Sky-Watcher EQM-35 Pro DEC Motor Fault: Grinding, Binding
and a Lesson in Mount Diagnostics
First of all, a huge shout-out to Adam at Rother Valley Optics (RVO).
And then to Dave Woods at Dark Frames Ltd.
Late on Friday evening I emailed both about my latest tale of
astrophotography woe. This time the culprit was my Sky-Watcher EQM-35 Pro
mount, whose DEC motor had suddenly decided that smooth tracking was overrated
and had instead adopted a worrying repertoire of jamming, binding and grinding
noises.
You can read about it in a previous post here: https://undersouthwestskies.blogspot.com/2026/10/skywatcher-eqm-35-pro-mount-grinding.html
As anyone involved in astrophotography knows, strange
mechanical noises coming from your mount tend to produce the same emotional
response as hearing an unfamiliar knocking sound from your car engine. You
immediately fear the worst while simultaneously convincing yourself it's
probably nothing.
Usually, it isn't nothing!
First Checks - Is the DEC Axis Mechanically Healthy?
Early Saturday morning I began investigating.
With the DEC clutch disengaged, I manually rotated the DEC
axis through its full range of motion. To my relief it rotated freely and
smoothly.
That was an encouraging sign.
If the internal casting, main shaft, brass bushings or
synthetic bearings had suffered a catastrophic failure, I would have expected
roughness, sticking, excessive resistance or obvious binding.
Instead, everything felt healthy.
That allowed me to narrow the search area considerably. The
problem seemed more likely to be isolated to one of three areas:
- The
DEC motor itself
- The
transfer gear assembly
- The
electronic control system
In troubleshooting terms, this was my first small victory.
The list of possible culprits had become shorter.
The Internet Rabbit Hole
A little internet research revealed something interesting.
Stepper motors can produce a truly horrible grinding noise
when they electronically stall. Rather than rotating normally, they effectively
lose synchronisation and shake violently.
One possible cause is inadequate power supply.
However, I was running from a fully charged Celestron
Lithium Pro PowerTank, so power starvation seemed unlikely.
Other possibilities sounded rather less appealing.
A broken cable.
A cracked solder joint.
A bent connector pin.
Any of these faults can cause a motor to lose one of its
electrical phases. The result is a motor that appears to have completely
forgotten its purpose in life, vibrating angrily backwards and forwards while
producing noises that suggest expensive things are happening.
Gulp.
Taking the Motor Apart... Slightly
At this point I decided it was time for a little controlled
exploration.
I removed the plastic DEC motor cover and detached the brass
gear mounting assembly, leaving access to the motor and its vertical output
shaft.
Now, this was venturing into territory normally occupied by
braver people than me.
Or perhaps more reckless people.
Possibly both.
My assumption was that the gear should not spin completely
freely. Instead, I expected a firm, uniform, clicky resistance as I rotated it
by hand, caused by the motor's internal magnets passing through their
positions.
Sure enough, that's exactly what I found.
The motor gear felt consistent and healthy.
The mating gear mounted to the metal casing also rotated
freely. The grub screws were secure, there was no wobble, no excessive play and
no signs of misalignment.
So far, so good.
Then came the important test.
With the gear assembly removed, I powered up the motor.
The vertical shaft spun smoothly.
Quietly.
Continuously.
No grinding.
No catching.
No stalling.
No unpleasant mechanical drama.
At that moment things started looking considerably more
positive. The motor itself appeared to be working perfectly.
Then a friend got in touch and suggested it was the motherboard - a common reason why skywatcher mounts will sometimes behave this way, apparently - was news to me - hadn't even considered that option.
Adam's Diagnosis
Later on Saturday morning Adam replied to my email and asked
me to send a video showing the mount's behaviour.
After reviewing the footage, his assessment was impressively
quick and reassuringly logical.
The likely causes were:
- A
fault in the DEC cable
- A
fault on the mount control board - which my friend had already suggested
- A
failure involving the bearings or magnets around the worm-drive motor
assembly
His recommendation was simple.
Before ordering parts or dismantling anything further,
determine whether the problem was electronic or mechanical.
A classic process-of-elimination exercise.
He suggested the following test:
"Unplug the DEC cable from the motor mount box and
plug the RA cable into the DEC motor instead. Then use the hand controller to
command an RA slew."
Simple. Elegant. Effective.
The Decisive Test
There were only two possible outcomes.
Result A
The DEC motor spins perfectly when commanded through the RA
output.
