The build manual. Follow it in order - later steps assume earlier ones.
Time: about 90 minutes, plus one drive to validate. Difficulty: no root, no soldering, no dashboard removal. Everything is a settings menu.
Do the whole setup parked. Several steps involve watching the tablet closely. None of them should happen while the car is moving.
- Phase 1 - Install software (10 min)
- Phase 2 - Configure the tablet (25 min)
- Phase 3 - Configure the phone (15 min)
- Phase 4 - Bench test (15 min)
- Phase 5 - Audio chain (20 min)
- Phase 6 - Physical install (15 min)
- Phase 7 - First drive (one drive)
- Install Open Headunit from its releases page. Use 3.2.1 or newer - earlier versions have Wi-Fi Direct and reconnect bugs that will waste your evening.
- Grant every permission it asks for: Location, Nearby devices, Bluetooth.
- Also grant Display over other apps - it is easy to skip and the release notes state it improves compatibility.
- Exclude it from battery optimisation: Settings → Apps → Open Headunit → Battery → Unrestricted.
Do not install Android Auto on the tablet. It is not needed and it is not how this works. If you have installed it chasing the app's "Self Mode", uninstall it - see Troubleshooting.
- Confirm Android Auto is installed and has run at least once.
- Turn off auto-update for Android Auto: Play Store, its app page, overflow menu, uncheck "Enable auto-update". Do this before anything else. An Android Auto update is what breaks this system, and it happens overnight without warning.
Do not install a wireless helper app. Android Auto 17.3 removed the connection intent they rely on, so they can no longer complete a connection. If you already have one, remove it: it sits permanently searching and costs battery for nothing.
This is the phase where care pays off. Nearly every "it does not work" report traces back to one of these settings.
Each settings sub-screen has its own Save button, and navigating away discards your change. A setting can silently revert with no warning at all - during this build, "Start on boot" reverted twice before it stuck.
The reliable procedure for every setting below:
- Change it
- Tap Save on that same screen
- Back out, re-enter, confirm it held
- For anything critical, force-stop the app and check again - preferences are only re-read from disk on a cold start
Second trap: the settings search box does not clear between searches. Typing a new term appends to the old one, producing nonsense like
auto-startdisconnectand an empty result list. Tap the X first, every time.
Settings → ADVANCED → Wireless Connection → Helper Connection Strategy → Phone Hotspot (Host)
The default is Google Nearby (Beta). If the phone Helper is set to Shared Wi-Fi / Hotspot while the tablet is on Nearby, the two use different discovery protocols and deadlock permanently - both ends listening, neither advertising. There is no error message. It simply searches forever.
If you fix nothing else, fix this.
Settings → ADVANCED → search audio → Audio Sink → OFF → Save
The in-app description: "If disabled, the headunit will not receive audio. Sound will play from the phone."
With this off, the tablet stops advertising the media (AUD) and speech (AU1) audio sinks during handshake, but keeps the system sink (AU2) - the protocol requires at least one to hold the connection open. So you keep display, touch and media controls, and lose only audio output. Verified live: the phone's Android Auto requests audio focus and immediately abandons it, and no Android Auto audio device appears in the phone's routing table.
Fully reversible if you ever want tablet audio.
Layer 1 - the app: Settings → BASIC → search orient → Screen orientation = Landscape (0°) → Save
Layer 2 - the system: on Android 12L and newer, large-screen devices ignore apps' orientation requests, so layer 1 alone does nothing.
adb shell cmd window set-ignore-orientation-request falseSide effect you should know about: this restores the ability of all apps on the tablet to lock orientation. Portrait-only apps may now force the tablet into portrait. Revert with set-ignore-orientation-request true.
Alternative if you dislike that side effect - lock the whole tablet to landscape instead:
adb shell settings put system accelerometer_rotation 0
adb shell settings put system user_rotation 1Why this matters so much: Android Auto negotiates screen geometry exactly once, during handshake. The receiver deliberately calls SCREEN_ORIENTATION_LOCKED because live rotation is architecturally impossible in the protocol. Whatever orientation the session starts in is baked in until you reconnect. Determinism before connect is the only available fix.
