Hi folks,

This modification is unique to the North American and possible the European, NHW11 model Prius, 2001-2003. It appears that these Prius, unlike the Japanese and later, NHW20 Prius, has an extended warm-up that in cold weather, really hits the mileage. The ICE must run, even if at idle, until the coolant temperature reaches ~70C. It seems to be common to Prius with the hydrocarbon (HC) emission system that is no longer in the US and never was in the Japanese Prius. [NOTE: the European Prius do not have the HC system and warm-up like the Japanese models.]

Thanks to an early experiment by Clive, one of the UK members of 'Prius_Technical_Stuff,' we learned that adjusting the ICE coolant thermistor can trick the NHW11 into going into fuel efficient, hybrid electric mode instead of running the ICE for 6-7 minutes just warming the ICE to 70C:


The S4/S5 [RJW correction] modes allows the ICE to auto-shutdown after meeting two additional requirements. Once the coolant indicates 70C, the car must reach a speed of 35 mph and later, the car must stop for about 10 seconds. A simplified explanation, these last two conditions allow the car to shutdown the ICE when it is not needed and save substantial amounts of fuel.

There are four approaches to this thermistor hack:

• diode-resistor - a simple hack, there is a delicate tradeoff since in cold weather, too low of a resistor can throw an engine code. Too high of a resistor value and the warm-up time is extended.
• diode-resistor, switch - this uses a switch to enable the diode-resistor once the coolant reaches 30C. The problem is the operator has to monitor coolant temperature and open the switch once the vehicle reaches operating temperature.
• transistor circuit - we don't have the details of Cor's hack but it uses one transistor to sense the 30-40C range and a second to enable the bias resistor (and diode?)
• uprocessor circuit - this is the approach I'm using. This allows software changes to the temperature set points and understanding how to use the MSP430-F2013 for future projects.

I'm still tweaking the box but some of it is described here:

http://hiwaay.net/~bzwilson/prius/pri_temp.html

BTW, this hack does not replace but augments the effects of block heater. But unlike a block heater, it does not require house power when the vehicle is parked.

Bob Wilson

 

Thanks for all the research and effort put into presenting this info for us.

I have an 02 Prius and every morning when driving to the gym I think of adding the tweak using the resistor however realize I don't know where exactly you hackers are placing the resistor physically in the circuit. I know where on the schematic, just no clue on the car itself. I hate to ask for a pic of where the simple resistor is located however without seeing it, I cant even begin to place it.

I have done the transmission oil change thanks to your detailed info, thanks. As far as I can tell, the oil had 70k miles on it.

Over the weekend I replaced the original plugs with Bosch Platinum +2's. Seems to run just fine but dont have any miles to speak of on them yet. Whatever mpg I had is dwindling because of my recent short trips and cold engine syndrome.

take it easy,

Tony

 

Hi Tony,
Today, I have two leads on one of the engine ECU that reach to the center armrest area. What I plan to do is add a 12 VDC supply and have a short lead to an RJ-45 female behind the glove compartment. Then I'll leave my thermistor hack in the glove compartment with a pig-tail, ethernet cable with a plug. This will be a fairly clean installation yet let me swap control modules as needed.

What I'd like to suggest is when I document the proper installation, you might use that as the 'template.' For cost reasons, I'll also suggest a diode-resistor-switch hack, possibly a latching relay so it will default 'off' when the car is turned off. This will give you complete control and default to 'safe mode' everytime the car is turned off.

Would you prefer to 'make your own'? Alternatively, for parts, mailing and a 'quality cuppa coffee', I'll put together a switch-kit for you. You'll still have to tap the vehicle wiring but I'll see if I can find a 'vampire tap' so you won't have to do any wire cutting and soldering.

Any observations about the oil quality? Did you drop and wipe the pan too?

Sounds like you're well up on the maintenance.

GOOD LUCK!

Bob Wilson

 

hey how long did it take to install the hack? does one require a degree to accomplish? How about if the person can change their own oil and transaxle oil?

03 Silver 58k miles.

 

Installation requires three circuit taps and the circuit. I only know three people doing work in this area, Clive in the UK, Cor in California and myself. We are and have tested five approaches, resistor, resistor-diode, dual transistor, switched resistor-diode and microprocessor. At this point, our efforts are R&D and not really working on making kits, this year.

After we get the bugs worked out, one or more of us will probably come up with a kit. But today, it requires cutting and soldering harness wires, populating and debugging a circuit and integration and testing with the vehicle. This is well within the capabilities of any ham or electronic technician. But before I offer a kit, I want to quantify its effectiveness.

If I offer a kit, it will be something that won't require soldering and can easily be handled by anyone who can follow a set of photos and instructions. The circuit will be fully assembled and tested. There will also be a set of instructions to back it out and how to test the circuit in the vehicle.

Bob Wilson

 

Hi Bob,

Even living here in Fl, where the temps arent actually that low or last for any length of time, my o2 Prius mpg gauge dives during warmup most all day long due to my driving schedule.

Because the temps are not that extreme here, I was hoping for maybe a simple resistor to be placed in parallel with the temp sensor. I was thinkin of simply placing it at a connector. Maybe even a variable resistor to fine tune and forget it, YET be able to yank it out should I need some sort of warranty work. Just dont need the techs to lose site of the real problem while wondering what the heck the hack is.

