[bwilson4web]

Monday afternoon, Dr. Stephan B. contacted me about a problem with his 2002 recently bought at auction. The 12V battery wasn't charging and the local auto shop recommended taking it to the Toyota dealer. So I took some personal time and we met at the shop.

We quickly determine: inverter pump was running and the 12V battery had enough charge. I had already configured my scangauge to monitor the 12V battery so we put it in his car and I followed him with the caution that it wouldn't work after it reached ~10V. He pulled into the driveway and as he was trying to straighten the car out, it stopped at 10.4V. He noticed the steering had become stiff and given the electric, power assist, especially at low speed, this makes sense.

If someone has a Prius that is not charging, it is important to minimize all electrical loads. One of the biggest is the power steering and in the NHW11, the daylight, running lights. The simple answer is to pull the fuses and ignore the error codes. The other load is the electrical pump for the brakes but this involves safety and safe vehicle operation. Hopefully, everyone knows no heater or air conditioner or any other avoidable electrical load.

He turned off the car and we opened the hood hood. There was coolant in the tank and since it had been a few minutes, he turned the car on, not to READY, and we verified turbulence. The battery had recovered enough juice so we turned it off and went inside to talk about the options.

We knew one of two faults were possible: (1) failed fusible link, or (2) failed DC-to-DC converter. Although the fusible link looked OK, we needed to 'ohm it out' but it does not pull out. It is solidly in the fuse box on the driver side, engine compartment. We needed to test it to make sure the DC-to-DC converter was bad.

We decided that having the car in the garage during the repairs would make more sense as rain was forecast in two days. I had my manuals, instruments and parts at home but it would take me three hours to return. So we agreed to resume at 7:00PM.

When I returned, I had my AutoEnginuity, maintenance manuals and a spare NHW11 inverter in the trunk that I'd bought several years ago. Stephen had the car in the garage, the 12V ground disconnected, and already loosened the bolts holding the fuse box but there was not enough room to get into the fusible link. But having nearly removed the inverter several years ago, we proceeded to shift it so we could get into the fuse box.

(my earlier effort)

Stephen turned the tools and soon the windshield wipers, cover, wiper motor, and firewall cover was removed. We then loosened the two bolts in the front, four side connectors, two battery cables, and two rear bolts. This was enough to shift the inverter and get the fuse box loose enough to work on it.

The first puzzle was there is a lower skirt around the fuse box. Prying the tabs allowed it to fall away but the fusible link remained in the box. But we found two tabs on the top that a short flat could depress and the whole, fusible link assembly slipped out the bottom:


Now the fusible link remains a somewhat strange, assemly with two obvious parts. But we couldn't persuade them to separate. But Stephen found a black-wire connector that could be loosened. We'd been working for over an hour and a half so we took a water break and I fetched my schematics and Vol. II of the maintenance manual.




While cooling off, I realized we only had to measure the fusible link continuity, it wasn't necessary to disassemble the holder. So we went out and using probe-through-insulation, soon determined the fusible link was good ... the inverter was bad. So we took out my spare, inverter:


Progress was rapid but only three of four star screws cooperated. He will have to get a bit that fits. We also need to drain the inverter coolant. So we called it a night after documenting the MG2 wiring:


So we'll get together soon and finish the inverter swap. Then with the AutoEnginuity, we'll carefully check out the electrical system and bring this $4,000, 2002 Prius back to operational state. I plan to do a forensic check of the DC-to-DC converter but I noticed he had the modified battery connectors and SAE terminal adapter.

He has two, 12 V batteries, used one at a time, which raises the possibility of an accidental, reverse connection or possibly one so discharged that it put too much of a load on the car. My UPS testing suggested that as voltages approach 11V, it could easily draw more than 100A. This may be a case where a $500 part protected a $10 fuse.

Bob Wilson

ps. Dr. Stepen B. is a nuclear engineering Phd who teaches at a local university. So how how many engineers does it take to fix a Prius? ...

[HighDesertHybrids]

bwilson4web said:

I plan to do a forensic check of the DC-to-DC converter but I noticed he had the modified battery connectors and SAE terminal adapter. He has two, 12 V batteries which raises the possibility of an accidental, reverse connection or possibly one so discharged that it put too much of a load on the car. My UPS testing suggested that as voltages approach 11V, it could easily draw more than 100A. This may be a case where a $500 part protected a $10 fuse.

He has 2 12V batteries installed, with SAE terminal adapters? Why not replace with a single Optima D51R?

[bwilson4web]

Sorry, he had two, separate 12V batteries used one at a time. I suspect he knew the 12V charging was FUBAR and bought it at auction thinking it was a steal ... from one point of view, it was. But then he has a new Prius buddy who happens to live in Huntsville. <grins>

Bob Wilson