Hello all. I Recently went to Autozone to have my 12v battery tested on my 09 Tahoe hybrid and they concluded the alternator was bad ( diode failing) my question is are the gas Tahoe alternators interchangeable with hybrid models. The guy at autozone stated he couldn't find the correct part in his computer. He mentioned the hybrid alternators are different due to charging an additional 18v battery.
Thanks all In advance!

 

yeah. nothing he told you is correct. you don't have an alternator of any kind and there's no 2nd 18v battery. he just made all that up.


if you look on the left side under the hood there's a big black cover. under there is an inverter. it takes the 300 volts from the hybrid battery that's under your rear seat and outputs 300v for the electric motors in the transmission that move the truck, (and charge the hybrid battery under braking) 300v for the electric ac compressor. 42v for the power steering and 12v for all the rest of the standard 12v powered stuff, like the engine ecm, headlights, radio. all the normal truck stuff you have. it also charges the 12v battery. there's no traditional starter motor on these either. so your 12v battery is used a little different than the non hybrid truck.

what as the reason you took it to be tested? the for them so say the alt was bad, I'm guessing they ran a test with the truck running. that won't tell you anything on these and no really much on a standard truck either. all those little hand held are good for is where you disconnect the battery, hook up the tester and let it run a load test. even then they are the most accurate thing, but if you have a battery that says it's 800cca and it's tests at 200 you know it's close to being considered a weak battery. a weak 12v battery can cause all kinds of electrical Gremlins, so it's usually a good idea to replace on a maintenance schedule not just wait till it dies like in a normal car.. how long that is depends on tons of thing like climate, type of usage, if it's been abused, like say key was left on and it ran the battery very low. they don't like that and it takes a lot of life out of them quickly. quality of the battery you got, things like that. but usually somewhere around 3-4 years is a good life out of today's car battery.
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About a week ago the vehicle had trouble going in reverse, the gears felt like they didnt want to shift. Immediatly following that and after shutting the car off it wouldn't start (no crank) but all the lights and radio etc worked. It's also worth noting the last few weeks the temperature gauge as well as the oil pressure gauge were both reading on the high side but only during short trips of 10 miles or less. Oddly enough Anything over 10 miles the gauges read normal. Oil, transmission fluid and coolant all filled to appropriate levels.

 

I've not dealt with that personally and haven't seen any posts about high gauge reading. not sure if it means anything or not. might be a sign of bad sensor or grounding issues, might not. you could put a mechanical oil pressure gauge on it to confirm if it's real or not.

only thing I can think of off the top of my head is if it does it again. pay attention to the little orange lights on the dash that indicate what gear you're in. those are lit up by the transmission module directly not the shifter. if they aren't lit, or are our it sync. it won't crank till it sees it's in park or neutral. these have lots of safety checks they go thru before spinning the engine over to start. they gotta make sure the 300 volts is contained and not anywhere it shouldn't be.

with all that said, especially if you're up north in the salt. wiring harnesses issues end up being what puts 90% of these in the junk yard. there's a few spots they are known to rub thru and short things out. as well as all these Gen gm suv have ground and starter wire commonly go bad, they tend to get eat up from the inside my corrosion. So they can look good but still not be. grounds are very common to be issues on the message boards too. as well as these quick release battery terminals not being as tight as they need to be.

intermittent issues are always the hardest to find. fingers crossed it was a onetime thing and doesn't start doing it randomly.

 

I'll start with checking the grounds/shorts and good recommendation on the shifter I will keep an eye out for the display to show if it's indeed lighting up the correct gear or not. I am in florida approx 3 miles from the ocean so it could be salt corrosion on the wires etc

 

there's a tsb somewhere showing where to check for wiring rubbing. but basically there's a big plug on the drivers side rear of the transmission near the pan. the harness there can rub on a exhaust heat shield that's right next to it. and under the hood. if you know where the non hybrid alternator normally sits, on the top drivers side front of the engine. on these they run a main harness thru that area and it clips to a metal bracket. I've heard it can rub thru on the bracket and short out wires. somewhere along the run I'd guess about 2011 they changed that bracket out with something else. so it's not as much of an issue.


just for info, I believe your year didn't go into auto stop in reverse, it started the engine any time you shifted into reverse. but newer ones would. but there's no actual reverse gear in the transmission. it's just the electric motors turning backwards. Just something to keep in mind. if it's like a really big hill you're backing up, some guys backing up trailers have had to use 4lo for the lower gear ratio.

on a side note, if you plan to keep it for a while. I always tell people one of the China clone tech 2's for around 300$ is a very worth while investment. the will run all the dealer level scanner tests and codes. it's very hard to find a shop that can fix this is the break.

 

Good to know, you probably saved me an hour of fumbling around under there tracing wires etc. Would shorted out wires be something I could check with a multi meter in this scenario ?

 

you could try but you'll have to know what you expect on the wire you're testing to know if it's open, shorted to ground or battery pos or another wire. I'm not really that good at it. so not sure.
​​​​​​here's one of the tsb about harnesses. I don't know if this is hybrid specific or all these gen suv's but that's the idea.

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The TSB references both hybrid and non-hybrid models.

 

Great. Thank you for sending that over. If I can't self diagnose I'll be forced to find a garage that knows their way around a hybrid which in itself is a challenge. I'll be sure to post a reply if I get a diagnosis for other people to see.