As frenchie stated in his comment, a co-founder position can't start on a wage basis and then equities later. As a co-founder you have to take your part of the risks. And that starts at the beginning of the project. You can't say: "for the moment I don't take any risks, but later if things go well I'll do". It's far too easy. So you should have equities at the beginning. It's good for them because they'll know they'll have an involved associate with them. And it's good for you because you are sure to be in the board.
However, I doesn't mean necessarily not to be paid at all on a regular basis.
Let me explain taking my own situation: people who I work with know I can't spend months to develop a project without any personal revenue. I have a child, a flat to rent, and so on. My part of the investment is not in money, but in time, motivation and technical skills. Your potential associates should know that. If they don't, then explain that to them. If they still don't understand, forget them, they probably think the technical part is secondary — and they're wrong. They can be great marketers or businessmen, they won't do nothing without a good tech engineer. Besides that, investors invest money for people first. Just for that: being able to live while the project is not done. This is commonly accepted.
Another thing is, starting in a startup project requires to have people which we can rely on. You can't start developing a project, and in a few weeks/months say you want to stop. Your associates need somebody developing the entire first step of the project. Not a part of it, otherwise they will loose a bunch of time finding another engineer, that will loose a bunch of time trying to understand and continue your work. And that's bad for a startup. Very bad.
At last, you can set up any contracts you want, but everything has to be based on confidence. You have to trust your partners, and they have to trust you and rely on you at any time.
Hope this helps. Another frenchie.