C124 speakers have all stopped working!

[denty]

I recently overhauled the stereo system in my 1990 300CE, fitting a Pioneer DAB head unit and replacing the front dash speakers for a pair of Crunch 130mm's and a pair of Infinity's for the rear. The DAB receives signal with a splitter using the existing electric telescopic aerial.

All was working fine for a few weeks and then one day the sound completely cut out through all channels. It came back on briefly in little bursts, usually with the car driving over bumps and potholes. But now - nothing.
I still had the old Kenwood stereo I'd taken out, so I refitted that to see if it was a fault with the amp on the new Pioneer head unit. But I got the same problem with that unit too.

The old wiring in the dash has been connected to the modern ISO system with bullet connectors, which are all labelled and the connections are all good, from the 16-pin connector at the head unit end back to the bullets on the old wires - I've checked these for resistance with the multimeter.

So I'm baffled! My only thought now is that there is possibly an amplifier hidden somewhere which could be having some trouble. Question is - where? And if I find it, would it be a case of simply disabling it, as the Pioneer head unit has an adequate 4x50w output? My car doesn't have the fader switch in the centre console.

Anyone got any ideas? Thanks!

 

[brucemillar]

Oh Oh. I have the same issue on my 124 and it drives me potty, as I cannot trace the fault. I am deeply suspicious of the fader switch in the center console and have pulled it out and cleaned it. So far so good?

 

[denty]

Haven't got the fader...

 

[grober]

First thing to check is whether your radio/head unit has a good earth- the mounting may not be enough. The existing earth lead may be missing/ making poor contact/ corroded/ broken. Likewise with the main power lead and its fuse[ corrosion again] radio display light up may be no indication as it often has a separate supply possibly linked to dash illumination. You need to refer to the pioneer dab head unit wiring diagram and check all its power leads are functioning. Some head units have permanently live + live on ignition supplies And these can get mixed up with odd results.

 

[denty]

Thanks for the suggestions Grober.
There is a wire on the radio harness for dash illumination but this isn't connected. If this was connected, the display would dim when the lights are on.
You may well be on to something with the live feeds though - there does appear to be a bad connection somewhere in one of the live cables (either continuous feed from the battery or the ignition live) as both the head units have lost all power when I've been moving them around whilst poking around in the radio aperture.
I'll look into this further.

 

[denty]

OK so after some testing with the multimeter I discovered the ACC live was bad. Having sorted this and tested the continuous live to ground I was happy that I was getting voltage. Plugged everything back in and for a couple of minutes or so I got my sound back. Then it went off again!

I then tested the speaker connections in the ISO connector for resistance. I got 0 readings on all but one - the front right, which was reading 7.
I'm no expert with electronics - does that rogue reading imply a short circuit? And if so, would that cause the amp in the head unit to cut out?
All the speakers fitted are 4ohm.

 

[grober]

That doesn't sound right. sure you are measuring across the correct connections? Speaker impedance is usually 4 ,6, or 8 ohms If the amp is rated for 8 ohm speakers then 4 ohm speakers may draw too much current and the amp will shut down. Final resistance may also depend on how exactly the speakers are wired. Any short between any positive and negative leads is bad news. How to Measure Speaker Impedance | Our Everyday Life

 

[denty]

Solved! I disconnected the front right speaker, where the bullet connectors had joined the original wiring to the ISO connector. Sound came back in the remaining speakers. I popped the offending speaker out of the dash and tested it for continuity across the terminals. That was fine, so then worked back towards the head unit, checking the 2-pin connectors in the dash and then both ends of the factory speaker cables. All checked out fine. This meant that the only possible fault was the bullet connectors themselves.

So I have put both front speakers onto terminal blocks (as I had these to hand) and everything is working fine.
I assume the reason I wasn't getting any resistance to the other three speakers when I was testing them was because the amp in the head unit shut itself down possibly to avoid damage.

 

[denty]

...and thanks for that last post and link Grober!