Nearly forty years on, three electrical faults: the speedometer, the fuel gauge and the boot lock. Nicolas, the car's owner, and Cédric, our president, spent a weekend investigating them, signal generator and soldering iron in hand. The video of the job comes out on 17 October on Nicolas's YouTube channel; here is the detail of the three diagnoses.
The R25 Limousine is the Renault 25 stretched by 22.7 cm by Heuliez: only 832 were built, from 1986 to 1988, and it is the model that carried François Mitterrand through both of his terms. Under the bonnet, naturally, a PRV V6 — the 2.7-litre 144 bhp or the 2.5 turbo 182 bhp. Nicolas's car — Ztyph on the forum, and the man behind the YouTube channel Zick'Family en camping-car — was suffering from three electrical faults: the speed side of the instrument cluster, the fuel gauge and the boot lock.
Together with Cédric, our president, he spent a workshop weekend tracking them down one by one. No parts swapped blindly: each time, a measurement to isolate the culprit, then the repair.
LA VIDÉO — « RENAULT 25 : COMMENT SAUVER L'ÉLECTRONIQUE APRÈS 40 ANS ! » · ZICK'FAMILY EN CAMPING-CAR
On this R25, the speed does not reach the cluster through a cable: a sensor fitted in the gearbox sends an electrical signal to the dashboard. That sensor is a Hall-effect sensor.
Discovered in 1879 by the American physicist Edwin Hall, the effect bears his name. Pass a current through a thin plate of semiconductor and bring a magnet close: the magnetic field pushes the moving charges towards one edge of the plate. A small voltage then appears between its two edges, proportional to the field: the Hall voltage. In a speed sensor, a target turns in front of the chip, driven by the gearbox — a toothed or magnetised ring. Each time a tooth goes past, the field changes, and the electronics built into the sensor turn those changes into a square, on-off signal. Its frequency is proportional to speed: the faster the car goes, the closer together the pulses. And unlike an inductive sensor, the signal's amplitude does not depend on speed: it stays clean even at walking pace.
To find out whether the cluster was at fault, we stood in for the sensor: a square-wave signal generator, set to 5 V, connected to the cluster's speed input. By varying the frequency, you simulate a car speeding up and slowing down. The cluster responded correctly: it was not the culprit.
That left the sensor. To put it to the test, we had it read an ABS target — a toothed ring like the ones on the wheels that give the anti-lock system its speed. That test brought the real cause to light: the sensor was badly positioned in the gearbox. Once refitted correctly, it gave the cluster its speed back.
Second fault: the fuel gauge, shown on this dashboard in green digits in the middle of the cluster. First check, the simplest one: continuity between the gauge and the electronic unit housed in the right-hand rear wing panel. The check was conclusive: the wiring was sound.
So the unit was opened up. It suffers from a design flaw: only half of its circuit board is varnished. Yet that varnish is what protects the circuit: it stops the solder joints from oxidising and deteriorating over time. So we decided to redo its solder joints. Last step: disconnect the battery, then reconnect it. The fuel level then started working properly again.
Last job: the boot, where two faults were piling up. The first, electrical: the boot relay. Same cure as for the gauge — unit opened, solder joints redone — and the relay came back to its senses.
The second, mechanical: a broken spring in the lock. Pending a permanent repair, the fix came MacGyver-style: a rubber band 😂. Temporary, but effective.
Three faults, three diagnoses, and dozens of solder joints redone: the Limousine has its speedometer, its gauge and its boot back. Thank you to Nicolas for the weekend and for the video, to watch in full above and on his channel.
Do you have an R25 whose electronics are playing up? Tell us about it on the forum: we'll look into it together.
Renault 25 — the car and its PRV V6s · The R25 V6 Turbo · M6's R25 HAS Turbo · 2026 R25 meeting on Oléron · The Zick'Family en camping-car channel
Diagnostics and repairs: Nicolas (Ztyph) and Cédric (machouse) · Video: Zick'Family en camping-car YouTube channel · R25 Limousine: PRV Concept's Renault 25 page · Hall effect: physical principle (Edwin Hall, 1879)
