According to FAR Part 61, flight instructors who hold an "instrument-airplane" rating only on their flight instructor certificate are authorized to give instrument flight instruction in single and/or multiengine airplanes for instrument certification, provided they hold single and/or multiengine ratings on their pilot certificate."
And it also says:
"15. (C) Ratings Limited to Instrument.
Instructors with ratings limited to instrument may not give instrument flight instruction to students who do not hold category and class ratings in the aircraft used, since this would be instruction for the addition of a rating that conveys other than instrument privileges. These instructors may not certify logbooks or recomment applicants for any aircraft category or class rating."
The way I read these is that you can provide instrument instruction in *any* aircraft for which you hold category/class/type as long as the student *also* holds category/class/type. So you couldn't give instrument multi to someone who holds only ASEL, but you could give instrument multi to someone who is adding an instrument rating to their multi private/commercial certificate.
Most (if not all) people get their instrument rating in a single *after* getting their private single, so someone with only a CFII could instruct that person for their instrument rating.
I would tend to agree with Frascaflyer. What you would not be "qualified" to do is give instrument instruction while teaching or simulating multi-engine procedures (ie simulated engine failure on the approach).
Good threads there. But I still maintain that you can give instrument instruction in a multi-engine airplane holding only a CFII and a Commercial Multi/Instrument *provided* the student has a private or commercial multi. Instrument instruction includes all and only the tasks set forth in the PTS for an instrument rating - and that includes single engine approaches. Is it smart to instruct in a multi with an engine shut down if you don't have an MEI? Probably not. Will an insurance company let you? Almost definitely not. But is it *legal*? I would have to say yes.
In those other threads there are two FAQs where John Lynch offers a concurring opinion, and one legal interpretation that seems to suggest that you need category and class ratings in order to give any flight instruction at all. If you look at the dates, the legal interpretation was issued in January 2004, and the FAQ answer (in response to Jedi Nein's challenge) is dated the end of 2004. Per Doc's FAR forum, and taking Jedi Nein at her word (http://www.propilot.com/doc/bbs/msgs//11190.html ), the legal interpretation is being changed to conform to John Lynch's FAQ ruling.
While I don't forsee a situation where I would give multi instrument instruction without an MEI, I believe that the FAA interpretations make it *legal*. And I definitely believe that I could train an applicant for a single-engine instrument rating (after their private) holding only a CFII. Think of it this way: if the student holds category and class ratings, either single or multi, presumably they are familiar and competant in all the areas of operation required for the practical tests in those areas. With instrument instruction, the new knowledge that you are imparting relates solely to things that they would not know - hence you can pull an engine in instrument training because they *know* how to deal with an engine failure, but not necessarily how to fly an approach with one inoperative. You're not teaching them how to deal with an engine failure; you are merely teaching them how to fly *instruments* with one shut down.
FF, I have, for many,many years, thought of the II in twins the same as you.
However, 61.195(b) says:"A flight instructor may not conduct flight training in any aircraft for which the flight instructor does not hold:
(1) A pilot certificate and flight instructor certificate with the applicable category and class ratings."
I don't know how to get around that. All I can go by is FAA legal interpretations and the FAA FAQ that deal with that particular FAR. Jedi Nein says that the inspector changed his mind and agreed with John Lynch, and the Eastern Region is bringing their interpretation in line with the FAQ. I agree, the regulation seems clear, but it appears that the FAA's interpretation is different. I'm inclined to think that their interpretation will stand, for reasons stated above - the things you teach in instrument flying have already been tested for the CFII. You aren't providing *instruction* in any of the elements that lead to an ASEL or AMEL rating.
But like Bushwick Bill said, it's all moot if you just get the CFI and MEI. That's the best course.