If that happened, the motor and gears were physically
healthy and the fault would lie elsewhere.
Result B
The same grinding and sticking occurs.
That would indicate a genuine mechanical fault involving the
motor, gear train or related hardware.
By this stage I was already leaning heavily towards Result
A.
And that's exactly what happened. The DEC motor ran
perfectly.
No binding.
No grinding.
No hesitation.
No drama.
I then repeated the test using the second cable.
Exactly the same result. Result A again.
Process of Elimination
At this point the evidence was becoming rather difficult to
ignore.
The motor appeared healthy.
The gears appeared healthy.
Both cables appeared healthy.
That left one increasingly suspicious candidate standing
alone under a bright interrogation lamp.
The motherboard.
Which, incidentally, was exactly where Adam's and my friend's experience had
pointed from the start.
Sometimes experience simply gets you to the destination much
faster than trial and error.
The Solution
A replacement controller board has now been ordered from
Rother Valley Optics.
With a little luck, swapping out the board will bring the
EQM-35 Pro back to life and return it to its proper role of tracking galaxies and
nebulae rather than impersonating an angle grinder.
Fingers crossed.
Going forward
At this point, having seen how smoothly my friend's EQM-35-Pro mount operates - it had been rebuilt by Dave at Dark Frames Ltd, I attempted to convince the boss that a rebuild was a good idea and having spent an hour bending her ear about all the advantages of doing it, over a coffee - I was thrilled to get the seal of approval.
A Final Word About Customer Service
Throughout this entire process Adam at Rother Valley Optics has been exceptional.
Multiple emails on Saturday.
Fast responses.
Clear instructions.
Follow-up emails first thing Monday morning.
This is exactly the kind of customer service that keeps
people returning to specialist astronomy retailers.
When things go wrong, it is easy to discover who simply
sells equipment and who genuinely supports their customers.
Rother Valley Optics have certainly done the latter and my
deep thanks and appreciation go to him.
I missed a phone call Monday evening from Dave Woods at Dark Frames Ltd - I'll explain why in a minute. But again, excellent customer service. Tuesday morning, a detailed email in my inbox - outlining what would be done to my mount, how much it would cost, what differences it would make to performance - excellent stuff.
Under normal circumstances I would have immediately contacted him and actually taken the mount up to him in the Portsmouth area. I mean I have seen the difference his work makes to a mount - astounding transformation.
But alas, things have conspired against this.
Welcome to the word of subsidence, unexpected collapsed patio areas, falling walls and lots of unbudgeted for expenses suddenly on the horizon.
It isn't a big hole by cornish sink hole comparisons but the walls are cracked, the patio is a goner - the drain shaft seems intact fortunately. Builders are now on site - concrete is on order and so the process of rebuilding starts. The emergency contingency fund is now down several thousands of pounds. New wall, new drainage pipes and channels, new concrete pad across whole patio area. New fencing. New patio! Ouch!
Sometimes you have years like this one - 2026 has been the year of a new dishwasher, new fridge, new microwave, new car windscreen, new side panel repair to motorhome, two collapsed trees in garden, a new mount control unit and now the patio and wall rebuild. And there is still November and December to go - aargh!
And so, much to my disappointment, the rebuild of my EQM-35-Pro is on a temporary hold. I'm not quite sure how long temporary is at the moment - but probably not this winter season. However, the boss has given it the go ahead - so Dave, it will happen but not immediately - unfortunately!
What I Learned from the mount situation
Oddly enough, I have come away from this 'failing mount' experience with
something valuable.
Confidence.
A few days ago, the idea of taking even part of my mount
apart would have filled me with dread.
Now I've inspected the motor, removed components, carried
out fault-finding tests and learned a fair bit about how stepper motors and
mount electronics actually work.
That's probably a dangerous development.
If I'm not careful, I'll start convincing myself that a full
strip-down, bearing inspection and regrease is a perfectly sensible weekend
activity. On the other hand - sending it to Dave Woods seems a far more sensible option frankly and so that remains the plan.
Let's not get carried away with an over appreciation of my 'assumed' engineering skills!
One astronomical crisis at a time.
Meanwhile, I'm back off onto the 'tea and biscuits' shift'. Its very cold outside, there is a brisk chilly northerly wind and our builders are being heroes.
And my deep thanks to Adam at Rother Valley Optics, a couple of members of the Plymouth Astronomy Society and Dave Woods of Dark Frames Ltd.
