88-94 Factory Stereo Info

[AK49BWL]

Turns out there were a lot of different part numbers for the stereo systems in these trucks, and since I seem to have started hoarding them (lol), I might as well put down what I know about them based on my own messing around with them. Note that I've made this based on only the units I've personally used or seen... Anything else is guesswork on my part at best.

CDM Units

88-94 GMT400 Radio CDM units (the amp under the dash next to the accelerator) come in two variations:
-- A 2-Band equalizer version that is usually mated with a radio head that contains Bass and Treb buttons, and may come with a mechanical-button-controlled cassette player, or no cassette player unit at all
-- A 5-Band equalizer version that is usually mated with a radio head missing the Bass/Treb buttons, because the 5-band equalizer controls are part of the electronic-solenoid-controlled cassette player unit.

CDM Unit Part Numbers

I have personally tested the following to be 5-Band EQ CDMs:
16072780
16147065*
16169561*
16172125*
Untested by me, but may also be 5-Band based on research only (List compiled by @unruhjonny):
16087080
16172061
12341718 (maybe?)

The following are 2-Band EQ CDMs I've tested:
16072800
16172115*
Untested by me, but may also be 2-Band based on research only (List compiled by @unruhjonny):
16141203
16147075
16169551
2231934 (??)
6208545 (??)
2501 (??)
89AZTU (??)
5358583 (??)
16055070 (??)

Part numbers with a * next to them indicate they have the updated heat sinks and moderately better power output (based on just listening to them, nothing scientific lol). No, I don't have specific years for the part numbers.. I didn't pay attention when I pulled most of them, or they were already removed from vehicles (one of the sets I picked up from an estate sale, looking like it's never been used!)

The various parts of the system are interchangeable, but some functions will be missing in certain configurations:
-- Using a 2-Band CDM with a 5-Band radio head and cassette player will work, BUT - only the first and last bands on the 5-Band equalizer will be adjustable, as they're basically just taking the place of the Bass/Treb buttons.
-- Using a mech cassette player with a 5-Band radio head will allow NO EQ adjustments using EITHER CDM, but will otherwise work.
-- Using a 5-Band CDM with a 2-Band radio head will work, but allow you to only adjust the first and last bands, and the 5-Band equalizer on the cassette player will be nonfunctional due to the 2-Band radio heads missing two wires that run from the back of the 5-Band radio heads to the equalizer on the cassette deck on 5-Band systems.

Note that I have not tried modifying any of these radios to try to do something outside the scope of what I've mentioned here lol. I have also noticed during my play tests that the 2-Band CDMs don't seem to put out nearly as much power as the 5-band CDMs. At 50% volume on a 5-Band CDM (with the EQ bands topped out), the output is equal to 100% volume on a 2-Band CDM with the Bass/Treb maxed! The 5-band units SCREAM at full volume. On that note, the 16072780 seemed to not be able to play nearly as loud as the later ones. The later versions also seem to have a delayed shutdown - press the power button, 1 second later the amp clicks off the relay and the EQ display goes out. Early versions, the power button shuts this relay off immediately.

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Radio Heads

The radio heads functionally changed only once (that I know of) between 88-94 -- 88-90 (91?) versions have an AM Stereo button, later models do not. The button layouts are also entirely different between these two versions, but aside from the AM ST button, have the same number of buttons operating the same functions. Most 1988 models have black buttons for all stereo controls, while every other year has gray buttons. The AM ST models' Tune and Vol buttons hang over the right edge of the radio bezel slightly, while the later models don't. There do not appear to be any differences in the displays between the years OR 2-band vs 5-band versions. The 5-band heads have two additional wires on a separate harness coming from the back of the unit - this goes to the equalizer input on the back of the cassette player.

