RESOURCE That Mystery Check Engine Light- Lamp Driver Repair Information For CCC Cars


Supporting Member
Jan 2, 2006
This discussion came up on another forum, and some good information came from it. Figured I'd share some findings here on this mystery lamp driver. If you have a CCC G-body car, you have this. From what I understand, it takes a 5V signal from the ECM and also boosts it to 12V to light the CEL when warranted. Basically, if the ECM loses its ground with the key on, the light comes on.

If you've ever had to troubleshoot a check engine light that is fubar, it may not be your ECM. It may just be that pesky little lamp driver for it. When you turn the key on, one of the immediate checks the car does is illuminate that check engine light (CEL) because when the car is on but not running, the ECM isn't grounding, so...CEL. Sometimes, that doesn't happen, and while the ECM may still operate the car, the lamp won't light up when you need it. The lamp driver also monitors the ECM and if it craps the bed, the lamp driver senses this and turns on the CEL. Prodedures in the chassis service manual has charts [Charts A5 (CEL lamp doesn't light) and A6 (CEL lamp stays lit) for B, G, and E body Oldses] which guides you to the issue.

Some of you may be like, "WTF? What lamp driver?" That's ok. For years I never gave that thing much of a thought, either. Until I had to replace it one day. Years ago when I was fixing up my 84 H/O, I didn't get a CEL, or SES, or whatever you want to call it. I changed the bulb, but that didn't work. So with very limited electrical experience, I turned to the factory manuals. Turns out, it was suspected the lamp driver was toast, so when I replaced it with a new GM piece, it was no big deal. Everything was good again. Simple. Find box, open box, replace electronics card, close box and done.

Where is it you may ask? Answer is: Depends. MOST of the time you can access it by removing the dash...ok, just kidding. You usually will find it behind the glovebox on the radio side behind the dash. It's typically a little bluish-green box with a snap on lid and wires coming out the other end. It's taped up in the harness, but big enough to see. If you're lucky, you can drop the glovebox door and access it right there to the right. If not, it could be up in the vent ducting areas which makes it more of a PITA.

You'll be looking for something like this little box, it's about 3.5 to 4" long-ish:

G-body Lamp Driver Housing.jpg

And just like Cracker Jack, the prize is inside. A small screwdriver or angle pick and you can pop the lid and with a pair of needle nose pliers or that angle pick in that open slot in the back, the card comes straight out:

Lamp Driver and housing picture.jpg

Now, the biggest problem is that those ACDelco $25 (used to be when I bought mine) lamp driver cards (p/n 12020004 covers MOST GM vehicles) are discontinued and the only way to get them is aftermarket. But the price of the aftermarket jokers range from about 51-76 bucks give or take. Good thing is they have updated electronics, using the new-style resistors and transistors and stuff, only taking up about 1/4 of the original real estate. It's under Standard Motor Products RY536.


Not much to the components on the card. 3 resistors, 1 capacitor (dual-legged orange disc), 1 transistor and 1 diode. TYPICALLY- but not always, the transistor is the thing that pukes out on the little cards, although any of it can get toasted. The transistor's the black, 3-legged little half round cylinder standing up on the OEM card. I can't dazzle anyone with my electronics knowledge here, so I can't even begin to explain this somewhat simple circuitry, but everything on the old card matches up with the new card. The transistor is an NPN (turns on at low signal) used for amplification. I cannot tell you how to test these components for failure, but you can do it. I'm not an electrician or electronics type of person, so I'll leave that to the experts to explain. But if your components turn out bad, you can still get some parts to possibly fix it. And that's what will be covered here. NOBODY else I know of has figured out this junk. And I'm not sure I'm there either, but darn close.


If you buy a new SMP aftermarket card, then stop. Put it in, and you're done here. Good for you, you fixed it. But if you don't want to spend a lot of money for one, and are handy with a soldering gun, you may want to try to fix your old card with cheap new parts.

Unfortunately, not much is known about the pieces parts. The internal part numbers on the diode and transistor doesn't match anything in a cross-reference. The through hole resistors are about the size of 1/4 watt resistors. You probably could use a 1/2 watt resistor if you like, but it's a driver for just a single lamp. Doubt if it would need much power dissipating properties.

Some interesting info can be found on the "new" card when you magnify the diode (F7, F1), transistor (G1), and resistors (362, 163, and 103). Not sure about the diode's dual info of F7 and F1. Maybe someone can chime in here that knows this stuff. But since it has the same components, only updated, it really should be the same ratings so as not to blow sh*t up. Those 3 little lines on the new card diode is the same as the silver stripe indicating which end is the cathode. To read the resistors, the first two (or three) numbers are the significant numbers of the resistor, and the last number is the number of zeros after it. Thus a 362 is a 3,600, or 3.6K ohm resistor. The 163 is 16,000, or 16K ohm resistor, etc.
SMP RY536 Lamp Driver.jpg

GM OEM Lamp Driver Card with components - Copy.jpg


(from top to bottom in the picture above) They look about the size of 1/4 watt resistors, but not sure how to tell just by looking at them. They're about 6mm body length, thus I'm guessing around 1/4 watt. They're small.
10Kohm resistor, 5% tolerance.
3.6Kohm resistor, 5% tol.
16Kohm resistor, 10% tol. (most of the ones I've found were gold band meaning 5% tolerance, which will work just fine)


RULE: KNOW YOUR PINOUTS. You screw this up and it won't work.

2SC1384/C1384 (was field tested to work) or 2N5551 old style NPN amplifying transistor (Found a cross-reference equivalent for the G1 to 2N5551). Pinouts are different on the 1384 transistor as follows: 1) Emitter, 2) Collector, 3) Base (aka ECB), but it will work if you twist it to put the legs in the right place. It’s taller so it needs to be bent over to fit back into the housing. 2N5551 pins are like the OEM pinouts (EBC) and should fit the same as original. My suggestion, use the 2N5551 as the pinouts match right out of the box.


Ceramic Disc 270 pF capacitor, 100vdc, Y5S temp coeff. (based on B 271K oem number). Closest equivalent I've found so far- Vishay capacitor part number D271K20Y5PH63L6R. Good thing about the capacitor is it doesn't have a specific orientation to it that I can find.


I would imaging a general purpose 1N4001 1A, 50v diode should function well here. Maybe too well. The factory numbers (SC834) don't cross to anything I can find. (The 1N4001 is also a good choice for the magnetic field diode for the electrical connector on the 80s A/C compressor connectors.) Just make sure when installing a new one, the cathode “band” faces the connector blades as shown in the picture. This is just a guesstimate because I'm sure it's got a rating, but nobody seems to know what it is. Again, if the electronics gurus can jump in on this, feel free. The way it's positioned in the circuit seems like it's used to block voltage from coming back from ground, but if it's supposed to turn on, I'm not sure what the specs would need to be.

Always open for feedback and extra information to make it easier for folks to fix their own on the cheap. If you have electronics background or further information, please join in!

This is more information that's ever been garnered before on this thing, but if you have more to input, please, let's hear it!
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