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Electronics Gurus? (resistors, diodes, etc)

I'm using an LED for my CEL and it isn't on unless there's a problem. I have noticed though that it will dimly light when the EGR valve is cammanded and that gets to be a nuisance to me.
 
After doing some research in my GM manuals I found some info and have a few questions. This is for ECM #1227747. First thing is that the SES lamp (along with several other circuits) is driven by one of two quad drivers. Probably something similar to this:
http://www.ti.com/lit/ds/symlink/drv8803.pdf
It also states that a dim SES light is the result of a damaged quad driver. Could it be possible that in your experimenting with resistors that you shorted it out or used too low of a resistor (too much current) and damaged it? I don't think that the quad drivers back in the 80's had built-in fault protection. To check it you could put the original bulb back in and see if it's dim also. The manual also says you can check the output from the ECM (pin A5) to see if it measures 50k ohms or more.
Also, where did you get the LED bulb from and can you give us a link to it? Is it like one of these:
http://www.superbrightleds.com/more...led-bulb-6-smd-led-wide-angle-wedge-base/211/
http://www.superbrightleds.com/more...e/194-led-bulb-6-smd-led-wedge-base-fdac/881/
 
Appreciate all the info guys, I had to work this weekend, no time to get back to this until now.

These are the style bulbs I'm using: http://www.ebay.com/itm/20X-T10-W5W...Parts_Accessories&hash=item27cb57b415&vxp=mtr If the link works, this is what they look like "underneath" http://coloradok5.com/forums/picture.php?albumid=948&pictureid=8664 Comparing to the superbright LED picture, the ones I have are a lot simpler/cheaper.

The '165 ECM (MAF TPI) is notoriously bad for at least one inherent problem, it very well may be that the '165 is the only one affected by this...I found one post awhile back from someone else with the same problem, and I'm going to take a stab that it was also a '165.

I can swap an incandescent bulb in there, but I've been running/driving the truck for a year or more since I initially tried an LED (then went back to incandescent as I wasn't ready to rewire the whole thing), and haven't noticed any malfunctioning...since I screw around with trying to tune the PROM in this thing, the bulb check (key turned to run, light flashes then stays on steady) is one thing I pay particular attention to, and the bulb check continues to work as it should. Post-LED before, the incandescent bulb did not glow dimly.

Yes, measurements were with the LED removed, and resistor in series with LED. I'll have to check the resistors again, I didn't write down the color scheme. Figured using the meter was easier. :) Unfortunately I have a large pile of assorted resistors, they came in a bag with the various values on the label, but none on the individual strips of resistors.
 
Found out some more interesting info while reading my manuals, again this is for ECM #1227747. When you first turn the key on the ECM performs a system check. One of those is to check the SES bulb and make sure it is working and can complete the circuit. Does it do this by trying to turn the bulb on and checking the current? If so then that's not the problem. If it does it by sensing the resistance then we've got a problem since the resistance of an LED when off is basically infinite. Then when you insert an incandescent bulb the code probably goes away. If you had a scan tool it might show code 26,28 or 56 which is a problem with one of the quad driver circuits. Don't know if it'd throw a CEL signal to the bulb when it thinks it's not working. It'd be interesting to know this.

Also,just as an FYI, I checked a 194 bulb and the values stated earlier are correct. The resistance when off is 5 ohms. When connected to 12.45V battery the current was 245mA, so the effective resistance was 51 ohms.
So that raises the question, what exactly is the SES light connected to? I doubt it's simply a quad IC with that much current. More than likely there's a transistor driving it, maybe with a diode and resistor connected to it. Hard to tell exactly what's going on.

I don't know if you already gave some of these values, hard to keep all the numbers straight with all the tests that have been done. Could you take these measurements:

1) value of the resistor that's connected to the LEDs. Use your meter since the values on the resistor are only accurate to about 5%

2) current drawn by the LED assembly at full brightness. Just measure the voltage across the resistor.

3) when dimly lit, what is the voltage across the LED assembly (both resistor and LEDs)

4) without any bulb (open circuit) measure the resistance and voltage of the ECM output (may be up to 50k or more)

5) can you tell if the LEDs are connected in series or parallel or a combination thereof? The way they're all packed in there it sort of looks like parallel.
 
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If you feel this is safe, I just thought of a way to bypass the SES bulb/circuit check on start up if that might be causing the problem. Put in the incandescent bulb on start up and then pull it out and put the LED in and see if it's still dim. Then activate the parking brake to make the light come on.
 
I can answer some of the questions, more when I get home. CEL bulb test is only that...it won't set a code if it fails, so the ECM isn't "measuring" anything. Bulb comes on/flashes, then stays on steady until engine starts. If that doesn't happen, you either have a problem with the bulb/circuit or ECM.

ECM grounds the CEL, so 12V ignition on one side of the bulb, single wire to ECM that is grounded under appropriate conditions.

