid the previous owner's twist-on wire nuts in any vehicle wiring. They tend to jiggle apart. Once you have the power situation sorted, consider replacing them one by one with crimped heat shrink butt connectors.

[nagykurva]

Thanks you! This makes sense. I guess I thought 2 wires were missing based on pictures of other people’s harnesses. I will test out the wires soon.

[notny41]

I can't remember which site I found mine on, but wiring diagrams are available online for your entire car in sections. mine (944 turbo) had a complete section devoted just to the radio and speakers. It even had different radio options and which pinouts of the main power connector with all of the wire colors and where they go (believe it or not the always on +12v pin was in a different spot on the plug depending upon which radio came with the car). Yes, Brown is ground.

I've printed each section out and taped them together and now I have a library of wiring diagrams that has come in handy many more times that I originally thought.

I think this is one year newer than your car but probably similar if not the same for the radio section:

https://www.manualslib.com/manual/39253 ... 260#manual

Here's a good tutorial on how to read them:

[stoneguy]

Stereo Wiring.jpg (531 KiB) Viewed 1917 times

Does this help??

[usury]

The @stoneguy post reminds me that your car may have an antenna amplifier which needs to have 12V when the head unit is on, otherwise your FM reception will be terrible.

Some head units have a "trigger" output, often used for activating a relay to power an external *speaker* amplifier. That trigger output can also be used to supply 12V to power the *antenna* amplifier.

If the head unit lacks a "trigger" output, the antenna amplifier can be powered by switched (ACC) power.

[nagykurva]

Thanks for the replies.
I glanced back at the option codes, 158: cassette-radio blaupunkt Reno SQR 46. Not sure if that gives anything extra to the discussion.
Seems though I have a lot less wires than shown in that diagram.

[nagykurva]

So I finally got around to cleaning up everything in there stereo wiring mess…
Also found the multimeter.

It seems mine has the following: 3 wires for power, the speaker wire clump , and the antenna cable.

From the pics the red white = constant power B+ confirmed with 12-13V without key on multimeter
Brown seems to be the ground as most noted
The red/blue I presume is the ACC or ignition switch but,

I cannot get a volt reading on red/blue when I turn the ignition key to on position.

Fuses all intact. Fuse 7 that reads “ radio , clock, cigarette lighter” is intact and the clock and lighter work.

Would the next thing I do to check the ignition switch? Would it be checking the wire that attaches to the switch?

Thanks in advance !

[Tom]

I could be way off on this, but I think there is a relay in the early cars (and probably the 924S) that switches the ignition-on wire. You might check for a radio (and sunroof?) relay and see if it works...

[944er]

As Tom has noted, the red/blue wire is powered through the radio/sunroof relay, which is clipped to the hvac box in the center console, up above the accelerator pedal. It's coil is powered by a separate circuit in the ignition switch which closes when a key is inserted. These are your top two suspects right now. The signal that powers the relay coil also goes to the warning chime for indicating that the key was left in the ignition when you open the door. So if that works, then the relay is the suspect.