After I posted about my MOT woes on the Open Inverter forum, the ever-helpful Damien Maguire chipped in. It seems my occasional loss of torque may be related to a temperature issue with the Prius Gen 3 inverter.

Damien, who runs this setup in his E39, has found that this inverter needs serious coolant flow to keep it cool. Without this, the remaining Toyota brain in it (after I transplanted its main brain for one of Damien’s boards), shuts things down temporarily if temperature starts to spike.

It turns out Damien used to run almost the exact same coolant pump I am running now (pictured) (a clone of Bosch units used in VWs) and had very similar problems. So I shall be shopping for an upgrade. Damien recommends the pumps from the BMW 1-series, so I shall try to source something of that type or similar.

Easing bleeding

While I’m fitting this, I’m going to take a couple of extra steps to ensure things stay cool. First, I’m going to raise the radiator (1) up slightly to ease bleeding. It can only go so far without fouling the bonnet but a couple of inches should make all the difference.

To do this I will have to re-route the power to the charger (2), from the front of the inverter to the side. The inverter has two ancillary power connections around the top of the case. One is for the air conditioning system, the other for the charger. I was originally going to use the side connection (A/C) for the heater, but with the plans I have for the battery box and high voltage switching (subject for a future post), this will no longer be needed. The result will clear space for the radiator and look a bit tidier too.

To relocate the radiator, I will also need to make up new brackets. Only my sixth, possibly seventh iteration…

Hose tidy

I will also try to shorten one of the hose connections on the side of the inverter, which is currently pressing on one of the charger mounts, making fitting difficult and creating a future leak risk.

The hoses have been removed in the picture but you can see how the outlet from the inverter (3) is directly opposite, and quite close to, the charger mount (4). Depending on the layout of the pump’s inlet and outlet, I may need to reroute some of the hoses in any case.

Motor cooling

One of the other issues I never really solved for the MOT second attempt was the oil leak from the motor or motor cooling system. While the whole car is apart I plan to leak-test the motor, which should see me nicely covered in ATF again. I’m really coming to hate that burned Ribena smell.

I suspect oil is weeping out of some aged gaskets and some threads, so I will make up some new gaskets where needed, sealing it with paper or RTV as appropriate, and apply some thread sealer where needed.

I am also going to relocate the whole motor cooling package. I originally had it placed down to the side of the motor, as highlighted on the image below. You can just about make out the circular brackets for the reservoir behind the angled motor mount on this early picture. This is a nightmare to access once everything is installed and made hooking things up and diagnosing the leak really tricky.

Instead I am going to relocate the pump, radiator and reservoir to the front behind the grill – the location I once had the water cooling radiator mounted. This didn’t work because it put the filler for the reservoir below the level of the inverter and charger. But with a bit of fettling, it should be roughly in line with the motor sump, so this should work nicely.

At the same time I may return to my original oil pump. I dropped this because of its insane flow rate that drained the reservoir in seconds. But the replacement sounds terrible – “like artillery” is how one fellow EVer described it. If I can control the flow rate with some sort of PWM input, or have the pump run intermittently, the original may be the better option – especially once the motor isn’t leaking everywhere.

People power

While I’m thinking about all this, two other forum members are tackling two of my other remaining issues. I posted my bent coupler off to one person who offered to have a friendly fabricator take a look. And I sent my battery management system off to another, who has already managed to get the CANbus interfaces working that I couldn’t. With a little luck I may be able to reassemble the car not long after I get back from a little jaunt.

Upgrades along the way

Depending on how soon those parts return, I’m likely to make a couple more upgrades before reassembly.

First, a redesigned version of the brake booster pump control board that failed should be winging its way to me in a couple of weeks. A voltage spike wiped out the regulator that should have seen the numbers in this picture at 5V rather than nearly 12. The new version has a bit more protection against such spikes.

With this in place we will also reconnect our brake reservoir, with a less leaky T-junction than the one that let us down on the first MOT attempt.

Second, I want to tidy up my adaptor plate. This was pretty much the first thing we made on this project and it is frankly, ugly. It is likely to need some modifications once the new coupler comes back in any case: a straight coupler may mean that the mounting holes for the motor no longer line up.

In retrospect, I wish I had tapped the holes rather than trying to weld on captive nuts. If I was remaking it from scratch, that’s what I would do. But I can at least fix the captive nuts properly. Many have fallen off, making assembly tricky. And I can trim off a lot of excess material, saving some weight. And I can tidy up the weld spatter and give the whole thing a nice coat of paint.

With a tail wind and some kind weather, we might be ready for another run at the MOT this time next month. And this time, we’re going to nail it.