Is it illegal to use "fake" signature?

If the signature was written by you rather than by someone else, then you had signed the document. If you were in court over a debt and you denied that you had signed a document when in fact you had done so, (whether or not the signature you had written on the document resembled other examples), then you would be committing perjury (telling lies under the witnesses oath), for which you would be likely to be jailed.

Altering your signature is not illegal. Lying under oath in order to avoid a debt is illegal. In any event, it's unlikely that it would work. You received statements for the loan at your home address? The money went into your bank account? You made payments from your bank account? Chances are that a court would believe it's your loan even if you claimed that the signature was not yours.

It depends on the circumstance.

If you are pretending to be someone else by using a fake driver's license, passport or social security card - and then sign a document using a fake signature, that would be identity theft.

Altering your own signature isn't illegal.

In the USA, under UCC regulating signature on things like contracts and financial transactions including loans, credit cards, leases, etc., you can legally use any symbol you want as your "signature". In fact, a "forgery" of someone else's signature, if you used it for signing, is actually "your signature", because that happens to be the symbol you have adopted to indicate your assent to or authenticity of the document,

Also, your "signature" can be a string of computer text, such as "/signed bcnu/" or whatever you might use to identify yourself as the source of a text or email. Under the laws of all 50 states, the authenticity of your signature, and the document you signed, cannot be challenged merely because they are in "electronic form", barring some specific regulation requiring an original copy, signed ink, notarized, or whatever.

Your "alternate" signatures are not "fake" -- they are just different. Any attempt to disavow your signature, merely because it doesn't match the others, could be evidence of forgery, fraud, check kiting, or other crimes. You can repudiate signatures you did not actually make --- not those you made using different symbols.

I know a guy whose entire signature is a drawing of a fish (loop, crossover at one end, dotted with an eye). His name is Fish. That's his legal signature.

It's called FRAUD. It wouldn't be that hard to determine that you benefited from it, but now refusing to accept responsibility.

*EDIT* - When you sign, you sign. There is no "fake" signature. But by your description, you want to change your signature for loans and such so you can later turn around and say you never signed it. That's where the fraud comes in. You intentionally doing it to benefit yourself while stealing form someone else.

You signed it. It's your signature, no matter WHAT it looks like. Trying to use a "fake" signature to get out of paying loans is just plain STUPID and COULD end up sending you to court.

Of course it is illegal if you are trying to scam people, although someone who is stupid enough to think that a signature is the only identifying information about you on applying for a loan, explain away your address, bank account etc that you also had to prove

Not illegal, you just wouldn't get out of it. You saying it's your fake signature is you admitting you signed it.

Yes, that's illegal. It's no different than forging someone else's signature. You should only have one legal signature.

Of course you do realize that when you sign papers for a major purchase like an automobile the business usually has a camera aimed right at you, so they have you on camera signing the note.

Burn enough lending agencies like that and you will find there are darn few left that will lend you money for any reason.