I have a 2004 Prius with 191K miles. When purchased in November of 2008 the 12V battery was replaced (mileage at the time was 70K). Over the last 2 weeks my alarm has been going off daytime, nightime with no regularity. I have replaced the battery in the FOB (I only have one) and did this yesterday. Last night the alarm went off 4 times between 11pm and 1am until I disconnected the 12V battery. Today....I went to the dealer and they replaced the 12V battery (stating it threw some codes...they did not tell me which) They did say that the alarm would go off then the 12V battery was low or showed a high charge, which the codes showed. When I tested the battery at home it showed 12.4V in ACC with 11.8V under load. With the motor running 14.4V. I am unconvinced the battery was the issue. To prove my point as soon as I got home the alarm went off twice. I went back to the dealer. Now he wants to check the FOB and get a new one programmed for $228. The battery was $248. The dealer will charge me for the new FOB but if that does not fix the problem they will take it back for no charge. So does anyone else have a thought?
How could the FOB make the alarm sound unless it is near the car? Make sure it is too far away to work the door locks etc. If it still happens, it is not the FOB. Your dealer doesn't seem to know what they are doing. You should have a seoncd FOB though, it you loose one, it will cost a lot more to get the car going again. Even if you get a black one, that will not work with the SKS you will be OK if you loose the SKS one.
If you paid for the work, then demand the trouble codes. When you disconnected the 12V battery, that clears all but permanent brake DTCs. So any codes at all would have happened between your house and the trip to the dealer, making them highly informative. Seems like you can test the FOB theory by removing its battery and seeing if the alarm goes off. You could ask them to turn the "warn by horn" function off rather than buying a new FOB. High resistance between the battery negative post and chassis ground can also cause this. The connection at the post ought to be tight, since you have been in there as well as a technician, but how about the nut that connects the cable to the rear quarter panel?
I am sceptical about the FOB issue. I wasn't aware the warn by horn function could be turned off. If that is the case I will do that provided to FOB does not work. I will have them check the resistance on the ground although the connections are tight. Thank you.
So FOB was not the issue. Alarm still goes off. I have noticed my MPG's have really dropped the last couple of weeks. I had the inverter coolant pump replaced a few weeks ago. Is it possible I am having a traction battery issue that is draining the 12V causing the alarm to go off. Would that account for the lower than expected 10+mpg drop? If it cannot be fixed at this dealer it is off to another city.
I understand that you have just installed a new 12V battery. Does the alarm have a glass-breakage sensor or some other sensor that is intended to detect body movement? If yes then I suggest that you disconnect those sensors and see whether you still have false-alarms. A traction battery issue would not drain the 12V battery but a bad DC/DC converter in the inverter would. However it does not seem that this is a problem. Certainly, a weak traction battery might cause worsened mpg. Your OP says you measured 14.4V when the car is READY (which seems high, BTW. I would expect voltage to be ~13.8V.)
Patrick, Yeah those initial voltages were correct. The dealer said that the alarm could go off if the battery voltage measured too high (around 14.4V) or too low (around 11.8V). There was a code triggered that the dealer said indicated that was the case. This is the Toyota theft deterent system which does have a window breakage alarm. I enabled the system and beat the literal crap out of the car and could not make the car alarm go off. So last night with a new FOB (I took the battery out of the old one) same issue. Car alarm went off 3 times over an hour period of time. I disconnected the 12V so that it would not disturb the neighbors overnight. I went to the dealer today and at least had the 'horn on alert' feature disabled. Patrick, it has been very warm here for the last 3 weeks, with the average temp 95-100F during the day. I was thinking the sensor might have some heat issues but that doesn't make sense since the alarm has also been going off during the night. I am thinking it is time for a trade. Any thoughts on your side?
I had the dealer turn the 'warn-by-horn' function off today. Thank you. I didn't know that feature could be turned off. It will make the neighbors a little happier and keep me from having to disconnect the 12V at night.
Hmm, this is quite strange. You've gotten lots of service from the car and if you feel like trading it in, that's up to you - but if you want to keep driving it, I would suggest that you remove the alarm and see if any other issues happen. Since the car is 7 years old and has logged almost 200K miles, hopefully it would not be a ripe target for theft or other mischief. The unusual 12V bus voltages are also of interest, though. This implies that the DC/DC converter is not properly regulating voltage on the 12V bus when the Prius is READY. Continued long-term charging at 14.4V may shorten the life of the AGM 12V battery. However, an excessively high 12V bus voltage when the Prius is READY should not matter when the car has become IG-OFF and the alarm is armed. Good luck.
Can you give more information on how the dealer disabled the "horn on alert" feature? I called 2 dealers today and neither had ever heard of this. One said they wouldn't do it since it's a safety feature and they would "fix" the problem instead of disabling the horn alert (i.e. take lots of my $$$ before telling me they don't know what's wrong).
