Car Stereos: Car head unit, vauxhall corsa, wiring loom

Question
QUESTION: Hi,
I've got a Vauxhall Corsa 1999 reg. It came with an aftermarket head unit that didn't work (clarion). My brother tried his (kenwood) and that worked so I bought a radio for it (in phase).

I used the existing wiring loom that seems to be the adaptor for the corsa.
The new radio works with the ignition on, but when I start the engine it loses power and doesn't restart until the engine's turned off.
My brother and i tried his radio and when the corsa's started it cuts power then the radio comes on again.
When trying the new radio in my brothers car (fiesta 2001) it works with ignition, cuts out when engine's started then comes back on after a few seconds.

So, both new In Phase and Kenwood work in my brothers car, but only the Kenwood works in my car.

Any ideas?

Many thanks

ANSWER: Hi Tom,

It's certainly an strange problem.  My guess would be that it's a combination of an unusually high alternator output voltage, and an unusually sensitive head unit.

As far as the radio power is concerned, the only difference between the engine running and the engine off is the change in system voltage produced by the alternator.  When the engine isn't running, the radio is powered completely from the battery, and the system voltage typically rests around 12.6 volts.  Once the engine is started, the alternator takes over the power production, and the voltage rises to 14.4 volts.  Most head units have an operating voltage range between 11 and 15 volts, and they'll shut down if the voltage climbs too high or falls too low.

It's possible that the alternator in your Corsa is producing a voltage level that's out of spec.  If so, the Kenwood head unit may be able to better tolerate high voltage than the In Phase unit.

That's my theory, anyway; the way to confirm it is to use a multimeter to test the DC voltage at the power wire in the radio harness.  It might be interesting to test the voltage level in your brother's car as well, to see if there's a difference.  It's possible that the In Phase head unit is just more sensitive to a higher voltage level than it should be; but if you do find an exceptionally high voltage in your car (over 15 volts or so) then you'll want to have your alternator serviced.  A high voltage output can cause damage to your car's battery and computer system, in addition to causing odd problems with a radio.

Hope this helps!

Brian

---------- FOLLOW-UP ----------

QUESTION: Hi,
I'm afraid I don't have a multimeter, I do have a voltage tester (the light bulb) and that only registers power at one socket in the ISO adaptor (Blue wire). Looking at wiring diagrams for the car it shows two positive wires going to the radio (both red), one via the fuse box (which is fine) and one via the ignition switch. Whatever position the ignition key is in, there is only the one wire that lights the bulb.
I don't know if this helps? I can get hold of a multimeter but it won't be for a few weeks so I wanted to see if it's anything else :)

Many thanks
Tom

Answer
Hi Tom,

I'm hampered by the fact that I don't have a good source for UK wiring diagrams.  North American vehicles are a bit different; we don't use the ISO-type plugs that are common in Europe, so just about every head unit installation requires an adapter for that particular vehicle.  That means I'm usually dealing with the after-market wire color standard; the constant power wire is generally yellow, and the key-switched power wire is generally red.  That only applies to the wiring that comes with the head unit, though; factory wiring doesn't follow that standard.  This means I don't know the constant and switched power wires in your Corsa.  However, if one or the other wire wasn't getting power, I don't think the Kenwood head unit would be working in the car.

If it's a problem with a high voltage, then the test light won't help you much; it can tell you whether voltage is present or absent, but can't tell you the voltage level.  For any sort of in-depth troubleshooting, a multimeter is essential; though it doesn't have to be an expensive one.  The cheapest digital multimeter you can find would work perfectly well.

Good luck!

Brian