Background:
A friend installed an aftermarket stereo in an '87 BMW that he bought and then found out that the factory amp in the trunk was missing!

If you will notice on the wiring diagram (right side of top dia.) that the BMW factory radio only has two output wires, 1 blue for the right and 1 yellow for the left with no grounds. It is then split into front and rear by an external fader and fed to the amp in the trunk which is designed to work with speaker level signals.

It is common to install aftermarket stereo's in older BMW's using only the 2 positive right/left front speaker wires and leave the speaker grounds completely unconnected in the new unit (not tied to the car ground). This is what the "Official" adapter harness does, and this arrangement works just fine.

The question:
Would it work to jump the speaker input pins to output pins in the factory amp's harness plug, bypassing the missing amp and connecting all of the speakers' ground pins together with the car ground pin? Or must there be circuitry in the amp that handles the grounds differently?

Speaker
Position - Pin# to pin#
Right Front - 20 --> 2
Right Rear - 19 --> 4
Left Front - 15 --> 16
Left Rear - 14 --> 1

Then jumper the ground pins 3, 5, 7, 17 to pin 18 (just twist them all together)

Radio diagram:

Speaker diagram:

 

Sorry, but from the way you phrased it, what you're saying is impossible -- there MUST be a return (2 wires), especially if it isn't using ground (differential / BTL).

That means either two wires going to a speaker, or from an output, or that the amp chassis is bolted/wired to the metal car chassis as ground/return (and one side of the speaker is chassis grounded as well).

An adapter may provide, one way or another, a connection to car ground, but a speaker will never work with only one wire.

 

Thanks for the reply, but if you take a little more time to read the entire post and the diagrams, I think you will find that I was not in any way tring to get a speaker to work with only 1 wire.

Of course there are two wires to the speakers, the second diagram clearly shows them going to the factory amp.

The single wire set-up (as opposed to 2 dedicated wires) is from the factory radio to the factory amp. Like you, I assumed that the return is through both the radio and amp being grounded to the car chassis.

Hundreds, if not thousands, of aftermarket radios have been successfully installed, without adapter harness's and leaving the dedicated speaker grounds unconnected, apparently mimicking the ground return through the car chassis as the factory does. (Here are the instructions for this is anyone is interested) That is not in question here.

The question is, can this same concept be carried through by jumping the inputs and outputs of the missing amp and connecting the speaker grounds together and to the car chassis ground to provide a return to the radio?

I must confess that, although being an aircraft mechanic has always required a bit of knowledge of electronics, I have never fully grasped the differences between different reference grounds and chassis grounds. For instance, I'm not sure what you ment by: "especially if it isn't using ground (differential / BTL)"

 

I can lisp in schematics for hours, but have no patience for wiring diagrams. I did quickly peruse your diagrams, but didn't get much from them, I'm going more by what you say, so the clarification helps. Maybe someone else will come along with more experience in this issue, but until then....

If I understand you correctly, you want to run the output wires from the radio, through the missing amp connections, to the speakers? If your radio has single wire outputs and is using the car chassis as ground, then I would say ...yes.

If the radio has two-wire outputs, it is possible that neither one is ground (therefore grounding one or the other would be a big mistake). There is a technique known as Bridge-tied-load (BTL) where the +/- of a speaker are each driven by an amplifier. This method is very popular for delivering ~4 times the power normally available from the automotive +12V. This method does not use a ground to the speakers, each side of a speaker is driven 180 degrees out of phase. A similar use of this method is with Class-D, or switching amplifers, which also do not use ground (speaker +/- are both driven). As I said, this is to get greater power output than one might normally with analog output circuits run from +12V and GND.

It is also possible with some amps (usually older designs), where the output is not relative to ground, but relative to the supply (+12V) -- so this would not use ground either (I think it highly unlikely this would be used in today's radios / cars). And, I'm not talking about positive ground cars.

So if you are sure your radio output is relative to ground (either one-wire relative to chassis, or two-wire w/chassis GND), then what you say about by-passing the missing amp seems reasonable. Just think about the number and quality of wires that you need -- a ground/return wire for each speaker that each goes into the chassis (for best sound quality).

 

I'm not sure if I understand the post, but I want to mention a couple of things.

As nomurphy said, it is very likely that the outputs are for a floating load, so it would be best to run discrete wires. Also, if you plan on running signals through the chassis, then I advise against it. There are a few reasons why you shouldn't do this, including poor signal quality (impedance chassis >> impedance signal wire) and transients from solenoids, etc.

Another thing too, pretty much all power amplifiers use a flyback boost converter to step up the voltage. I think that they do connect the grounds of the supplies, usually through a low value resistor. 12V is pretty useless across an 8 ohm load, even if in bridge configuration (72W across 8ohms)

Steve