Fail! But in a good way

So, as predicted, the car failed its MOT. But not on what I was expecting. Or rather, the stuff I was most worried about (the conversion, my welding) absolutely sailed through. In fact, Jimmy the mechanic called the conversion “impressive”. The smile hasn’t left my face since.

Big issues:

  • Tracking is massively out (not surprising since I don’t have the tools for alignment) and you could tell this on the drive over there
  • Brake servo isn’t working. Clearly my fix of the vacuum leak didn’t hold.

Little stuff:

  • Bit of sharp-edged rust on the body (soon to be replaced) needs covering up.
  • Needs a new sidelight bulb (found one of these in my parts corner. I am such a horder)
  • Couple of nuts on the seat mounts need nipping up (whoops)
  • I need to reset the airbag light (forgot about that – plugged the battery in with the seat removed).
  • Oh, and my oil cooling loop for the motor is *still* leaking

Aim is to get all this fixed over the weekend and get it back for its retest next week. In fact, as I write this, I’ve already sorted the seat mounts and turned off the airbag light (thank you forums for…access…to the relevant software). When I can make some noise (it’s a bit early right now, I’ll get it up on the stands and take a look at the leak.

Alignment-wise, I need to get it to a decent level before getting it to a specialist. Plan is to experiment with my phone’s angle measurement app to try and get them reasonably aligned.


I’m still absolutely buzzing about the drive over to the test centre. Incredible feeling driving something you’ve built. But there’s a few issues to sort with the tuning. It wasn’t great on power – especially when I tried sticking it in second. And the car stalled a couple of times with over current events and wouldn’t get off the rolling road without a shove. I suspected there were just some simple setting tweaks to sort this though and I was right.

For a start, I had the overcurrent limit setting far too low. I had it at 300, since I have a 350 amp fuse in my junction box and I didn’t want to blow that. But I hadn’t done my maths. The number you need to plug in here is the peak current not the RMS. Being brave, 350 times square root of 2 (1.414) = 494. Adding some slack as suggested by the guide on Open Inverter takes me to 550. So I’ve plugged that in.

Then there’s the throttle ramp: how fast current ramps when you press the throttle (slew rate). I had mine set too high I think: apparently Johannes (Open Inverter originator) reckons you need it between 40 and 60 for the Field Oriented Control firmware I am running. So I’ve dialled that down to 50.

Throtcur I think I’ve mentioned before: how much current is applied for every percentage of throttle. I ran at 4.5 on the way to the shop and I’ve bumped this up again to 5. May go up again yet.

Then finally there is curki: current controller integral gain. I guess the inverter has a PID controller? My engineering degree was over 20 years ago so I’m not going to pretend to really understand this and I can’t be bothered to do the reading right now to remind myself. But others on the forum have suggested the default setting of 20,000 is way too high for my motor. Instead I will bring it down to 3000, which apparently is optimal.

These won’t be the final tweaks to the tune, but they should improve things a few notches.

Coupler squeak

One thing that might need a little more work is the coupler. Despite me packing it in grease on the last reassembly, after a little drive it was starting to squeak.

The coupler is made up of a length of thick tube with two clutch centres welded into it. On one end is the original BMW clutch centre and on the other is a Ford clutch centre that happens to have the same spline pattern as the electric motor. There are lots of reasons it could be squeaking. Poor alignment. Poor fitment to the motor splines. It might not be properly straight (I made it in the back yard with a hammer, a grinder and a welder from a skip, so it’s not exactly precision engineering).

I might stick with it for now and hope. But ideally it needs to come out (which means dismantling the battery box and pulling the motor), and get run through a lathe with a dial gauge. Then repacked with grease – maybe copper rather than lithium this time. Maybe it even needs to be remade. We shall see.


2 thoughts on “Fail! But in a good way

  1. The headline fuse rating is the current it can carry continuously. It will also have an I2T rating, current squared times time. This rating is the difference between slow blow, anti-surge, fast blow, ‘semiconductor’ etc. fuses. You should easily be able to pull 200% of the fuse rating during acelleration without any likelyhood of it blowing.

    For lubricating the coupler; Graphite or Molybdeneum grease.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.