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Sonusthree 6th July 2007 02:27 PM

How do I diagnose Faulty head unit?
Hi everyone,
My friend is bringing his car over later for me to look at ......

It's a MK4 (I think) vauxhall (GM) Astra. The head unit is original and has four speakers.

Both of the left ones are not working.

I know how to check the speakers and wiring but how can I test if the left outputs are blown? I'm quite new to car audio but I assume that they use class D? Would there be a short if it is blown?

You can probably tell that I'm in a muddle about this. I may only have access to the speakers until I can buy a specialist removal tool for the HU. (Already ordered from Ebay.)

What's the checklist?

All help would be very gratefully received,
Martin :)

Lsharptec1 6th July 2007 02:43 PM


I suggest the following:

1. Have a pad and pencil to document the results for each step of troubleshooting.

2. First, verify that the Balance & Fader controls are set in the middle. You'd be surprized how many times this has been the case.

3. Next, verify that only the right speakers are working.

4. If the HU is connected to a wiring harness, make sure that one side is not loose (losing connection).

5. Tag the speaker wires, and swap the right channels with the left. If the left channel speakers now work ... the problem is most likely in the HU.

6. If the left speakers still don't work (connected to the right speaker wires), the left speakers may be defective.

7. If the HU is found to be defective, replace it with a good quality aftermarket HU.

8. I recommend one with an SD card slot and USB port for playing MP3 files from memory drives.

Good luck.


Sonusthree 6th July 2007 04:29 PM

Cheers Larry, I'll do all of those things. What comes after that?

I'm looking to go a little further and find out if the outputs are blown, the pot is defective or the HU has dry joints.
After that ......

I'll advise him to get a cheap GM (vauxhall) headunit until he can get the money for something better and the required ISO connector, and fascia.

Can I check the output transistors with a DMM on the speaker wires?
Will the outputs be shorted (Zero Ohms) if defective?

What ballpark resistance woud a healthy channel read? (I can't remove the head unit just yet to find out for myself until the removal tool arrives. I may have to remove the door panels to access the speakers too.)

Basically, I don't want to remove the head unit and open it up if the output transistors are blown. If they seem OK then I may have a stab at the pot or dry joints.

Martin. :)

Lsharptec1 6th July 2007 04:49 PM


Typically, most HUs use 4 channel IC amplifier chips for speaker outputs (not separate transistors).

Also typically, if the IC shorts ... none of the outputs will have any sound.

Since this problem involves only the left speakers, I suspect a wiring problem rather than a defective amplifier IC.

Troubleshooting a car audio problem CAN BE very frustrating.

Your best test equipment is a good set of eyes. Look at everything closely.

You may find the root cause through observation, rather than checking for shorts with a meter.

Again, good luck.


Sonusthree 6th July 2007 05:29 PM

larry, I can't thank you enough.
I'm gonna be working on it in about 30 minutes time and I was getting a bit stressed.
The people around here seem to think that I can fix just about anything and I always feel a pressure to 'perform'. I get a massive buzz from fixing stuff and feel equally dispondent when I can't!

Your advice is very much appreciated and will help me immensely.

Long live DIY!

Martin. :)

Sonusthree 6th July 2007 09:26 PM

Well, My friend was overjoyed!
There was a slightly obscure balance control!:smash:

He wasn't aware that there was a balance control since it wasn't clearly marked. It was the type ( quite common) where you pull the volume 'knob' out but, unusually, the balance pot is more like a switch because each side may only be switched on or off.

Cheers Larry, you were right! and I'll know what to do even if the next problem is a little more complex. ;)

Thanks again,
Martin. :)

Lsharptec1 6th July 2007 09:36 PM


My compliments on solving the problem for your friend so quickly.

During my many years in electronics, I have learned to never overlook the obvious.

Glad to be of assistance.


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