Modernising Classic Hymer Tank Senders: Using the Simarine STR-1 Puck with Original Analogue Meters

A recurring headache on classic Hymers is the failure of the tank level monitoring system. The factory setup relies on five stainless steel bolts drilled into the side of the tank at stepped heights (0%, 25%, 50%, 75%, and 100%), using the fluid’s conductivity to switch discrete resistor steps behind the dashboard gauge.

In the fresh water tank, this system often survives for decades, though spade terminals eventually corrode, snap, or shake loose from wiring harnesses. The grey waste tank, however, rarely escapes trouble. Decades of grease, soap scum, and tank sludge cake over the sensor bolts. Once bridged by conductive crud, the meter either reads permanently full or stops registering entirely. Because these tanks are frequently tucked into double floors or boxed under furniture, dropping the tank or reaching the bolt heads for mechanical cleaning is often practically impossible.

The Simarine STR-1 non-contact tank sender offers a permanent fix. At around £80, it isn't the cheapest module on the market, but it is proper marine-grade kit built to IP68 standards. Because it shoots high-frequency radar through the top skin of the plastic tank, it requires zero drilling, has no moving parts, and never touches liquid or crud.

Before ordering, check access to the top of your tank. You need a flat, horizontal area clear of floor ribs or pipework with enough clearance to accommodate the puck, which measures 62 mm in diameter by 32 mm high.

Once installed, you have three distinct routes:

1. App Only: Leave the dashboard untouched and simply check your tank level, volume in litres, and percentage on your smartphone via the Simarine Connect app.
   
   
2. Drive the Original Hymer Meter: Tap into the STR-1's analogue output to bring the vintage dashboard needle back to life with a smooth, linear readout instead of the old 25% jumps.
   
   
3. Both Simultaneously: The analogue output runs continuously in hardware while the wireless link remains fully active. You get the authentic period cabin gauge working for quick glances, plus full app access on your phone for precise volume figures and calibration.
   
   

How the Meter Interface Works

The original fascia uses a classic German moving-coil galvanometer (typically a DIN 2.5 mA or 1 mA Full Scale Deflection movement made by GMW or Deuta-Werke). Rather than trying to interface with the old transistor ladder on the Hymer PCB, you isolate the two meter terminals and feed the gauge directly using the STR-1’s 0–5 VDC analogue output (the green wire).

Because 5.0 V into a low-impedance coil movement will overdrive the needle, a small variable trimming resistor (a potentiometer or "pot") is placed in series to calibrate full-scale travel.

Required Components

• Simarine STR-1 Radar Tank Module (~£80)
  
  
• 4.7 kΩ or 5 kΩ 10-turn potentiometer (see appendix below for sourcing and easy no-solder options)
  
  
• Basic wire connectors (crimplets or mini screw terminal block) and insulating tape
  
  

Step-by-Step Installation

1. Mounting the Puck (Tank End)

• Locate a flat, horizontal surface on the top exterior of the plastic tank directly above the deepest fluid section (confirming you have the required 62 mm diameter and 32 mm vertical clearance). Clean and degrease the surface thoroughly.
  
  
• Tip: Do not peel the backing off the 3M adhesive pad right away. Rest the puck flat on the tank top dry (or tack it temporarily with a strip of tape) while you experiment with location, sensor calibration, and signal stability in the app. Once you are completely satisfied with the readings, remove the liner and press it down permanently.
  
  
• Use the existing 5-core tank harness to carry the supply and signal forward to the dashboard, avoiding the need to pull new cabling through the vehicle:
  
  
  • Core 1: Red wire (+8–35 VDC supply)
    
    
  • Core 2: Black wire (GND) and Blue wire (COM) tied together at the tank end
    
    
  • Core 3: Green wire (0–5 VDC analogue output)
    
    
  • Insulate and stow the unused White (0–190 Ω) and Yellow (240–33 Ω) leads.
    
    

(Note: If you only want to use the smartphone app, simply connect Red to 12 V, Black to Ground, insulate the rest, and you are done.)

2. Isolating the Meter (Panel End)

• Remove the control panel fascia and locate the back of the relevant tank meter.
  
  
• Identify the positive and negative terminals of the moving-coil movement.
  
  
• Cut the PCB traces or desolder the factory connections feeding the meter pins to completely isolate the movement from the original Schaudt/Hymer 5-bolt comparator circuit.
  
  

3. Panel Wiring

• Connect Core 1 (Red) to a fused +12 V source behind the panel.
  
  
• Connect Core 2 (Black/Blue) to the panel’s common 12 V ground, and extend this to the Meter (–) terminal.
  
  
• Connect Core 3 (Green) to terminal 1 of the potentiometer.
  
  
• Connect the center terminal (terminal 2 / wiper) of the potentiometer directly to the Meter (+) terminal.
  
  
• Turn the knob/dial to high resistance before powering up so the needle doesn't slam violently.
  
  

4. Sensor Setup & Calibration

• Open the Simarine app on a smartphone and connect to the STR-1.
  
  
• Enter the physical tank parameters: overall height (sensor face to tank floor) and total volume.
  
  
• With the tank fully emptied, capture or set the empty baseline (0 V output).
  
  
• In the app, simulate a 100% full reading to drive the green wire to its full 5.0 V output.
  
  
• Slowly turn the potentiometer knob until the Hymer needle aligns dead-on with the 1/1 (100%) mark on the original gauge face.
  
  
• Clear the simulation; the needle will drop cleanly back to zero under its internal hairspring.
  
  

Secure the potentiometer behind the fascia. The conversion retains 100% of the period cabin aesthetic while delivering a reliable, non-contact tank level system that works seamlessly on both the original needle and your phone.

Appendix: If You Don't Know What a "Pot" Is

A potentiometer (or "pot") is simply an adjustable electrical restrictor valve for electricity—much like an inline radiator tap or a garden hose nozzle, but for electric current.

Because the Simarine sensor outputs up to 5 V when full, connecting it straight to the old dashboard needle would push too much current through it and slam the needle hard past the 100% mark. Placing a pot in the wire acts as an adjustable brake. Turning the shaft lets you fine-tune the exact amount of electrical flow reaching the meter until the needle sits precisely on the 100% line when the tank is full.

You do not need an electronics workshop or soldering skills to wire one up.

What to buy:

Search eBay, Amazon, or AliExpress for:

WXD3-13-2W 4.7K potentiometer (or WXD3-13-2W 5K)

Why this specific model:

• Big and easy to handle: It is a round unit about 20 mm across with a metal shaft and a turning knob included.
  
  
• 10-Turn Precision: Instead of turning all the way from zero to full in half a spin (which makes needle adjustment hair-trigger sensitive), this type takes 10 full turns of the dial from end to end. Setting the needle to the line is calm and progressive.
  
  
• No-solder terminals: On the back are three large metal tabs with holes stamped through them:
  
  
  • Terminal 1: Connect your incoming Green wire from the STR-1 here.
    
    
  • Terminal 2 (the middle tab): Connect the wire heading to your Meter (+) terminal here.
    
    
  • Terminal 3: Leave empty.
    
    

To connect the wires without soldering, you can simply strip 15 mm of bare wire, thread it through the hole in the tab, twist it tightly around itself, and wrap it securely in insulation tape. Alternatively, you can crimp standard small 2.8 mm female spade terminals onto your wires and push them straight over the metal lugs. Once the needle is adjusted, tuck the body behind the control panel out of harm's way.