Old Man Musings on a Day of Prius Repair

I wrote in this forum in late 2025 about overheating problems on my inherited 2010 Prius. The vehicle runs fine but on long mountain slopes in warm weather engine temp will pass 220 degrees F. At all other times the temp is regulated fine at around 200 degrees. The vehicle has a new water pump and thermostat, so these are not the explanation.

And this posting is not about my overheating problem. It's about trying to do DIY repairs on vehicles designed by multinational corporations who are focused on lowest assembled cost in their factories.

Today I launched into an effort to replace the radiator, having concluded that my overheating problem is caused by debris adhering to the upstream side of the radiator or else it's plugged internally.

In recent days I acquired the necessary parts and was surprised at how cheap they are. You can get a new Aisin radiator for less than $100 delivered. I bought a new fan assembly while I was at it: only $150.

But getting the radiator and fan assembly off the car was a major ordeal. The front bumper cover needs to come off. The vehicle actually has three finned cooling assemblies: the radiator that cools the motor, the radiator that cools the inverter, and the condensing coil for the AC.

Toyota has essentially unitized these three heat exchangers into a single assembly using the plastic housing that holds the two electric fans as the "superstructure" that binds the heat exchangers together.

"Binds" is probably a little harsh here on Toyota, but the plastic fan assembly doesn't come out unless the other three parts are lifted up. But it would not be harsh on Toyota to say that the guys that designed the front of the vehicle were clearly confident that the electric fan and radiator would never need to exit the vehicle during the warranty period.

And when I finally got the radiator and fan assemblies out, I could see the lonely Aisin inverter cooling pump parked deep in an alcove underneath the inverter. Its rusty mounting screws told me that this was probably the vehicle's original inverter coolant pump.

Being a Gen 2 owner also, I know that Aisin got its beehind scorched severely in the early 2000s by premature inverter pump failures in Gen 2 vehicles. A recall was necessary. But in a Gen 2 an experienced Toyota tech or DIY'er can replace the inverter cooling pump in less than 45 minutes.

So today I decided I would preemptively replace my Gen 3's inverter cooling pump while I could easily see and touch it. That was an erroneous conclusion on my part. Unlike the Gen 2, changing Gen 3's inverter pump is a nightmare. Two of three 10mm mounting bolts must be accessed from the underside of the vehicle using a long socket extension and wobbly joint--and this is after removing the inverter to expose the rubber hoses.

Why would Aisin, after its painful experience with Gen 2 pump failures, allow Toyota to place Aisin's Gen 3 inverter pump in a location that is easily three billable shop hours of work to access, including dismantling high voltage wiring harnesses?

I once worked for a Tier 2 automotive supplier, and I know that warranty costs are back-billed to suppliers, including labor. Even a small production hiccup can cost a supplier millions of dollars and make that supplier wish he'd never entered the automotive market.

I could see at least a half dozen locations low on the motor generator's housing where the inverter pump could have been mounted for easier post-factory access. I know that Toyota wants this pump low in the liquid loop so that an air bubble won't cause the pump to cavitate.

Are automotive suppliers given a say-so in where their hardware gets mounted, knowing that the supplier is on the hook for warranty failures?