Gears & Such: November 2021

My sincere hope in buying a newer used car was to get something that was so reliable that I didn't have to work on it on a monthly basis to keep it rolling. After several years of 2-3 days of month lost under a car, I was really looking forward to getting a little time back, and getting a little more confidence in driving further that I could walk.

It turns out that was a bit of a pipe dream. I bought the C30 in June and 5 months later I've basically spent some time every weekend wrenching on it, meanwhile every other car has required comically little attention (if you call dropping an exhaust system and re-welding all the leaks "little").

In terms of time and money I would have come out ahead by doing nothing.

One of the problems that has been difficult to solve had with the Volvo C30 is that the right side taillight/parking light, and the left side city/parking light sometimes won't stay on. They will illuminate for a few seconds when switched on (light switch from "off" to "parking" with the key out, or when the car is first started, but then they turn off.

The P1 Volvos (V50/S40 II /C70 II/C30) all power these lights using an electronic system (Central Electronic Module, CEM) instead of a traditional switch/fuse system. The CEM powers the taillights and park lights by providing a controlled voltage output using PWM and that is current monitored instead of fused. The upshot of this is that if there is a short in the system, the CEM stops power output, and it can also detect a bulb out and put a warning on the dashboard or store a diagnostic trouble code. The behavior of the system is also software defined, so bulb brightness, blink rates, and such can be easily tweaked for different jurisdictions, and the module can be reused in different vehicles. The downside is that it is difficult to troubleshoot and not very resilient, in particular, if it detects a short, it will stay off until either the car is restarted or (when key is off/out) the lights are cycled.

The taillight problem started a week or so after I got the car and was very active for a while, but then suddenly vanished, and has only intermittently manifest.

Troubleshooting notes:

During the few seconds it was on, voltage at the right tail lights were lower than the left by about 0.2-0.5V.
Prior owner had installed LEDs in the front city and running lights, some of which had failed. I replaced these with OEM spec bulbs (W5W and WY5W amber).
I removed the CEM module (after disconnecting the battery!): no corrosion/water signs visible, I cleaned the contacts and added zinc electrical grease.
The headlight housing internals look fine as does it's connector and pigtail.
The taillight bulbs and bulb holders look fine: there no obvious way for them to short.
The system provides current from the CEM so the grounds are (probably!) fine, especially since things that share the grounds points (headlight, break light, backup light, turn signal) also work fine.
The CEM still cuts power if the headlight or taillight plugs are removed. If there is a short, it's not in those assemblies.
I found an intermittent short in the license plate light harness, which I thought fixed it, but that was apparently a red hearing. But the plate illumination is now 100%.
The key fob has a parking light illumination button that is extremely helpful for testing
Once the lights are working, it seems to go great until I find a big speed bump.

System Design Notes:

The wiring diagram shows that the CEM internally bridges pins C:12 (right rear tail light) E:13 (left front parking/city light) and C:13 ( License plate lights).
Without ripping the carpets and trim out of the car, I'm not completely sure, but it looks like the right tail light wires go up the passenger side under the passenger seat, the license plate light wires go under the drivers seat, and the left parking light/city light go through the fire wall.
Based on the skbowe CEM teardown, I suspect these CEM uses the Infineon BTS443P 25A Smart Highside Power Switch. Each of the tail light bulbs is about 8W/1A, and the next most powerful high side components in the CEM are limited to 3A: it's unlikely they are ganged up.
The BTS443P has load sensing, current limiting, thermal shutdown, and overload shutdown features.
Overcurrent sense is internal. While typically 65A is the cuttoff, it can range from 35 to 110A (!)
It's not clear from the datasheet or the Volvo documentation I have if the CEM reads the overload pin, or how the BTS443P responds after a short is removed.
There's a big loop of wire for the license plate lights that's wiggling and dangling behind the rear bumper. Maybe this is stretching/wearing over time.