# Unidentifiable SMD Component

#### Anna Swinemar

Joined Oct 31, 2017
29
Hello,

I am working on my 2018 GMC instrument cluster. It has a parasitic current draw that drains the battery within 3 days. I found this part which to me, looks like and SMD diode. It is reading "OL" in circuit and out of circuit. See attached photo.

I have tried looking through the SMD Code Book and no luck. It reads "39C 8AA"

#### Tonyr1084

Joined Sep 24, 2015
6,898
Check Craig's list. I've found a replacement cluster for an 09 Tacoma for $20. MY reason for replacing the cluster was because the OEM cluster wasn't reading speed properly. When it said I was going 65 little old ladies were passing me on the highway and flipping the bird at me for going so slow. Thereafter I started time trials and GPS speed recordings. That proved that when the speed was saying 65 the actual speed was 53 MPH. With the transmission in drive (1:1 gear ratio) engine RPM predicted the speed was slower based on a given RPM over a distance of one mile (a.k.a 60 MPH). I took this evidence to the dealer but they refused to address the issue. The transmission driven gear for the speed sensor was a 30 tooth gear. I replaced it with a 29 tooth gear. This adjusted the speed (and with proper tire size; the tires were under sized when purchased) to within 2%. At 25 indicated MPH actual speed was 26 MPH. At 75 indicated MPH actual speed was 74 MPH. So I surmised that at 50 MPH my speed was accurate according to time trials and GPS speed reports. Being 1 MPH under speed at highway speeds was by far better than being 12 MPH under. In the end I didn't use the cluster. Funny thing was that when stopped the speedometer said I was going -2 MPH. Hence, the 2% assumption. But that was MY issue. The point is that you can probably find a cluster from a wrecked or junked car same model, same year. One caveat: You'll either have to record the mileage of the old cluster and the mileage of the new cluster at the time of installation. OR you'll need to have the replacement cluster flashed to show the correct mileage. In the US it's a crime to misrepresent the mileage a vehicle has traveled. #### MisterBill2 Joined Jan 23, 2018 11,941 Wow, that is bizarre. The power locks, who would've guessed that? A friend who owns a garage, who specializes in finding electrical issues, did have a oscilloscope hooked up to multiple modules and found it was the instrument cluster drawing 600mA when the vehicle was off. Of course a new instrument cluster can be purchased for$2000 but I find it a hard pill to swallow when the vehicle is fairly new.
OK, now we know that there is a 600mA draw with the ignition system power off. If that is power being consumed in the cluster, it should be possible to determine what component is still getting warm in the power off mode.
That will not be the failed component but it will show which system is failed. OR it may point towards a control for an external device.
So once again, is it possible to power the cluster outside of the vehicle?
If the same draw is not present when the cluster is externally powered and other external connections open, then it is either powering an external load or the actual fault is an external input, such as a door switch or a seat-occupied switch.
So the next analytical too needed will be a list or diagram of the external connections to the cluster.
I am guessing that the determination of the current was made by measuring the current at the cluster fuse position, since that is a common point for diagnostic measurements. So now there is a simple way to watch the draw while disconnecting various parts to see if it is an external source keeping some input active..

#### Tonyr1084

Joined Sep 24, 2015
6,898
Something like this might be useful. Insert it in place of a fuse in the panel and solder a couple wires so you can connect an amp meter to measure the draw of current. I don't know what types of "Resting" currents different items draw, but I'd imagine they're pretty low. It means fabricating leads to connect to your meter but that's an easy tackle.

Then by removing a fuse and inserting the tester you can determine which circuits are drawing what amperage while in the "Vehicle Off" situation. I don't know if it will tell you which part of the cluster is offending or not, but it may be a useful tool. Again, I DON'T KNOW if this will help you figure out what part of the circuit is malfunctioning. Lets see what others have to say about it.

#### Anna Swinemar

Joined Oct 31, 2017
29
OK, now we know that there is a 600mA draw with the ignition system power off. If that is power being consumed in the cluster, it should be possible to determine what component is still getting warm in the power off mode.
That will not be the failed component but it will show which system is failed. OR it may point towards a control for an external device.
So once again, is it possible to power the cluster outside of the vehicle?
If the same draw is not present when the cluster is externally powered and other external connections open, then it is either powering an external load or the actual fault is an external input, such as a door switch or a seat-occupied switch.
So the next analytical too needed will be a list or diagram of the external connections to the cluster.
I am guessing that the determination of the current was made by measuring the current at the cluster fuse position, since that is a common point for diagnostic measurements. So now there is a simple way to watch the draw while disconnecting various parts to see if it is an external source keeping some input active..
I wouldn't know where to begin in powering the instrument cluster outside of the vehicle without proper schematics. I have an idea in my mind that could use a thermal imager to look at the cluster with the vehicle off and see which component/circuit is still active.

There is a large connector to the cluster and a small micro USB connector, I am assuming the oscilloscope was connected to the micro USB connection.

I am also in a time crunch. We need the vehicle by April 8th for travel purposes.

I really appreciate your replies. It's nice to have other analytical thinkers putting thoughts together.

#### Anna Swinemar

Joined Oct 31, 2017
29
Something like this might be useful. Insert it in place of a fuse in the panel and solder a couple wires so you can connect an amp meter to measure the draw of current. I don't know what types of "Resting" currents different items draw, but I'd imagine they're pretty low. It means fabricating leads to connect to your meter but that's an easy tackle.

Then by removing a fuse and inserting the tester you can determine which circuits are drawing what amperage while in the "Vehicle Off" situation. I don't know if it will tell you which part of the cluster is offending or not, but it may be a useful tool. Again, I DON'T KNOW if this will help you figure out what part of the circuit is malfunctioning. Lets see what others have to say about it.
Thanks for your reply. I unfortunately don't have the vehicle. It is still at the shop a ways away from me. Just the cluster and no drawings. So I don't know how I would power it out of circuit. I was really hoping I would find a shorted component somewhere in the cluster but that hasn't been the case.

#### MisterBill2

Joined Jan 23, 2018
11,941
How about the friend with the diagnostic ability? They may also have some circuit drawings.
If the 600 mA draw was measured at a fuse point, one diagnostic option would be to pull that fuse out and run the car and see what functions do not work. That could be one way to locate the guilty components.

#### Tonyr1084

Joined Sep 24, 2015
6,898
one diagnostic option would be to pull that fuse out and run the car and see what functions do not work.
I unfortunately don't have the vehicle.