The LED parallel/series is going to be near impossible to check I suspect. I will look, but it's near microscopic.

I will try and take the time tonight to re-check my measurements in the system, and answer additional questions. I spent last evening looking up the values of all the resistors, and bagging/labeling them appropriately, so it will be much easier to track what I'm doing now.

Tested what I could:

11.81 reference volts.
When bulb is "on", after resistor:
240 ohm resistor: 11.71V
978 ohm resistor: 11.76V


When bulb is "dim":
240 ohm resistor: .06V (14.7V reference...truck running)
978 ohm resistor: .31V (14.88V reference)

With LED, after the resistor already on LED: 5.06V. Before: 5.03V

No way to check parallel/series, the circuitry appears to have a colored cover over it, almost like paint.

After all this testing, I think I'm going to leave it unless someone wants me to keep going. I don't plan on running the '165 ECM forever, and if the problem doesn't exist even with the '747 (TBI) ECM, probably an isolated issue with the '165. Not surprising.
 
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Sorry, didn't see your updated edited post.

I think we have a little miscommunication going on here or I'm just bad explaining things. From the picture you showed of the disassembled LED I only see one resistor. I think the two resistors you're measuring are the 'test' resistors that were mentioned earlier. If so, could we forget about those for the time being?

So let's try this again if you don't mind re-measuring:
1) Need the value of the internal resistor connected to the LEDs.
2) With one probe of your DVM connected to chassis ground measure the voltage between the resistor and LED array at full brightness. Also measure the voltage coming from the ECM (should either be 0V or maybe 0.7V)
3) Repeat #2 except when the LED is dim.
4) With the LED removed measure the resistance and voltage at the ECM

From one of the measurements you gave I think I have an idea how the LED array is configured, but if you feel like it you could always use your ohmmeter to figure it out.

I found the ECM schematic for the '165. Just open the 'Outputs' schematic.
http://www.exatorq.com/ludis_obd1/1227165schematic.html

U6 is the driver, it's part number is Delco #16042368. There's a post here (I think you're a member there) that talks about the driver and gives links to an equivalent datasheet.
http://www.thirdgen.org/techboard/diy-prom/298411-quad-drivers.html

The SES light is connected to a TVS (transient voltage suppressor) to prevent any damaging voltage spikes. I'm not familiar with TVSs, but from what I've read it shouldn't have any effect on what you're doing. Maybe someone else knows.

Also found this pic. It says it's of the '165 but some of the ICs don't match up with the earlier schematic, so who knows. If it is correct then that TVS should be located just to the right of the red cap.
http://wbo2.com.au/~techedge.com.au/vehicle/1227808/1227165ECM.jpg

So what am I saying? If the quad driver/TVS is the only thing connected to the SES lamp and no other circuitry, and the quad driver isn't damaged (which I doubt it is) then I can't explain why it's doing that. Maybe the quad driver isn't clamping the voltage to 12V, which is why I'd like the voltage measurement of the ECM when the LED is removed. Hopefully with those voltage readings we can figure out a way to trick it. I'd like to figure this out since I want to do this mod one day too.

As a side note, in my research I also saw that other people had the same problem with the '747 so it doesn't sound like it'll go away.
 
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Alright, so I *think* I measured what you were asking. We will see. :)

1) LED resistor: 101 ohms
2) (11.86V reference) 5.9V between LED's and resistor, .08V after. .
3) (14.8V reference) 9.8V between and 5V after.
4) .03V when CEL is on, .05V when it's "off". Resistance 13,500 ohms (13.5@20K meter setting, correct?) when CEL is "off".
 
Just let me know if it's worth figuring out.

I probably wasn't clear before, I meant that not everyone using '747 with an LED CEL had problems...but if you found otherwise, IF there is an easy/cheap fix, I can keep going. No guarantee the '727 ECM I will be using eventually won't have the same problem.
 
dyeager535 said:
Alright, so I *think* I measured what you were asking. We will see. :)

1) LED resistor: 101 ohms
2) (11.86V reference) 5.9V between LED's and resistor, .08V after. .
3) (14.8V reference) 9.8V between and 5V after.
4) .03V when CEL is on, .05V when it's "off". Resistance 13,500 ohms (13.5@20K meter setting, correct?) when CEL is "off".
Click to expand...

For 2 and 3, you are listing battery voltage, then the voltage after the resistor, then the voltage after the LED pack? #2 seems reasonable. You have 59mA through the LEDs. But #3 is weird. How can the ECM have 5V on that line? Consistent with ~2V less drop on the LED assembly, the current has dropped to 49.5mA. Do you actually see the LED dim a little when the engine starts? Have I misinterpreted your numbers?

I'm taking # 4 as probing the ECM. The schematic says there is no pull-up, so these results aren't real surprsing. I actually still think the pull-up may be the simplest end solution.

