Head unit send poor sub signal??

[Takid455]

Issue: Sub is very low regarding sound with amp on full and head unit(HU) indicated to sub ON and max level.

Test performed:
Car A
HU, Amp & speakers installed
fronts & rears work well, sub is practically not working. Improved when amp is on 4ch, but it is a 5/6 ch system
Switched RCA cables around to test is one may be bad. no difference
RCA cables all have 0.4-0.5 ohms. used a jumper lead to test both ends at once since they were installed in car thus the added connect could have increased resistance, but it is very low as tested.
Hooked amp hook up/ signal wire the goes from HU to amp directly to 12v bat to amp. No difference.
HU appears to be functioning as is should otherwise which tells me the HU wiring is good or adequate at the moment.
Hooked up a ground from HU case to body. No difference.
Connections are all secure



Car B
F,R & sub w. amp installed & working properly
Installed RCA cables from Car B into amp/ sub combo & battery of Car A. In essence, Car B has a stand alone sub system & F&R system w/ common HU. All works fine. Tells me Amp, Sub and wires from bat to amp and wires amp to sub and battery in Car A are good.


Next step is to find another JVC HU (use same rear plug) and test for bad HU.

System components

Speakers: All CDT
Amp: JL 900/5
HU: JVC KD-R820BT



Any Thoughts or suggestions??

[Kenny]

Need more info....... Have you used the h/u rear channels to the sub input to see if SPL increases? Are the subs and/or voice coils in phase?

We had a competition car that used coils for x-overs and mounted them inside the box since we were using a x-over to help control the speaker load on the amp(can you say old Rockford Punch days?). We had a very similar problem and during troubleshooting except the output was distorted. It actually played with one of the speaker wires unhooked at the amp! Since this was a pre-competition thrash, my installers had been up for way too many days in a row and were removing the amp for repair while it was playing, he removed one of the amp mounting screws and it quit playing.............. He had actually ran a screw through the amp frame into the choke(x-over). It was actually grounding one speaker lead to the amp chassis. Once he reconnected the speaker wire we were rewarded with 153db with 4-12s driven with 300 cheater watts. It is amazing how many times my novice guys could run a screw through a speaker wire when they were buttoning up a big system.