Problem SOC levels on 2002 Prius

I wouldn't based upon a rate of one incident per year but without a reproducible symptom or hard diagnosis, the "golden board swap" is all that is left . . . one of the most expensive and least successful ways to handle an intermittent problem. Based upon your rate, one device swap every two-three years and wait another two-three years to see if the symptom is gone. This can take a little time.

When I have a rare, intermittent problem, I start looking for some way to continuously monitor the system so when it comes back, it can be captured and hopefully give a clue. The problem is there are not that many systems that we know can monitor the NHW11 fast enough to be of use.

Ideally to document the problem, the polling interval needs to be half of the event duration. So for a 0.25 second delay, we'd like to see 0.125 seconds to poll all interesting data:

• ~0.100 seconds - Graham miniscanner polling, high speed, time between each data value. This is too slow if the acceleration delay is 0.25 seconds.
• ? seconds - Auto Enginuity, I've simply not tried to see how fast it can poll. However, the 10.5 kbps ISO 9141 rate, is not terribly fast, about 1,000 bytes per second.

My recommendation is to see if you can find a way to replicate the problem. Since it seems to be associated with a '25% SOC' display, take a half day and see if you can recreate the problem if not at will, at least often enough that adding instrumentation makes sense.

Now if I were trying to replicate your results, I would probably use my Brindley Mountain hill climb, 525 ft. and 8% grade, at a speed of say 75-80 mph. This will force the traction battery to a low state of charge. Then at the top of the hill, shift into "N" to defeat battery charging and pull off onto a road to try your acceleration tests. But I'll need a better 'test script', one that is more likely to replicate the symptom, before I can commit to trying to replicate the symptom.

Another way to bring the traction battery SOC down is to back into a parking curb, put the car in "R", and apply the throttle enough that the car tries to backup and shuts down the engine. Backing up a hill would have the same effect but I've found that experience a little 'exciting' and it tends to be noticed. Isolation for safety is recommended.

I would also point out that sticky throttle plate, throttle plate gear motor, and as others have pointed out, dirty MAF, have been associated with similar symptoms in the past. That is not a diagnosis but just something that makes one wonder.

Bob Wilson

 

Good point:

There are often two parts of which one is the symptom, the pause versus another, what leads to conditions that cause the pause.

My understanding is your concern was 'the pause' happening in traffic. Humans are notoriously poor 'time pieces' and if we can replicate the pause by forcing a controlled discharge of the traction battery, we can use instrumentation to measure:

• traction battery SOC - the battery ECU reports the SOC in unit percentage. This normally ranges between 40-80%. Knowing how low it has to be to replicate your symptom is very useful as others can do a controlled discharge to the same level and try to replicate the "pause" or "lag."
• acceleration "pause" or "lag" - we can measure the accelerator position, ICE rpm, MG1 torque (Graham miniscanner), and MG2 torque within 0.400 seconds. If the "pause" lasts longer than this, better still twice as long, we can quantify how much delay. For example, the recent ZVW30 'brake pause' turned out to be about 0.800 seconds in duration.

I treat most failure modes as having that possibility. When we find the ICE has trouble starting, we use historical data and symptoms to share previous failures with the same symptom: sticky throttle plate, gunked up MAF, failing throttle plate motor and gears. But I don't rule out other causes until after the more frequent ones are cleared first (aka., "low hanging fruit.")

Now Patrick Wong and Doug are the experts in engine and hybrid vehicle ECU version numbers. I don't remember seeing a posting of what the most recent version is and the these are fairly accessible to check (sounds like an excellent STICKY candidate with the resulting fame, wealth and good fortune.) It may be you have an older, dated ECU and we know there were changes documented in the various TIS. This would be a "low hanging fruit" test of your vehicle systems.

But to diagnose what causes this problem, it would be useful see what happens when replicating it. For example, what is the relationship between battery discharge current and SOC of your battery? This can be measured in a controlled discharge. The slope of this line can be used to estimate your traction battery capacity, Ahr.

We can also propose "testable hypothesis" such as:

• ICE off, low capacity traction battery, low SOC, air conditioner compressor enabled - can this combination of effects delay ICE starting? Can it delay power to MG2 to supply electric vehicle acceleration power?