Did you ever try putting a 1K resistor in parallel?
 
Yes to battery voltage...pretty significant change when the alternator is working obviously, which would skew pure voltage figures.

I never paid attention to what the bulb acts like as the engine starts, whether its on, then off, then dim, or if it simply goes from on (key on engine off) to dim (key on engine on). I assume the dash cluster circuit isn't a run/crank hot though (as opposed to run only), but I've not verified that. Not misinterpreting the numbers, I double checked all of them. #3 got me too.

Correct on #4. I have not tried a 1K resistor in parallel. I can easily do so, but it may take a few days, long/busy weekend coming up. A quick/easy fix is the only way this is going to be useful for anyone else running into this problem IMO. Could be cured when the bulb is being swapped out, but if it requires first seeing if there is an issue, then ordering additional parts, multiple solder jobs, etc. if there is, people just won't want to deal with it.

You think the parallel resistor might very well solve the issue?
 
Man I am still partially hammered from the doctor trip. It is taking me way too long to figure out what I am seeing on the numbers.
If the quad driver for that bulb is a Texas Instruments TPIC2402, which it seems to be, then it should not be doing that.
Under worst case it might leak a couple of ma, but thats all.
However, we don't know what GM might have put between the output of the chip and the connector.
I'm going to take a nap and see if I can get some sense back. I'll jump back in later.
Did you say you had a pic of the LED unit with the cover off? I can't get the link to work.
Also, a TVS should have no effect. Think of it as a Zener on steroids.

The bypass resistor might work. Still liking my relay idea more and more though......
 
Yep, that was what I was needing. I was holding out hope that the resistor in the assembly was actually a regulator chip that looked like a resistor.

But, with only two legs, it can't be.
But that does simplify things.

Now if I can just get my head back straight.......
 
lmao

I didn't say the project had to make sense did I?

Seriously though, this is being done so that the next time the cluster has to come out, it's because something has physically failed, not because a bulb burned out, or came loose, or connector wasn't making good contact. As little as the CEL is on, bulb should last forever, but if doing 99%, might as well eliminate the 1% as well if I have the chance.

A few months to re-wire the instrument cluster? I think that's reasonable. ha
 
The nice thing about a simple resistor fix is you can build it into the plastic base of your LED assembly. I would even build a simple 1-transistor switch into that space. The trick is designing it so it doesn't need a full-time ground - just the battery voltage and ECM signal.

What is the ground reference for the meter when doing these measurements? If you really had a ground shift from ECM to body you would have bigger problems. From the cluster to chassis or whatever should screw up all the gauges. But this is strange behavior.

And I do like the relay idea, the packaging is just tough. If you don't want to cut the wires behind the harness, you could make the bulb base like a plug and run all the relay wires out one side of it - to keep the whole thing a plug-in replacement. Where you hide that thing is up to you.
 
Hiding the PWM setup I cobbled together, or the wiring that goes with all of this, packaging isn't too much of an issue so far.

You mention ground reference...during testing I was using the stock ground wire for the cluster assembly. Is that what you mean? I have yet to plug everything in at once (testing individually it all works of course) with the re-wired cluster, but I don't foresee any issues that will crop up that weren't there before...I didn't change anything but the connector ends and eliminate the plastic circuit "board" from the cluster.
 
So yes, I agree, #3 is odd. But so is #4 - I was hoping that when the ECM signal was off that it would be 14V.

Let me see if any of this makes sense or if I'm way off base. Hope you're able to follow this 'cuz it's confusing to me too.:confused: If we use the equivalent output circuit that's on page 3 of the TPIC2404 datasheet (which is probably different for the original Delco part), the output might be needing a reference voltage instead of floating. The incandescent bulb gave that, which would be 14V when the ECM is off. The LED is like an open circuit until turned on. So in an open circuit you said the output of the ECM was at .05V which would turn the LED on. But then it at the moment it does turn on the driver now has a reference voltage (originally supplied by the 194 bulb) of 5V so it turns off. If this theory is true is it possible that the driver is actually oscillating - constantly switching of and off? If the LED is oscillating and it had say a 50% duty cycle then it'd explain why it's dim when it should be off. I don't know if your DVM would show a DC voltage if it was. You might be able to set your DVM to AC voltage and see if you get a measurement at the ECM output when it's dim. Maybe I'm grasping at straws, but it's the only thing I could think of that would explain this behavior.

But like mentioned earlier, I too was thinking you should try adding a resistor in parallel to give the driver a reference voltage. With a 1k resistor it'd have to be rated at 1/4 watt. You probably have a bunch of 1/8W resistors so I'd use a 2.4k and see how that effects the circuit. If it works then you can increase the resistor value so it cuts down on the heat dissipation.

By the way, from the 6V drop across the LEDs I think they are arranged in 2 rows of 4 LEDs.
 
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