If the main relay is same as I had looked at in a newer Civic, the
main relay is how a tiny transistor inside electronic controller turns
on and off power to major engine functions. I noted this newer Civic
would not trip its relay when the car was used in hot weather and after
running for a while. If the transistor is not quite large enough, then
it will not output enough current to trip the relay. Once the relay is
tripped, less current will hold the relay closed. The main relay
would fail to trip often after the car was only shut off for a few
minutes or less - when the transistor was warmest and therefore may
have been outputting less current.

This transistor that drives the main relay is inside the controller -
would be same (marginal) by design.

Unfortunately the car did not remain long enough for me to prove this
analysis by putting an 'amplifier' between that transistor (inside
controller); thereby eliminating the problem. I do know for a fact
that the main relay would not trip during warmer temperatures AND when
the car was running only seconds perviously. After sufficient time for
electronics to cool (anywhere from 5 seconds to less than a minute),
then the main relay would trip - and car would start.

Jonathan Upright wrote:
> Car: 1990 Honda Civic LX, 5-speed, 1.5L SOHC DPFI.
>
> Question: Can anyone explain, or point me to a site that explains, the
> exact functions the main relay is responsible for?
>
> Here is the reason why I ask. My main relay is screwed up, because it
> will not start when it's hot. I have to reach down and squeeze the crap
> out of it while pressing the clutch and turning the key for my engine to
> fire. (Yes, I have unbolted the relay from its original position and
> have it dangling beneath my dash. Otherwise, I couldn't drive my car
> anywhere) Once I do get the car to start, while it does run, it
> sputters when idling. (The "rhythm" of the sputter is quite irregular)
> ...