I have a transformer that converts 120v AC to 24v DC.
Can I use the 24v output to jumpstart a 12v truck safely and without damaging the battery?
No.
No. You would probably cause severe damage to the truck AND blow up the battery in a matter of seconds.
In the old days, we would jump start a vehicle with a 6 volt system from a 12 volt source with no problems. I started my 1950 Chevrolet pick-up this way several times. However, we don’t live in the “good old days”. With the solid state circuitry and processors, you will likely damage something. DON’T DO IT.
Your 24 volt device will be instantly overloaded and something will be damaged…
I have jump-started a lawnmower with a car, which is using a 12 volt source to jump a 6 volt system, but I was told to do this you had to be very quick about it (as in, turn the key on the mower the instant the jump circuit is completed) or the 6 volt battery would probably overheat or explode in short order. Either way, it’s not a very good idea, and the OP’s idea is probably worse. I wouldn’t use a 24 volt source to try to jump-start a 12 volt system. It’s too potentially dangerous in my opinion, and this is coming from somebody who uses ether and a torch to get stubborn tire beads to seat to the rim.
The starter may survive a quick jump but the electronics may not.
I went the other way–I would start my 6 volt Chevy truck from my 12 volt lawn tractor. The lawn tractor had a 12 volt car battery. I remember service stations in the late 1950’s and early 1960’s would jump start the older 6 volt vehicles from a 12 volt source.
One really had to watch the polarity because the Ford and Chrysler 6 volt cars as well has some independents had a positive ground.
Thanks to everyone for the quick answers. I didn’t think about the electronics, but they would certainly be at risk.
Are you sure your 24V transformer converts 12vac into 24vdc ? unless you have a diode rectifer on the transformer you will have 24 VAC coming out the other end. Your starter will draw anywhere from 150 amps DC+++ to turn the engine over depending on engine size.
My 59 T-Bird draws about 250 amps to crank the engine with a 430 cu V8 on a warm day
Not only is this a bad idea, but how big is the transformer we’re talking about? If it’s any smaller than a cinder block, you’re likely going to overload it beyond belief and cook it as well as any devices in the car that can’t handle the higher voltage being supplied. (albeit possibly briefly)
I don’t know for sure what the effects of 24 VDC would be on 12V electronics but I know for sure what higher voltages do them.
One of the “tools” in one of my toolboxes is a household extension cord that has been shortened and fitted with a pair of alligator clips.
As per the Subaru factory rep at one time, if any electronic parts are replaced under warranty make sure they are bad before submitting them under a warranty claim.
So what better way to remove all doubt than to connect the clips to the part in question and plug it into a standard 110V wall socket. 
I can understand the CYA motivation for insuring any replaced parts are definitely bad when they get back to the manufacturer but have you considered how that may sabotage efforts to collect meaningful failure data? Especially during warranty periods, manufacturers are typically very interested in examining infant mortality failures to understand if there is a deficiency in the part or way it’s being used. Imagine the reliability engineer who is baffled by the complete destruction of the parts you returned. Even NPF parts that come back from the field can provide valuable information about potential deficiencies in the diagnostic tools and procedures provided to the field service people. Assuming shotgunning isn’t the mechanic’s primary mode of operation- which I know is not the case here! Just another perspective on the suggested practice…
We used this same practice back in the 70’s when I worked in a Chrysler-Plymouth dealership. Chrysler began using solid state voltage regulators, and many of them would intermittently fail. When we sent them back under warranty, if they “happened to work” when Chrysler’s warranty center ran them through a test, then both the dealer and we mechanics would not get paid.
We tried to resolve this issue through Chrysler’s regional tech representative. He understood our concern but was unable to help us.
We learned quickly that the only way to get good working regulators in customer’s cars and to get paid for it was to “cook” the regulators with 110v before shipping them back to Chrysler.
I agree it’s critical for manufactures to be able to collect meaningful failure data, but the process to make that happen takes both parties. If one won’t play, the other is forced to resort to survival skills.
I had planned to add the rectifier, but I can’t get anywhere near the amperage you mention. The best I could find (in my price range) was 35 amps – no where near enough.
A 35 ampere 120 volt to 24 volt transformer had to be rather expensive. Back in the 1950’s when I was in high school, I purchased two aircraft receivers for $2 apiece. For each stage in the receiver, there were duplicate tubes. Across stages, pairs of tubes were wired in series (the tubes were 12 volt tubes). Aircraft electrical systems were 24 volt at that time. As I remember, when I did locate a 120 to 24 volt transformer to power the tube filaments, it was rather expensive.
For what a large enough transformer to do this job would cost, along with a huge rectifier, you could buy a good battery charger with the high amperage “boost” feature. However, I admire your spirit in trying to build something to do the job.
Joe Mario is quite correct.
In the case of Subaru two things should be kept in mind; one is that the Subaru factory rep said to do this and two is that few if any warrantable parts were ever analyzed to begin with.
Regarding the latter, all defecive warranty parts were tagged and held for the factory rep’s examination. Reps for the most part do not have great mechanical skills and their analysis consisted of nothing more than eyeballing the part to make sure it was physically present followed by consigning it to the dumpster. These parts were never sent off for testing by engineers, etc.
Shotgunning is distasteful but often there simply is no othe way of doing it. This is very true of electronics (especially with intermittents) and the more complicated cars became the more this is done. Even the car manufacturers state in the service manuals this may be necessary.
You can’t jumpstart the truck with this “transformer”, but if you have a 24 volt rectifier and another 12 volt battery you could charge the dead battery.
You’d have to connect the dead battery in series to the second battery. That is: connect the positive terminal of one battery to the negative terminal of the other.
DON’T connect the other terminals together or you’ll have one 24 volt battery short-circuited end-to-end.
You’d then connect your 24 volt charger to the unconnected terminals of your now-24-volt-battery and charge the whole thing. Note that this will not provide enough current to jumpstart anything, but it will charge the dead battery if you’re set on using your transformer to do it.
Personally, I’d sell the 24 volt transformer and use the money to buy a heavy duty charger capable of jumpstarting the truck.
That’s a clever way to use my transformer, but a store-bought charger with a charge control feature is safer and not that expensive.
I bought my transformer for $5 at a Habitat for Humanity store, and could get the rectifier for $4.50 from AllElectronics. It would have been nice to have a 110v jump starter for $10, but I’ve abandoned the idea based on what I’ve learned from this discussion.
I’ll find something else to do with the transformer, or sell its copper for scrap.
This could be dangerous with bits of molten metal flying through the air.
I hope you wore eye protection or at least put the victim/part behind a barrier.