Sporadic "dead reckoning" issues

The dealer's comment was rubbish, but that's often the case.

Just so I understand your problem, let me some some statements. You correct them as necessary:

1) Your Nav system works fine after recalibration as long as you have a good GPS signal.

2) Over time, the calibration drifts off and the Nav gets flaky, even with a good GPS signal.

If these are true, then that's very odd. The dead reckoning portion of the Nav system should only come into play when you loose the GPS signal. For example, on first starting the car, you may see yourself driving through a field or going off on an angle. This is because the GPS is still aquiring the satilites. As soon as a good fix is obtained, your position should snap onto the current road and all will be well.

I'm suspicious that your aren't getting a good signal at all times. Dead reckoning is only marginally effective, and only for short distances. If your GPS signal drops out for more than of few seconds things will get progressively more wierd.

Tell us a bit more about where and when the failures occure.

Tom

 

It does sound like a gyro issue, although I'm surprised that a bad gyro signal can influence the GPS signal. Does this happen out in the open, away from trees, mountains, and tall buildings?

Tom

 

I can't see where that would help at all. From your description, I think you have a failed or failing component in the Nav system. Have any error codes been registered?

Tom

 

It's done at the shop, or with any of the scan gauge type of instruments. They plug into a socket under the dash.

Since you aren't already familiar with this, you probably need to take your Prius to a shop or parts store and have them pull the codes. Some of the Prius systems are very specialized, so it's possible only the dealer will be able to help. I would take this one to the dealer.

Tom

 

Sorry I can't help with this issue but with my TomTom XL ONE GPS sometimes the maps aren't 100% accurate especially in remote areas or a road has been relocated and that can see you "driving through fields", but mostly it is spot on. 4 or 5 satellites seems pretty poor, I normally pick up 8 to 10 with the TomTom. I never heard of a gyro in a GPS but I don't get out much.

 

The gyro and speed sensors are in use all the time. If they're sending the nav system false data, it will believe them, until the conflicting data from the GPS overrules.

The system continuously monitoring the sensors, the GPS and the map information to determine what's happening. The sensors are higher-resolution than the GPS, and give orientation information the GPS can't. But they have long-term drift. The GPS is low-resolution, but can give absolute position with no drift. The map is a useful clue - you're more likely to be following a road than going cross-country.

The combination of the 3 information sources are put together to achieve the best results.

For example, if it detects a 90 degree turn from the sensors, it WILL take your cursor straight off of the road the map says you're on, as it can sense you're not following the road. A non-sensor-equipped unit wouldn't do that if it didn't know the road turned onto - it would have to wait a while until it could be sure that the apparent deviation from the road wasn't just a measuring error due to the low-res of the GPS.

If your sensors are wrong, it will perform much worse than a GPS-only device, due to the conflicting information. It sounds like the user-visible calibration options aren't helping. I don't know whether you could bugger it up with the service menu. But it sounds like there's a real fault, and probably not something you could calibrate your way out of. I wouldn't expect the sensors to suddenly drift out of calibration but still be functioning adequately.

 

We have a similar problem with our Prius. For us it seems to occur a bit more often in the mountains/forests but it sometimes occurs in the flats. It is very intermittent, maybe happening once or twice a month, but it can repeat several times in one day. It seems like a sat signal problem to me because it has us running down the wrong road, spinning aimlessly in the middle of a field or heading off cross country. I've not tried any of the service menu options.
Anyway, is there any chance you have a lockpick unit from coastal electronics installed? It's a unit that allows you (your passengers) to enter destinations while you are moving. It does so by disconnecting the gps signal for a short period of time to make the car think it's not moving. We have one and I'm considering removing it to see if that if it may be what is contributing to the problem. Unfortunately, it really is a nice feature and living without it for long enought to diagnose the problem will be inconvenient. Just wondering. If you don't have one then I may also looking for the other solutions.

 

Just a casual observation, it sounds to me like it could be an intermittent / shorting connection or a bad component as previously suggested. Have you checked all of the connections to the GPS unit? Perhaps some of the wiring harnesses are not properly "seated" and are only making sporadic contact, confusing the signals to the GPS unit. Once again though, I am grabbing at straws just as much as the next guy, but it would be the first place I would start checking. BTW I am going to see a "Cirque" show in a couple of months in the DC metro area at National Harbor, will I see you there?

 

That's a good suggestion. I just tried reseating the connections on the DVD unit under the driver's seat. For us, that's where the Lockpick add-on is. I didn't see anything obvious but we'll wait a while to see if the problem re-occurs. Maybe it won't.

 

The GPS unit is located underneath the driver's seat. You should be able to access all of the wiring harnesses / connectors by sliding the seat all the way back. I would also check to make sure that all of the individual wires are properly seated inside the harnesses, it wouldn't take much to cause the kind of behavior that you are experiencing. I would also be concerned that some of the individual components and / or sensors may not be properly soldered, much like the MFD problems that some owners were experiencing in earlier models. If that is the case, you will either have to submit it for warranty / recall or take the highly technical and electrically irresponsible method of fixing it yourself.

My fiance and I will be attending one of the Saturday performances of Kooza in October, if you'd like, PM me and I'll give you my information; maybe we could meet for lunch while you are in town.