Intermittent starting problem 2002 Prius

Hi all,

Greetings from the proud owner of a 2002 Prius. We bought it about a month ago, and it seems to be in great condition with 163k miles. We bought it from a mechanic who picked it up cheap at an auction with a bad HV battery. He had the battery repaired, and did some mechanical work himself before flipping it.

The only issue we've had is that it sporadically will not start. The dash lights up and everything seems normal, but the READY light doesn't appear. The first time it happened we assumed we simply didn't know the secret trick to start our strange new vehicle. After several tries it started up and ran great so we figured it was our fault.

That night it happened again. We had vacuumed it earlier that day, and left the key on ACC with the radio on for 30 minutes. I didn't think that would drain the battery, but the PO had mentioned that he didn't actually drive much so I thought it could have already been low. The battery tested at 12.1 and dropped to 11.8 when the key was turned to START. I trickle charged it overnight, but it still wouldn't start in the morning.

The other suspect was the key. We had it duplicated, not realizing the chip inside was required to start the car. We tried the duplicate key first (didn't work), but then tried the original and that also didn't work. That afternoon I tried it again (with the original key) and it started fine and ran normally. The problem disappeared for a few weeks.

Two days ago we drove 400 miles to a wedding. We stopped for a break 50 miles from our destination. When I got back in the car it wouldn't start. Once again the READY light just didn't appear, everything else seemed normal.

I had an inverter with a digital display, which showed 12.1 at the power point. I hooked it directly to the battery and it read 12.5. Shortly afterwards I turned the key and our Prius started right up. We made it to the wedding on time, and had an uneventful trip home.

I want to fix this, but I'm new at Prius troubleshooting, plus I can't replicate the problem. Here are my thoughts, hopefully you can steer me in the right direction..

I know AGM batteries are a little different, but 12.5 is still low for a battery after 350 miles of charging. Will a weak battery cause intermittent failure to start? And if it is the battery, why doesn't charging help (at least temporarily)? This Prius has been modified to accept Miata batteries, in case it's relevant.

The only way the key could be involved is if the detection gizmo in the car intermittently failed to detect the chip in the key. Doesn't seem likely to me. We have two chipped keys. Neither works when this problem occurs.

My last suspect is that some sensor somewhere is malfunctioning, like a shifter position sensor or brake sensor. Do those ever cause trouble?

What else could do this? I assume if there were codes to read a dash light would come on?

Thanks for your time, I'm glad to join the community.

 

Good to know, Davestr.

I thought a weak battery might cause electrical chaos, but I was hesitant to jump to that conclusion since it doesn't actually turn over the ICE. I'll pull it out, check the water (if possible), charge it, load test it, and see if how it performs.

When I mentioned the weak battery to the PO he dismissed it, saying "that battery just runs the computer and closes relays.. The car would start with a 10v battery." Maybe that's why he put it in our car before he sold it to us.

 

No, the dash lights don't dim. At least not that I noticed.

This weekend I'll test the battery to see what voltage drop I get at different amperages. If I drain the battery a bit I might be able to recreate the problem too. It could also be that the conversion to accept the miata battery resulted in higher resistance.

According to this site (Prius Startup Current) the load on startup is usually 30A or less, with a highest recorded at 60A. Neat article.

Thanks everyone for your input, it's great to see this is such an active forum!

 

Turns out, that battery is young. From the sticker it looks like it was purchased the month before we bought the car, and manufactured a year before that. So I'm going to try pepping it up before replacing it.

After a few hours charging at 2 amps it was at 12.5 volts. I applied an 8 amp load and it dropped to 11.6. I charged it at 2 Amps for about another 16 hours. It's at 12.7 now (after removing the surface charge).

I'm going to do a bit more research on testing AGM batteries and I bet the auto parts store can do a more definitive load test. Unless you all have ideas..

 

Here's the conversion setup for the battery.. There wasn't any resistance in the terminals or negative to ground connection.

 

Alright, I came to my senses and ordered a battery to pick up tomorrow. I went with a Deka miata battery. Bit pricey, but it has a 3 year warranty.

I just hate giving up on batteries. I think I'll hang on to this one to use in another project. We are building a tiny house on a flatbed trailer, and I have a uninterruptable power supply that could use this AGM to keep the lights on. Long term I'd like a good battery for that purpose too, but until I design the solar setup (and know our energy needs) I don't want to buy a dedicated deep cycle battery.

It will be good to have a battery I trust in our #1 car (the #2 car is a true #2 car in all senses, and is barely road worthy).

Thanks everyone for your help. I'll certainly post here if the problem persists, though I doubt it will.

 

Update: The car has been starting great since I put in the new battery, seems that was the culprit.

Anyone with similar problems would be wise to check their 12 volt.

Thanks to everyone who weighed in.