[All] [Generic] Really Confused about sound system

[dantheman]

Not on an Mr2 but im sure theres someone who can help me anyway.

My setup is a JVC headunit. 4 door speakers and 4 tweeters with their own mini amps, a 2 channel amp and a sub. Someone professional installed it for me in another car and i copied the setup exactly when i moved it over to my current car. despite my complete lack of knowledge it worked first time.

fast forward a year and i notice im not getting any base. ive tried basic diagnosing but my lack of knowledge and no obvious faults leaves me stumped. heres what ive tested so far:

-when sub is supplied with direct battery voltage it makes a noise. this tells me the sub works.
-ive bought a brand new amp. when connected with a power, ground, rem, and 2 line in jack cables the power light on the amp comes on.
-if i remove rem amp turns off, if i remove both line in jack cables amp turns off.
-the voltage across power and ground is 6.5 volts.
-the voltage across rem and ground is 5 volts.
-lightly touching line in cables against the jack housings on the amp makes other speakers in the car make a tapping noise. these cables have a very small voltage across them ~ 0.2V
under all conditions the voltage across the both channels or bridged is 0
-headunit is set to high amp power, but with normal settings or super bass on and bass boost turned to max theres still no bass.

so as far as i can see all the cables have some affect, the amp works and the sub works. i hadnt touched the head unit when the bass stopped. i havent pulled it out it check everything is ok because it was a massive chore to install it in the first place. likewise i havent checked the cables over their full length for the same reason.

so, whats the problem? anything else i can test?

[SonicSW20]

Checked all your cables for continuity? Also, the power to ground PD of 6.5V, is that for the amp? It should be ~14V if the car is running.

[dantheman]

that measurement was taken without the car running, only with key turned 1 place. when i started the engine it went up to 7.1V.

Does the amp not work at that voltage? it gets its live fused feed directly from the battery and ground is onto chassis. i checked battery voltage and it was 14V with engine running so i can see why you say it should be 14V. I dont understand why it would only be 7 at the amp? if there was a wiring problem it would surely get 0V?

[SFLee]

depend how you wire it

almost all car amp work of 12V to 14.4v

also headunit have pre out and sub seperate
so if your sub pre out on the head unit is not plug into your amp

your amp will only do door speaker and no bass

[dantheman]

well the sub worked before so the way its wired into the head unit hopefully isnt a problem?

[mrfil13]

Power on the headunit, use voltmeter to check voltage between the + terminal and an earth point on the chassis, should read 12v or 14v with the car running (give a little).

If thats ok then you know the correct amount of juice is getting to the amp.

If thats ok you can disconnect the - terminal from the amp, set your multimeter to measure OHMs, stick one end where the earth is connected to on the chassis and the other to the end that was connected to the amp.

Should be low resistance unless you have a bad cable, connection.

[dantheman]

sorted the problem out this morning. Looks like the bit of chassis i removed the paint from to get a clean grounding point had rusted, making a poor connection hence the 7V. When i grounded it on the battery negative and gave it 12V everything worked nicely. Thanks for the help guys