Radio Head Part Numbers

As mentioned above, there are 2-Band units and 5-Band units. I don't have any 2-Band heads for the early versions yet, so no part numbers for them from me.
2-Band, AM ST: ????????
5-Band, AM ST: 161413851
2-Band, no AM ST: 16189885
5-Band, no AM ST: 16189895

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Cassette Players

I can't tell any immediate differences between the years for the two versions of cassette deck available, aside from the 1988 models having black buttons over the gray of the rest.
-- The 2-Band cassette player has an electronic button for tape side selection, but mechanical buttons for Rewind, Fast Forward, and Eject. This version also has a cubby to the right that fits cassette tape cases or whatever else.
-- The 5-Band cassette player has electronic buttons for all functions, including a Track Seek and Repeat feature and a Chrome/Metal Tape selector button. This version, obviously, replaces the right side cubby with a left side 5-band 8-position graphic equalizer.

Cassette Player Part Numbers
-- 2-Band Mechanical: 16156555
-- 5-Band Electronic: 16156545 -- This one is missing the Dolby B NR logo on the cassette flap, but still has a Dolby Trademark sticker on top.
-- 5-Band Electronic: 16170535

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Typical configurations

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Playing some Alan Jackson

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My entire collection.................so far

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[unruhjonny]

Great post!!

This would have ben super helpful for me when I started looking into my trucks stereo issues!!

It is worth adding the that the later CDM's with the blue tape covering much of the sticker (which has the part number) should have a small sticker on the end showing the last four digits of the CDM part number;
this is a big deal because at this time, if the blue tape is still present on the large sticker, you will not be able to remove it and read the part number... unless you're REALLY lucky.
I have also seen later CDM's with blue tape over the part sticker, where the small sticker was absent - BUT(!) sticker residue showed the presence of a small sticker with the four digit part number on the end;
this small sticker is about the size of a conventional price sticker.

Please notice the CDM in the foreground of this picture for an example of the small sticker:

 

[AK49BWL]

Your thread is why I decided to post this lol

 

[TechNova]

has anyone figured out the fix for when these go bad?
Previously, someone said it was a filter capacitor, but I have not found which it is or how to test it.
I have a friend who repaired radios in his electronics shop in the 70's and 80's but he cannot find the problem.
I think it is because mine was intermittent.

 

[unruhjonny]

that's a good question;
I think that you would need to first diagnose if it's the CDM that went bad, and once it has been confirmed that the CDM is ok, then you can move forward.
As I see it, these "radios" essentially have dummy controls and the real guts of the radios are the CDM.

 

[TechNova]

It is the CDM.

 

[DerekTheGreat]

Great post, dig all the info. Will have to keep the following CDM numbers in mind when I go searching for another one to replace my dead unit: 16169561* 16172125*.

Also, 1989 was an odd ball year like '88; Silver face but with black buttons. The best looking combination IMO.

Anyway, a word about equalizers and how to use them: (I saw your zany eq curves and just had to comment )
Resist the urge to adjust above zero or flat. Why? This overdrives the signal and thus over taxes the amp. You are not going to get more output from the system by doing so. There's a bunch of electrical engineering stuff you can read about how much harder an amp has to work to drive something say an additional +3dB vs how much easier it can work when you attenuate -3dB of all the other available frequency ranges. So if you want more bass, attenuate the mids and highs lower. The resulting sound will be much cleaner and more dynamic since the amp will have head space to better reproduce the transients and nuances. Even the crummiest of amps will benefit from this practice. Of course, if these amps had the guts & reserve then you could boost the bands above flat/zero but they do not... Exceptions to the rule are when trying to compensate for crummy source material, like FM radio.

 

[AK49BWL]

The EQ settings were for display only in this case lol

 

[Lorenzo]

Any ideas on where to get replacement decals for face of these radios? Mine are sorta worn.

 

[AK49BWL]

I might have found both an 88 and an 89 with the stereos still in them. They aren't anymore lol.

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I agree with the black button/silver face being a really good look for these! Needs a lot of cleaning tho. CDMs are both 16072780, 89's cassette deck is 16084945, 89's head is 16084245. The stickers were ALL missing from the 88's head and cassette deck so dunno if they're the same or not. I'd have to assume not, based on the difference in aesthetics, but the CDM was the same part number so who knows lol