gmos-04, grand prix, no onstar and more

So i'm using a GMOS-04 on a 2004 Pontiac Grand Prix with onstar and the Monsoon Sound System, added a clarion vx400 and steering wheel control. the problem is, when i press the onstar button nothing happens (its not activated, just for diagnostic info) the main problem is im not getting any sound from the drivers door speaker (although door chimes are working) and all the speakers cut out for about 1/2 a second every 3-4 seconds. i ruled out the radio being the problem by hooking a speaker to it's speaker level outputs and the cut out wasnt occurring there.

i tried another known good gmos-04 and it did not fix the problem. there was an extra plug in the gmos-04's harness that i dont recall seeing before on other gmos-04's it's a small black 8 pin plug. it has a headphone cable coming from it as well as several other wires. whats that for? is it part of my problem? i dont believe there was anything about it in the manual.

my assumption is that the onstar system has something to do with this problem since the onstar is supposed to use the front left speaker.

also, the SWI told me to connect pin a6 from the main factory harness to accessory power. well, at first i t-tapped it and it was keeping the radio on at all times. then i saw that the gmos harness had jumpered constant 12v over to that wire so i cut it.

something tells me that the onstart in that vehicle was an add-on system. GM "cars" didnt typically come with onstar from the factory. so my guess is that it just isnt compatible. i could be wrong, that is just my first impression.

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pretty sure it's factory, its built into the rear view mirror and all. the owner is the 1st owner and he said he never had it activated so i dont think he would pay to have it installed.

the black plug im talking about is for steering wheel controls so thats not the problem.

I want to say you need a GMOS01 if you have the monsoon system.

nfrank89 wrote:

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pretty sure it's factory, its built into the rear view mirror and all. the owner is the 1st owner and he said he never had it activated so i dont think he would pay to have it installed.

the black plug im talking about is for steering wheel controls so thats not the problem.

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no GMOS-01 is for non amplified systems, although now that you say that im not sure if the monsoon system is fully amplified, it might just be a factory subwoofer. so you may be on to something with the gmos-01. you can test if you want, all you need to find out is if the speaker wires run straight to the back of the radio. check the plug for four sets of wires: blue /lt blue, GREEN/ lt green, BROWN / yellow, and tan/gray. if these pairs exist on your plug then you dont have a true amplified system.

and by saying that it was an add on doesnt necessarily mean that someone had it deliberately installed. it could have just been something the dealership decided to do. right around 03-04 was really when onstar first came onto the scene and became popular. and like i said it was mainly equipped in suv's. that's why im still under the impression that it is an add-on.

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thought the 04 was specifically for Bose applications only, which is why you would get distorted audio, bose uses low level and monsoon uses high level...?

Yea i think you could be right about using the gmos-01

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INSTALLING THE GMOS-04

1. With all connections completed to the aftermarket radio, plug the 24 pin and the 12 pin connectors into the vehicles corresponding wiring harnesses.

2. Reconnect the negative battery terminal.

3. Cycle the key by turning the ignition on then back off, then back on again to test the radio.

Sometimes with these interfaces I have had to disconnect the box, wait a couple seconds and redo step 3, some times a few times before everything works correctly. FYI if you have not tried that!

the monsoon is fully amplified. there is no factory sub but there is a center dash mounted speaker.

i ended up noticing that every time the audio cut out the lights on the onstar mirror were flashing on momentarily. i pulled the onstar fuse to see if this would fix the problem and it did! no more audio cut outs! i checked the wiring diagrams in alldata for the onstar fuse and the obd2 port also gets power from that fuse so i chose to disconnect the onstar unit itself. the unit can be accessed by reaching from the driver's footwell up just behind the center console below the radio. there are 2 white plugs. remove both and this will deactivate the onstar system.no panels need to be removed to access these plugs, they're right there.

i chose to simply disconnect the onstar unit for 2 reasons, one, it's an old analogue unit so it can never be activated anyways. two, the customer has never used it.

so, this resolved my problem. hope that it will help others in the future!

oh! and the no sound from FL speaker issue was because the pin in the gmos04 harness had come loose.