Replacing a Burnt-Out Wheel Autopilot Motor

The Raymarine SPX-5 Wheel Autopilot 

The SPX-5 wheel autopilot installed
on Johanna Rose.  

A SPX-5 wheel autopilot is currently installed on Johanna Rose, a C&C Landfall 38.  SPX-5 wheel autopilot system from Raymarine includes a ST6002 controller, SmartPilot X-5 course computer,  ST4000 mk2 wheel drive unit, and a fluxgate compass. Designed for yachts with a laden displacement of up to 16,500 lbs it puts this unit at the limit for a C&C Landfall 38.  I originally purchased this autopilot for my C&C 29 mk1 but at the time when I sold the C&C 29, the autopilot computer was not working, and so I removed the whole unit and put it aside as it was no longer under warranty.  I eventually decided to send the course computer to Raymarine for repair(or at least to see how much it was going to cost).   Even though the unit was past warrantee, Raymarine fixed the course computer and upgraded the computer firmware all free of charge.   It turns out at that time I was looking at various options for new chart plotters, instruments, and transducers.  Needless to say, I ended purchasing all from Raymarine.


Burning-Out the Drive Motor

Having already installed the autopilot on the C&C 29, it was really not much of a challenge to install the unit on the C&C Landfall 38.  There were some minor changes to installation.  Like making use of the NMEA0183 interface to power a Raymarine  i40 speed system, mounting the ST6002 controller on the starboard cockpit bulkhead.   The wheel pilot was performing fine for several months.  I would often tell folks that I prefer not to rely on the autopilot.  Using the autopilot is nice while motoring and raising the main single handed.  But all that said, it is time fess up and eat crow.  I made a big mistake while sailing by myself on a really nice day.  I was sailing close hauled and I had locked the wheel.  I think I was enjoying the day's blue sky too much because after about 15 minutes, I noticed a change in the wind direction, so I reached down, turn on the autopilot, and then jump over to trimmed the sails.  Lesson to self: don't use autopilot with wheel locked.   I only realized what I had done the moment the controller beeped and read "MOT STALL".   After placing the autopilot on standby and freeing the wheel, I reengaged the autopilot and the motor appeared to work fine but within several minutes it stopped working--the damage was done.    I burned out the motor.  A  lesson learned.  I was told by someone that to fix the autopilot, I needed to either replace the wheel drive with a new unit or send in the wheel drive to Raymarine and pay to see if it can be fixed.  The cost of a new ST4000 mk2 from defender is $550.  Before going down this costly path, I decided to search online and I came across the ST4000 mk2 service manual.  It explains how to test the motor and list part numbers.

Testing the motor with a 12V supply in
parallel to a lightbulb.

Removing the Original Drive Motor 

Removing the drive motor was easy.  The drive cable was disconnect.  Then the plastic collar nut was unscrewed from the drive plug socket.   The motor was removed by unscrewing the whole motor tube from the wheel assembly and then gently sliding the motor out from the tube.  The motor was tested by placing 12V across the leads and declared dead.  I should also mention that it was clear from the smell of the motor that it was burnt.  The model marking on the motor where RS-555SH.  A Google search shows that the motor was made by the Mabuchi Motor Company in Japan.

The original ST4000 mk2 motor. A
Google search shows that the motor is
made by Mabuchi Motors model
number RS-555SH.  

The original ST4000 mk2 motor
showing the gear and single locking
screw.

The original ST4000 mk2 motor showing
the 1nF bypass capacitors and soldering
connections.

The original ST4000 mk2 motor with
wire loom and motor end cap.

A Replacement Drive Motor 

Mabuchi motor (left) purchased from
Skycraft shown next to the original
Raymarine A18086 motor. 

Raymarine sells a replacement motor, part number A18086, for about $100.  The Raymarine part come with the motor gear (18T gear teeth) and with bypass capacitors already installed.  But it turns out that the motor is rather common and can be purchased online.  The problem is that the motor does not include the gear nor the bypass capacitors.

One can purchase a Mabuchi motor RS-555SH-TD050112 from Skycraft Parts & Surplus  for $5.95 ($12.20 shipped).    This motor is not an identical replacement as the shaft comes with a flat sided shaft which turn out not to be an issue as the flat part of the shaft is later trimmed off so that the final shaft length is the same length as the original motor's shaft.  For the new motor, the shaft was measured, marked, and cut to length using a simple hacksaw.

Original motor (above) and new motor
with gear installed (below).

Finding a new T18 gear of the same dimensions did not prove fruitful, so the original gear need to be removed and pressed onto the new motor.  This proved to be the most challenging part of replacing the motor.  It is easy to mangle the gear teeth if one is not too careful.    With help, I was able to pry a small screw driver under the gear,  lifting the gear and shaft, exposing the underside of the gear.   At this point, a good hobby pinion gear puller can be used to remove the gear.    The gear was installed on the new motor by heating the gear with a torch and quickly pressing the shaft down onto the gear and finished by compressing down with a C-clamp.




The Soldering Bypass Capacitors and Loom Connector 

Wiring the new 2 nF bypass capacitors

Electrical brushed motors create large amounts of electrical noise.  This noise can interfere with nearby electronics and cause problems.  For noise suppression, it is common that DC motors have up to three bypass capacitors soldered onto the motor: one across the terminals and one from each terminal to the motor case.   This is exactly what Raymarine does to their motors.  The capacitors on the original motors are little ceramic capacitors with the number 102.  Looking this up, 102 is code for 1nF capacitor.

I also removed the original capacitors, and measured the capacitance with a borrowed meter.  Expecting to find 1 nanoFarad, I was surprised to measure 4 nF.  It turns out that a precise value is not too important.  Since I had a readily available supply of 2 nF ceramic capacitors,  I used these for making the new bypass capacitors.

The bypass capacitors soldered in place
on the new motor.

New motor complete with wire loom and end cap (top)
with old motor (bottom).   The sleeve to the left
originally came on the new motor which needs to
be removed before installing new motor.

New motor installed in motor tube ready
 for installation onto wheel drive. 

Installation and Operation 

Before installing the new drive motor, the drive cable was connected and the autopilot tested to see the motor spin forward and backward.   Installing the new drive motor took less than a minute.  First the drive motor was installed into the drive tube assembly, then as I tightened the motor tube to the wheel assembly, I engaged the wheel clutch, rocking the wheel backing and forth so that the gear box would align with the motor while screwing on of the motor tube.

In little time it was tested, but first without engaging the clutch, then engaging the clutch but double checking the steering wheel lock first.   It performed fine at the dock. The seatrial testing has been limited to just holding a course heading.  The sensitivity seems a bit high for my liking but I plan on redoing the seatrial calibration next weekend if weather permits.

 I came across a tossed out ST4000+ wheel drive (black unit)
at my marina.  I took apart the drive motor assembly, and
while the motor looks like an RS-555SH, the gear and gear
collar clearly are different. But the end cap and wire loom
plug assemble are the same as that used for the ST4000 mk2