Persistence caveat: the system-level override is written to /data/system/display_settings.xml, which is unreadable without root, so persistence across reboot is unconfirmed. If orientation regresses after a tablet restart, re-run the command.
Settings → BASIC → search density → Pixel density (DPI) → L (218) → Save
The default 172 DPI gives about 1515 dp of width. Real car head units are 1000-1200 dp, so at the default everything renders roughly 30% smaller than the UI was designed for - the on-screen keyboard especially. At 218 DPI touch targets are ~27% larger.
The in-app help confirms the direction: "A higher DPI makes Android Auto icons and text larger, so less fits on screen." Go to XL if still too small for you.
Takes effect on the next connection.
Settings → Auto-start settings
| Setting | Value | Why |
|---|---|---|
| Start on boot | OFF | |
| Start on screen on | OFF | Would fire every single time you use the tablet normally |
| Auto-start on Bluetooth | Not set | See the warning below |
⚠️ The bug that will annoy you most. During this build, "Auto-start on Bluetooth" was left pointing at the phone. Result: the head unit app launched itself over ordinary work whenever phone and tablet made momentary Bluetooth contact. The logcat trail:OPENHU: AutoStartReceiver.onReceive | BT Device connected: <phone> OPENHU: MATCH! Starting AapService via Bluetooth Auto-startOnly ever set this to a device that exists exclusively in your car. Never your phone, never a home speaker.
To clear it, use the picker's Remove button - you cannot untick it.
Settings → BASIC → search fps → Debug → Show FPS Counter → OFF
On by default in some builds. Useful for diagnosis, distracting while driving.
Recommended: OFF.
With it ON you must manually reopen the app every drive, and the measured benefit is zero - a backgrounded receiver already holds no wakelocks. The app itself warns that ON is unsafe in combination with "Start on boot", because a sleeping (not rebooted) tablet never fires BOOT_COMPLETED.
adb shell am force-stop com.andrerinas.headunitrevived
adb shell am start -n com.andrerinas.headunitrevived/com.andrerinas.openheadunit.main.MainActivityRe-open settings and confirm every value above. Cold restart is the only honest test - preferences are re-read from disk only on a fresh process.
Settings → Mobile Hotspot → Configure → Band → 2.4 GHz / "Compatibility"
Not "5 GHz preferred", not "Performance". This is the single fix for macroblock corruption while driving.
The band preference is not applied while the phone holds a 5 GHz station link. Right after saving, the hotspot may still report 5 GHz. Cycle the hotspot off and on, then verify:
adb shell dumpsys wifi | grep -i "frequency"You want the STA and AP frequencies to be in different bands. Same number on both = you are still broken.
Also turn "One-time password" OFF so the tablet can rejoin without a prompt.
Samsung's "Switch between networks" and the settings behind it make the phone move itself to another network mid-session, which drops the projection. The receiver app warns about this by name when it sees a disconnect.
adb shell settings put global auto_wifi 0
adb shell settings put global sem_wifi_switch_to_better_wifi_enabled 0
adb shell settings put global wifi_watchdog_poor_network_test_enabled 0Set these on the phone. Note this changes general Wi-Fi behaviour on that phone, not only in the car: it will no longer move itself to a network it considers better.
The hotspot subnet is not stable. The same phone handed out three entirely different private subnets across three sessions, so the head unit had a different address each time. Nothing in your setup should hardcode an address; discovery has to find the phone each session.
- Power the dongle from the car's 12 V (via its converter).
- Pair the phone to it. Name it something unmistakable - you will reference it in automation.
- Confirm music plays through the car speakers.
| Setting | Value |
|---|---|
| Auto Start Service | On Bluetooth Connection |
| Bluetooth device | your car audio dongle, and nothing else |
| Try to auto reconnect | ON |
| Stop on BT disconnect | ON |
Check the device list has exactly one entry. During this build a leftover home Bluetooth speaker was still ticked from an earlier test, which would have launched projection every time that speaker was used at home. Untick everything except the car device.
