what would cause this?

With my system fully installed in my vehicle, I have a pioneer head unit controling my mids (3 way speakers) on the regular front and rear channels. On the preamp rca outs, I have those running to my amp. My amp is a a MTX thunder 342. It is two channel but it can also be run bridged into one channel. I currently have it in two channel mode running two JL 6.5s. My problem is this. My subs come in and out. Most of the time when I turn my radio on, the subs are not even on. They are moving very little, like they are only getting 10% of the amp's output. If I mess with the volume (turn it up) or mess with the EQ settings in the head unit (turn the bass up a tad), they will usually kick in like they all the sudden have full power. They will play like they are supposed to for a while and cut out again, like down to 10% again. Again, if I mess with the level settings or volume, full power will usually come back. The mid range speakers being run by the head unit's front and rear channels are not affected. This is only the sub out channel. Also to note, the sub channel did the same thing when I was running it bridged into one channel with a 10" JL driver. I was thinking that maybe I have a grounding issue or something. I figured I may go and try to get some multimeter readings on the voltage at the amp. I'm not sure what to check. I guess it could be the amp. Any ideas?

well you never said what amp you are using for the sub? which rca output are you getting the signal from(rear or sub)? as an experiment you can switch which rca you are using for the signal and see if it does the same thing, if it does the same thing then it is likely that the rca pre amp is not the problem. pioneer radios are notorious for having issues with the rca pre-outs. also while the system is playing you can go around and wiggle the connections behind the radio and at the amp and see if you can get a response from the system, this can help point out a loose connection if it exists. how do you have the sub wired to the amp? what is the ohm rating of the sub and is it a dual voice coil or a single coil?

I did say what amp I was using. It is a MTX Thunder 342. I have had it for years. It puts out 60x2 rms into a 4 ohm load or 240 rms bridged into 4 ohms. My subs are 6w3v3-4 JLs, 4 ohm single voice coil. So they should be getting about 60 or a little more rms to each. There is only one set of rca outs on the back of the head unit, they are for the sub channel. The other front and rear channels are all in the regular wire bundle. It is a newer unit..I just bough it about a year ago. Voltage at thr amp is at 12.6 with the truck off, and 13.6 with the truck running.

ok sorry i misunderstood, i thought you were using the JL 10" not a pair of 6" subs. anyway, you can swap the sub rca with the rear rca and make sure the rear crossover setting is off when you do this and the sub should play normally. if you do this and the same thing happens then i would start to suspect your amp. when you check the voltage at the amp there is a process i like to use. first check the voltage on the positive and negative terminals at the amp, then move the ground lead to a different spot that is a known good ground like the outer shield of a cigarette lighter plug or anything that is bare metal. there should be very little or no difference between the two readings. if the voltage reads higher when you move the ground to a different spot then you need to re-do your amp ground. you should also check for a voltage drop across the power terminals with the sound up to listening volume and using some music with a decent amount of bass. the voltage should not drop below about 13v with the car running.

Ok, thanks for the tips. I wil double check the ground like you said, I did not think of that. The head unit is about the cheapest one you can get from Pioneer..I bought it last year because the one I had in there messed up and would not eject the cd. I had the old one for about nine years. At the time the old one went out, my truck was not a daily driver, so I went and bought the cheapest cd player I could find just so I would have some sort of head unit that worked. At the time, I didn't even have my amp and subs in there. I recently switched back to this truck as my daily driver, and I put the amp back in and added the two 6.5s. The head unit is a DEH - 3400UB. Looking at the manual, it does seem to have two sets of preouts. I am certain that I am plugged into the right ones. I am not crazy about the lower line head unit, but it does work. It just isn't as versatile and easy to set up as the higher end models. Changing audio settings isn't easy, and the settings are not as detailed. The only other thing that is suspect at this point would be the rca cable itself. It is a cheap one. This is about a 10' cable. Maybe I should get a better rca cable?

From what you have described I think you could be running into a common issue with your AMP. I am willing to bet that amp has some sort of internal issue with solder joints. Some older MTX Thunder amps and Kenwood amps would develop that problem as the go up in age. Is there a electronics repair place near where you live that does car audio??? You may wanna have them take a closer look at that amp for you. Or how about this, if you can get your hands on another amp to switch into that car and assuming that amp works properly I bet you issue goes away. Another thing you can do it try that amp in another car and see if the issue persists.there can be only one.......

see i keep finding out that i misread your post. i was under the impression that you have an amp on your normal speakers too, thats why i thought that you had more than one RCA output on that radio. well either way, if the voltage checks out to be ok and your connections are good then it can either be the radios output or the amps input. the only way to test this without going out and buying expensive equipment would be to either find a different amp or radio that is known to work perfectly ans swap out your radio or amp and see if the problem keeps up. its not likely that your rca cable is the problem but if you just happen to have one laying around then you can swap that out first and see if it helps the situation. .