Sporadic "dead reckoning" issues

[Lutzbone]

Hello all,

I have a wonderful 04 Prius with almost 100k miles (!) on it. We've been around the US as I'm on tour with Cirque du Soleil. I have an issue with the nav system - the "dead-reckoning" system seems to be really flaky. Sometimes it tracks perfectly, and other times it has me going thru fields, buildings, and even bodies of water! Sometimes it even says I'm turning in a circle when the vehicle is STOPPED. As long as I have GPS signal (most of the time), it will usually fix itself within 20-30 seconds, but many times this causes me to miss a turn or two. I've searched for an answer to this problem, but either I'm not typing in a good search string, or this isn't a common problem. I have tried both of the calibration options in the Setup menu (Tire change and Position/Direction), and they usually help for a while, but it eventually repeats the problem. I asked the Toyota dealer about this when I took it in for the last 5k service, and he said that he's heard of this problem with people who travel extensively - something about the nav "getting used to" a particular area and not working as well when away from "home." I looked at the Location screen of the Service Menu, and saw there were a few options that could be reset: "Distance per pulse," "Gain coeff." and "Map Matching." Anybody know if it would help or hurt to reset any of these values? Or should I just keep recalibrating thru the Setup menu? Any help is greatly appreciated!

[qbee42]

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

[Lutzbone]

Hello Tom, and thanks for replying!

I don't know if it's a signal issue; the GPS screen in the service menu says I'm locked into 4 or 5 satellites most of the time (3D status). I will check on the signal strength tho. What is the normal range of signal strength?

The thing that has me thinking it's a dead-reckoning problem is that it will randomly start turning when I'm going straight. In fact, it often says I'm turning when I'm stopped at a red light. Then when I go, it thinks I've turned and puts me on the wrong road going the wrong way. After a few seconds, it always corrects itself (unless I don't have GPS signal, in which case it will of course continue to wander aimlessly). It really seems like a gyro and/or compass issue to me. What I'm wondering is how to recalibrate that part of the system. anyone know if it can be done, and how?

Thanks again!
Jim

[qbee42]

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

[Lutzbone]

Yep, happens everywhere. Hey, just noticed you're from Northern MI! I grew up in Mt. Pleasant and went to school at CMU... small world! My parents also drive a Prius around Mt. Pleasant...

[Lutzbone]

Anyway, got off topic there... Any idea if I could do any good (or harm) from the Location screen in the service menu?

[qbee42]

Originally Posted by Lutzbone

Anyway, got off topic there... Any idea if I could do any good (or harm) from the Location screen in the service menu?

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

[Lutzbone]

I'm not sure... after a quick search of the forums, I couldn't find an easy and free way to retrieve error codes. Can you point me in the right direction on this?

[qbee42]

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

[patsparks]

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.