Head Unit Woes

[Talasas]

Hi all

I'm having some trouble getting a head unit working after fiddling around yesterday.

It's a Kenwood KDC Series, like the MP333 but the older model with blue lights if that helps (I'll get the proper model if anyone needs it).

Here's the rundown of what's happened:

- Head unit has beed installed in the car in question for about 3 years without trouble.

- It recently stopped turning on. The unit has a flashing red light when the face is removed and is a good way of seing it has power. The red light was still flashing, install the face and it doesn't turn on.

So yesterday we took it out to do wiring for rear speakers and check out what's going on.

- I pulled the 10A fuse at the back of the unit and put it back in, this seemed to reset the unit and it powered up and worked fine.

- We switched it off and I soldered the rear speaker wires on.

- When that was done we tried to switch it on again and.... nothing. No red light, no power on at all.

After going over everything and making sure it was getting power I noticed it sparked on when the head unit housing touched the car body. I checked the housing against the car body with a multi-meter and found that there was now 12V on the outside of the head unit!

Thinking this can't be right I've opened up the unit and there's 12V all over the place where there should be earth - what the hell happened and what does this mean?

I've gone over wiring many times and it all checks out, the only possible thing I can think of is my soldering iron fried the unit somehow?? Doesn't make sense.

[The Witzl]

What colour wire have you earthed the head unit to?

[Mos]

After going over everything and making sure it was getting power I noticed it sparked on when the head unit housing touched the car body. I checked the housing against the car body with a multi-meter and found that there was now 12V on the outside of the head unit!

Thinking this can't be right I've opened up the unit and there's 12V all over the place where there should be earth - what the hell happened and what does this mean?

If the head unit does not have any sort of ground, then the only voltage potential *everything* will be at is the *only* voltage applied - in this case 12V.
Internally the unit will have many connections between 12V and ground - they will all be high resistances, but in the absence of another voltage all these high resistances will "transfer" the 12V to all the other parts - that's normal and is the basis of zapping people with static electricity.

Measure voltage between the ground connection on the plug at the back of the unit and the chassis of the car - if there's 12V there it means nothing is grounding it. If you make sure the ground connection on the plug *IS* grounded and everything else is at 12V, or you're blowing fuses, then you have a problem. It could be as simple as the ground socket in the plug not engaging with the pin on the plug on the back of the unit... or as bad as a number of components shorted in the circuitry.

Mos.

[Talasas]

What colour wire have you earthed the head unit to?

The head unit earth is the black wire and I've tried earthing it to the body, engine block and the battery directly.

It could be as simple as the ground socket in the plug not engaging with the pin on the plug on the back of the unit...

I double checked this also because it's happened to me once before, the plug is sitting nice and tight on the pin. There is no potential difference between the ground pin and the car chassis. Basically I've pulled the unit out and tested it on the bench with 12V in and earth and I get no sparks indicating a short circuit when wiring it up. Same result.

Also, strangely enough using the continuity tester there is no connection between the headunit body and the earth pin and the back of the unit.