In my 1991 300E, I have the signal from the head unit sent to the Amp in the trunk. This signal goes out to the two rear speakers and the two in the front doors.

I have a whine only in the rear speakers the fronts are quiet!!!

Why?

Thanks.

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Esquire:

1991 300E (Autobahn Modified)
1986 Trans Am (Race Modified)

If you are using the stock Becker head unit and an aftermarket amp for the rear, that may be what's causing the problem. The head sends a line-level signal to each of the speakers, but the front speakers each have their own amp in the enclosure case, while the rears share an amp located in the trunk (somewhere).

MB has isolated noise problems with all of them, but in your case, if you are using an aftermarket amp, you may have inherited noise from the wiring path you have chosen.

This narrows the problem down to either the positive or ground lead to the amp. I would vote for the ground lead...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

G Benz:

Thanks for the thoughts.

I have an after market SONY ES headunit with 4v preouts. There are what has been hooked up to the amp.

I pulled the input wire - from the head unit to the amp - for the3 rear speakers and the whine stopped. Does this mean that the4 feed simply stopped or that the head unit (wiring) has the problem? If so, what type of problem?

I checked into both the ground and power leads. Both run very heavy duty gauge wire and the ground lead is short and runs to a frame panel. The power lead is fused. However, all wires coming out from the amp (ground, power and output) run together for about two feet or so before splitting in all directions.

Am I destined for a ground loop isolator?

Many Thanks,
David

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Esquire:

1991 300E (Autobahn Modified)
1986 Trans Am (Race Modified)

I am ruling out the head unit, since the noise is only being transmitted through the rear speakers...

...your wiring seems to be in good order, but I am not a guru in isolating and suppressing all of the various noises that can result in an installation. But it does seem like the fault is in the rear amp.

You can try a couple of things:

Output wire should NEVER be run parallel to power conduits...even a two-foot run could be sufficient to introduce noise into the signal path. If the two should ever meet, do so at right angles. Otherwise, find a completely different path for the speaker wires.

In some cases, the amp chassis can introduce a ground loop because the metal case is grounded to the car frame. Most manufacturers have rubber grommets installed into the sheet metal screw install holes to insulate the amp chassis. Check for those as well.

You may also be plaugued by "radiated" EM noise...after all, cars are chock full of electronic circuitry, and I believe there are some black boxes that reside in the trunk area. That problem would require "shielding" the amp in some fashion in order to block the radiated noise. This is a bit beyond my scope though...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

The best way to clear out noise in early 90 MBs if you plan on using anything aftermarket to clear out anything factory. Aside from that make sure power goes down one side and signal carrying wires go down the other. and NO CHEAP WIRE. pre 95 MB's have all sorts of noise generating electronics.