I dont want to hi-jack your thread here too much so I will briefly say, Yes, I did drop the pan and the whole innards were very clean with exception to the magnet which was doing a great job of collecting stuff out of the fluid. The tiny magnet was about the size of a large jellybean before cleaning it off. The intake screen was perfectly clean. I was also able to reuse the gasket with no issues at all. I used the original fluid so my mpgs are about the same, no change was really expected. The fluid looked clean with no burnt or real bad used odor.

The plugs with 70k miles looked like new. They were double tipped iridium type. I almost put them back in but decided to try the +2s in the Prius. I use them in my other cars with very good performance.

Also got some new tires. They make it a lil more difficult to get the mpg the Bridgestones did with ease, but it is possible to do so, just not as easily. To me it was a no brainer to get tires warranted for 65k @ 50$ with a rating of 51psi max pressure.

Sorry, I DID hijack the thread, I will now get outta here.

Tony

 

Ok, let me share some of the details, enough anyone willing to try can at least get to the right signals and have a reasonable chance of success:

http://hiwaay.net/~bzwilson/prius/pri_temp.html

The full story can be found in "Prius_Technical_Stuff", YahooGroup, looking in the November 2006 postings for "clive1471", "bwilson4web", "Hobbit" and "Cor". The connectors you want are on the engine ECU:

"In our left-hand drive US Prius, it is located behind the glove
compartment. In the US model, the temperature sensor comes in on:

Connector E8, pin 14

This connector has three rows and from the latch side going down:

1,2, null, 3,4,5,6,7
8,9,10,11,12,13,14,15,16
17,18,19,20,21, null, 22,23,24"

The 'brown' wires are ground and the thermistor wire is white with a stripe.

I would recommend reading the associated postings in "Prius_Technical_Stuff" to understand how the diode protects the high-temperature readings and the problems of enabling the hack at too low of a temperature. You may want to consider using a switch to turn it on after the ICE has a chance to reach 30-40C, 40C may be a better starting value.

Thanks! This is good information to share anyway. I remain interested in the transaxle oil and you've provided another data point. I have long suspected that part of the early transaxle oil problems are exhaserbated in dusty environments. Florida with its almost daily shower, would be relatively free from airborne dirt and grit. Good buy on the tires too.

Bob Wilson

 

Hi folks,

Integration and test revealed a power-on, race condition that software now corrects. What happens is the engine ECU can 'boot-up' after the temperature hack circuit. Without ECU power, the lack of voltage on the thermistor briefly makes it look like the engine is already, very hot. Low voltages are a hotter temperature while high voltages are cold. This fooled the temperature hack into putting the bias resistor in 'feed-back' mode too early and causing the ICE to 'throw a code.'

Unique to the North American Prius, the HC system has a valve that may play a role in the low MPG during warm-up. By partially blocking the exhaust, the ICE would be 'choked' and suffer significant performance problems from high back pressure.

There have been reports and I'm starting to see high current drain in the first minute or so of operation under high power. This makes sense if the vehicle is trying to keep the gas flow through the exhaust at a minimum. Keeping the speeds low, 25 mph and less, in the first minutes of warm-up is likely to be part of the answer and now we know why.

It is too early to give specific fuel efficiency improvements but I've compared my 2005 Nov-Jan GreenHybrid data to my 2006 Nov-Jan data with the earlier manual hack. Some part of this improvement MAY be due to 'learning curve':


Had the manual temperature hack failed, I would have stopped earlier. Until the temperature hack is fully operational, I'm not ready to quantify the effect. But the early data looks promising.

Bob Wilson

 

Monday evening, I tested the hack in 27F / -3C weather. At the end, I ran fast enough to get the ICE coolant to 81C, enough to peg the adjustment to off and then began an extended 'stealth' or EV mode. The hack keeps my NHW11 in hybrid, battery management mode and the coast-down from 50 mph ensured the battery was at a reasonable, peak charge:


Ordinarily, the 'stealth' mode would have ended in 1 minute 15 seconds because I was using the heater to cool the ICE and would have been below 70C. However, the hack let my NHW11 continue in 'stealth' mode for 5 minutes and 45 seconds. I wasn't trying for a maximum 'stealth' mode, just recording the data to evaluate the thermistor hack. When I parked, the ICE coolant measured 47C.

Bob Wilson

 

Hi folks,

The brass board works well enough that I'll use it the rest of the season until I build the permanent replacement. I can easily build extra copies if there is an interest. Based upon what I know, they should run about $100-200 each.

If you have an interest, send me a PM. This will give me an idea of how many parts to order and the technology to use. I'm not after deposits, just contact information and a 'request for a quote.'

SPECIFICATIONS:
1) Thermistor monitor and adjustment system.
2) By default, adjusts temperature to 70C after reaching 40C. (note 1)
3) By default, limits temperature adjustment below 82C. (note 1)
4) By default, startup delayed 5 seconds. (note 1)
4) Records temperature adjustments for playback.
5) Upper, lower temperature settings and startup delay can be changed.
6) Operating range -20C to 85C.
7) User installable without special tools

Note 1: Default parameters are specific for NHW11, USA Prius.

Bob Wilson