EV mode not working correctly, engine always running

First let me start of that yes, this is on a Highlander Hybrid but as the long battery rebuilding thread showed early on these vehicles are very similar in how they are designed so I'm hoping to draw off the Prius community knowledge (second to none) to fix my issue.

Not sure when this started but sometime last summer the vehicle stopped acting "normal" under EV mode and started exhibiting it's current symptoms. Basically if I'm stopped at a light with absolutely nothing running (no fans, defrost, etc) it will enter EV mode (shown on the display). It will only enter this mode after the vehicle is fully warmed up (totally normal). The issue is the moment I turn on the HVAC fan (regardless of setting temp to cold or hot or if the defrost is on) the engine kicks on.......when I first got the highlander it would cycle the HV battery down while the fan/defrost were on and only kick on the engine when the battery voltage had dropped low enough. When I do this the hybrid battery is certainly fully charged and I have verified this both through the display and through voltage readings taken directly from the battery itself (I have a Hybrid Automotive charger hookup on it). Also the vehicle will not enter EV mode while rolling unless I don't touch the throttle at all (ie just coasting and again with no HVAC items on in the car).

Now I have done a few things to try and fix the issue. I bought a very low mileage (60k) HV battery to see if my original one was the issue. I swapped in the batteries first and installed and used the Hybrid Automotive charger system, no change. Then I swapped the modules in (relay and battery voltage monitor modules) and again no change. I suspected it might be a 12V issue so i also replaced the AUX battery as well but again no change from before.

So I'm at a bit of a loss right now on what to try next. I haven't had time to hook up Techstream so I'm not sure if there is anything I might spot on that. I have been looking at similar Prius threads and I saw someone that needed to replace a hybrid ECU but not sure if it might be that or the inverter or possibly something else. Any suggestions on what to look at would be most appreciated.

 

Well I spent some time the other day and I think I figured out what's going on while I waited for my son's hockey practice to be done. First of all I'm in Canada so it's pretty cold right now (-10 to -20C is not untypical for daily temps). I hooked up Techstream and watched both the Hybrid and Air Conditioning screens to see if I could spot anything.

At first I was thinking there might be an issue with the DC/DC converter since the battery voltage showed 13.4V on the NAV screen (maintenance mode). Before leaving my son jokingly suggested that maybe the temp control might have something to do with it so with the car warmed up I gave it a try. I looked at the AC screen and saw the rear HVAC was set to max hot so I turned that down to 3/4 heat, then did the same on the front. I was surprised because as I turned the temp down EV mode did kick on even with the defroster (which uses more HV battery amps, so I knew that wasn't the issue).

So watching the HV battery it started around 65% SOC and the aux battery voltage stayed right around 14.0V but eventually the engine kicked back on after just a few minutes. So then I decided to watch the engine coolant and when I did it became obvious that the engine coolant was loosing heat very fast in the cold conditions so it would kick on at a predetermined calculation (engine coolant temp vs HVAC temp setting). So at around 3/4 heat the engine would kick on every time the engine coolant dipped below 160F. So long story short is that, yes, it does appear that all is working well and that winter cold is the enemy of my quest for better MPG (my son was quite amused at that when I told him after practice). I'll try and post some Techstream screenshots so people can see what I mean when I get a chance.

If anyone has any tips for how they conserve heat on their Prius I'd be all ears.

 

Any links or pics of that? Sounds like it might be promising.

 

Think I found it.......

2012+ 3rd gen Prius grille blocking strategy | Balancing Act

Doesn't look like anyone has done a Highlander so looks like I'll need to get creative.

 

Winter I'm getting around 21-22 MPG (11-11.5 l/100km) but summer it's closer to 26-29 MPG (8-9 l/100km).......we have lots of snow and hills so that doesn't help either.

Can't wait to try that grill block and hopefully get my winter mileage up a bit too.

 

After running with the new HV and AUX battery for over a week now, the car is running very well and my MPG has settled around 10.5 l/100km (22.4 MPG) without any mods to retain heat. It even went into full EV mode when I pushed the button which surprised me but you don't get much acceleration like this before too much pedal causes the engine to kick on.

 

Well temps are back up near 0C (32F) and so is my mileage (now around 8.6-8.7 l/100Km or around 27 MPG!!!!!) which is excellent for this time of year and not to mention I have over 300k kms (190k miles) on it as well. I think a key was to make sure to only leave your HVAC temp set to 3/4 heat, full heat will run the engine constantly in colder temps to keep the coolant as hot as possible.

I also wanted to post some screenshots from Techstream ( mini VCI ) since it may help other with their diagnosis as well. The first 3 screenshots are of the HV battery so you can see what to look for to know your battery is working well (things like coolant temp, outside temp, aux battery voltage, HV battery SOC, HV battery charge/discharge currents, HV battery min/max voltages, etc). The last shot is the AC screen which shows all the specifics for your AC while in operation (stuff like coolant temp, outside temp, heater settings front/rear, etc). There is so much info in Techstream it is an absolute must IMHO for anyone doing DIY work on these hybrid vehicles. Anyway, hopefully others find this useful as well.