I won't rule this out but I have to treat it as a 'guess,' a hypothesis, until we can add instrumentation and document what is going on. Notice that a testable hypothesis requires a "test," some way to replicate the problem so we can measure it. I also call this "reality training." <grins>

Right now, you have a vehicle that about once or twice a year, exhibits this symptom. If you can find a script, a protocol, that replicates the symptom, others (Ok, me) can try the same script. If I'm able to replicate the symptom, I can use my instrumentation to document what is going on. If not, I can rent a Graham miniscanner or you could get an Auto Enginuity and with a modified OBD, and measure what is going on.

Sorry for the length of this posting but this is how diagnosticians approach a problem like this. Fortunately, you have given the local Toyota shop the first crack and an extra day at this problem. Once they give you a 'clean bill of health,' the big dogs can start the hunt. <GRINS>

Bob Wilson

 

Just the ECUs. If a traction battery module was at fault, the problem would not be intermittent. I agree with Bob's suggestion that unless you can figure out how to induce this intermittent condition, you won't know whether any action that you took has improved or has worsened the situation.

Regarding Bob's suggestion to check the current ECU part numbers: the engine ECU is located behind the glove box and is accessible when you drop the box down (same procedure as when you are replacing the cabin air filter.) The HV ECU is under the carpet by the front passenger position. The traction battery ECU is in the traction battery case.

Engine ECU: 89661-47054 is the current revision, and should be installed if your car has no-start problems and you've addressed the obvious maintenance issues: cleaned the throttle body, replaced the iridium spark plugs with the correct NGK or Denso model, and ensured engine oil is not overfilled and is 5W-30 viscosity.

Hybrid Vehicle ECU: I believe that 89981-47062 is the latest part number. Note that if the HV ECU is replaced, it is necessary to make the car IG-ON (not READY) for a 30 minute period (hook up a 12V battery charger so that the 12V battery doesn't go dead during this time) so that the HV ECU and the immobilizer ECU will sync up. The reason I suggested replacing this is because the HV ECU is responsible for interpreting accelerator pedal position, deciding how much power is required from the gasoline engine and the MGs, and directing the inverter to take or provide power from/to the traction battery.

Traction Battery ECU: I do not know the part number for this, and suggested replacing it because of your observation that the SOC level moves up and down quickly when the intermittent condition occurs.

 

None of the above faults will cause low HV battery SOC on a vehicle with a running engine.

 

Your instincts are correct. The tests that were suggested to you would not help you diagnose the issue.

 

No, it's not. It's how parts replacers approach a problem like this.

 

Gen 1 Prius vehicles can indeed act as stated in stop-and-go traffic. The first step is to determine whether or not the issue is caused by the vehicle's own strategy, or by a battery issue.

With the right equipment, battery state of health can be tested. Let me know if you need more info.

 

Excellent!

We all need more info so post away.

Bob Wilson

 

I think you can and should trust them. But your symptoms remain and should not be ignored.

My recommendation is to see, as you are able, if there is someway to replicate the problem in a controlled fashion. Share that protocol with us and as I am able, I'll give it a stab. Once we can replicate it at will, resolution is all but trival.

I'm not doubting your symptoms ... I have to take original reports at face value. Rather, we need some help, a way to replicate the problem and then the problem is 'toast.' <grins>

Bob Wilson

 

I doubt they ran any tests. More likely, they did what they said they did: checked for TSBs, checked for codes, probably took a look at live battery data, and called it a day.

This is not a criticism: if the complaint can't be duplicated, and no codes are present, there's not much the average technician can do, as OEM diagnostics are designed around symptoms and codes, and most HV battery issues are associated with DTCs. However, the control strategy of the Gen 1 Prius isn't perfect, particularly in extended stop-and-go traffic. In addition, a marginal battery pack may exhibit degraded performance only if specific operating conditions are met.

To clarify this issue, you will need to find someone who has experience with no-code Gen 1 battery failures, and who can design a state-of-health test based on your vehicle's current data.

 

did you check Delta SOC ?