The fact that the SCREEN went black or blank is a red flag to me that the HEAD UNIT itself is Faulty. Even if the Audio part of the head unit was wired wrong, I've never experienced a Blank Screen or intermittent display, UNLESS the power wiring (-) and (+) were done erroneously/inadequately.

I would make sure that they are NOT using the (-) Ground wire from the OEM HARNESS to the Head Unit's (-) Ground wire or for the MOST adapter's (-). They should run a separate (-) Ground from the chassis or direct to battery for both.

That head unit has a 15-AMP FUSE so it will need at least a solid 15-amp (+) 12v switched ACC power connection which the OEM stereo harness probably cannot supply (most are around 5-10 amps MAX). This means that the (-) Ground must also be capable of handling 15-Amps, and most OEM harnesses will Not handle that much IME.

They also may be trying to power the (+) power input for the MOST adapter by using the Head Unit's "Remote Turn-On" (+) wire, which is only for low current to operate a small turn-on relay in an aftermarket amplifier, and it will NOT power the MOST adapter adequately.

When you connect an aftermarket MOST adapter, you also need to make sure that any interrupted optical connection or optical "T" connections still complete the entire MOST loop throughout the vehicle. The adapter's MOST connection has to be a Pass-Through, you can't leave any of the OEM Most connectors disconnected, they must all be connected in a loop.

But to me, it seems like a faulty Head Unit. OR incorrect/insufficient (+) & (-) power wiring to the Head Unit and/or MOST adapter.

The power connections they made might not have enough current (amperage capacity), which would allow the head unit to work for a little while until it over-heats due to under-current/lack of power and then shuts off due to that low power state and from thermal overheating. Unfortunately you can't put a CD disc in the unit and eject it after this happens to see if it is super hot. It shouldn't be.

Are they using a MOST adapter like this one?....

Fiber Optic(MOST) adapter for Mercedes-Benz/Porsche/BMW : Aftermarket Navigation Car Stereo, Android Navigation DVD Player, Car Navigation Head Unit

Aftermarket Navigation Car Stereo Fiber Optic(MOST) adapter for Mercedes-Benz/Porsche/BMW - This Fiber Optic(MOST) adapter allows you to retain the factory(Mercedes-Benz/Porsche/BMW) MOST amplifier audio system(Harman/Kardon or BOSE) when installing ANY aftermarket radios. The output from all...

www.carnaviplayer.com

Go to Peter's PSsound Facebook page or Instagram and send him a message. He's in the UK and works on a lot of BMW & Mercedes and would most likely know the probable causes. He's a busy guy so, don't expect an answer right away.

https://www.facebook.com/pssounduk/

 

bbfoto.....brilliant reply and thank you for taking the time.
My installer is convinced its a power issue although he thinks its to a different module ( does CAN mean anything to you? Sure he mentioned this! ) ....he believes a direct power from the ignition supply would suffice however it would bypass this module meaning I'd loose the minimal steering wheel controls . ..not an issue if it worked!

The MOST module is the same as you show ...as a reference the kit I've used is from a UK company called Connect2 and the part is CTSMC008.2 specifically designed for my vehicle with harmon kardon fibre optic.

I've had a reply from someone saying it is probably a speaker issue in that Harmon Kardon usually have 2 ohm speakers where as most are 4. He says the headunit has a thermal relay and this is kicking in causing my issue .....I haven't a clue if he's right but he mentions being a kenwood installer for 24+ years!

My only worry is most folk I talk to who install etc, they have never had this issue with a kenwood headunit making me wonder if its my set up causing the issue. I can only hope it hasn't damaged the head unit as its currently on its way back for a warranty change. If I get a new unit and my car causes that to do the same im sure the selling company won't exchange ....

I'll send an email to the guy here in the uk.

Thanks again ...

 

Ahhh...you didn't mention changing the OEM speakers for new aftermarket speakers. Yes, I believe the OEM speakers are 2-Ohm (except for the tweeters), so if the aftermarket speakers are 4-ohm this might be affecting the OEM H-K amp, but I doubt it. A 4-ohm load is easier to power, though you would typically get just half of the wattage from it to the speakers (slightly less volume).

And that shouldn't affect how the Head Unit was acting. Hopefully it was just a bad head unit.

But again, the shop needs to MAKE SURE that the switched ACC circuit they are tapping into will supply at least 15-amps on both the (+) & (-) to the head unit.

They may need to use that switched ACC circuit to trigger a Turn-On Relay, and the relay would then be wired directly to positive (+) and (-) from the battery to supply the head unit with power. That circuit (the + power wire) should be protected by a 15-amp in-line fuse near the battery.

Most modern vehicles are Either CAN-bus (copper wire) OR MOST-bus (digital fiber optical). AFAIK, you will never have both types of communication buses in the same vehicle.

 

Lol! No you've misunderstood me, they are the factory speakers ....ive changed nothing but the head unit.
My fitter thinks its a power issue ....Im sure your info re a relay would be the best bet...
Headunit arrived back at the shop this afternoon so will see what they think!
I'm worried now about plugging a new one in incase its my car causing the headunits problems!!

 

Yup, I was confused about the speakers with what you wrote in the previous post. 😊

"Your System" is a OEM factory system. YOU are not responsible if the Kenwood shop/installer that sells you the head unit installs it wrong and causes an issue. The shop should be responsible if they cause any issues when installing the system. You touched nothing. If it happens again with the second head unit, I would demand another replacement and then take it to a different shop to have it installed.

Good luck!