If your isolator thinned the bass, or you just want a different sound in the car, scope an EQ profile to the car - full walkthrough in 04 - Audio Chain.
Short version: build a routine with If: Bluetooth device <car dongle> connected → Then: Equaliser = Custom. The automation platform adds the revert automatically, so it returns to normal when you park. Your headphones stay untouched.
Do this at home, parked, before you rely on it.
- Turn the phone hotspot on.
- Let the tablet join it.
- Open Open Headunit on the tablet → tap the Wi-Fi tile.
- Confirm the receiver is listening:
adb shell "cat /proc/net/tcp /proc/net/tcp6" | grep 14A8
14A8is hex for 5288. A row in state0Ameans listening. - Connect the phone to the Bluetooth dongle (or press START in the Helper).
- Within a few seconds the tablet should flip to the projection UI.
- Confirm a live session - state
01is ESTABLISHED:adb shell "cat /proc/net/tcp /proc/net/tcp6" | awk '$2 ~ /14A8$/ && $4 == "01"'
| Check | Expected |
|---|---|
| Picture | Landscape, full width, no cropped edges |
| Touch | Responsive, no offset |
| Audio | Car speakers - not the tablet. Play something and confirm. |
| Keyboard | Comfortably hittable at arm's length |
| Maps | Renders and follows GPS |
| Frame rate | Smooth, no stutter |
If any of these fail, go to 07 - Troubleshooting before continuing.
- Listen for noise first, before installing the isolator. Engine running, LED strips and accessories on, volume up, nothing playing. A whine, hiss or beep that changes with engine RPM or with your lighting is a ground loop.
- If it is clean, stop here. An isolator costs you low end. Do not fit one you do not need.
- If there is noise: fit the isolator between the dongle output and the stereo input. Verify the connector genders before ordering - a dongle with a male output needs an isolator with a female socket on that side.
- Re-test. Noise should drop dramatically.
- Now check the bass. Play something you know well. If the low end has gone thin, that is the transformer, and it is expected. Fix it in this order:
- Amplifier Sensitivity trim toward the 0.3 V end
- Amplifier Low-pass trim to 120 Hz
- Phone EQ compensation curve - see 04 - Audio Chain
Mounting rules, in priority order:
- Never block your view of the road. Below the dash line, not on the windscreen in front of you.
- Never in an airbag deployment path. A tablet becomes a projectile.
- Reachable without leaning - if you have to stretch, you are looking away too long.
- Angled away from glare.
- Cables routed so nothing fouls the gear lever, handbrake or pedals.
Check your local law. Screen placement rules vary by jurisdiction, and some are specific about what may be visible to the driver.
Power: the tablet needs a USB-C charger for anything beyond a short trip - a projection session is a sustained load. Prefer a socket that dies with the ignition so it cannot flatten the car battery overnight.
Before you leave, arm diagnostic logging so you get data from the first real drive rather than the third:
powershell -ExecutionPolicy Bypass -File scripts/Start-DriveLogging.ps1Then drive normally and note anything odd - especially in the first few minutes near home, when the channel collision would show up if it is still present.
Afterwards:
powershell -ExecutionPolicy Bypass -File scripts/Pull-DriveLogs.ps1This pulls the captures, prints a summary, and clears the device side only after verifying the local copy. What the numbers mean: 06 - Diagnostics.
Getting in: hotspot on (or automatic) → open the app → tap Wi-Fi. About five seconds.
Getting out: press the tablet's power button. Screen off = zero wakelocks = zero drain, while the receiver stays armed for next time.
If you have a PC on the same network, scripts/Start-AndroidAuto.ps1 automates the whole handshake for bench testing. It is a development and diagnostic tool, not a car workflow - it needs a PC on the network, which you will not have on the road.
See 05 - Protocol Notes for how it works and why it cannot replace the native flow.
Next: 04 - Audio Chain · 07 - Troubleshooting