does dei 610t relay have coil diodes?

The only way I can think of that you can test to see if there is a diode would be to get a 1/2 amp fuse put it inline with the power wire, and connect the coil backwards.  If it blows the fuse, there is a diode.  If it does not blow the fuse, there is no diode.

Yes you can use a smaller fuse.  A 500MA fuse is just easier to find than a 250. 

You could add a series resistor of (say) ~47 to 820 Ohms and connect both ways across 12V. Measure the voltage across the solenoid. If it is greater than ~0.7V (and the same) in both polarities, there is no diode.

For the cost of a diode, why not play safe and install externally.

Which could be placed at the whatever (alarm etc) outputs.... Of course quality equipment should have such protection... (Speaking of quality - Ford Aust have just recalled some models after a cruise control took over - the driver could not turn off ignition, change gear, etc. That's what I call a BAD design. People wonder why I don't have any automated controls without overrides.... - eg, ABS, wipers, etc)

Bingo... oldspark ! I think your method is 100% correct. Basically create a voltage divider and check to see how the +12v source gets divide up. The forward biased diode should not exceed approx 0.7v across it give the exponential VI profile for a diode. I will test this method tonight but I am sure it will work! Simple concept and better than the fuse because its non-distructive to all elements under test.

smoketest wrote: ...(create) a voltage divider

Dang! That's the tech-word for it! Talk about knowing concepts but not (caring about?) the detail! Maybe next time I'll say "try a voltage divider". (That'll keep anti-verbose experts & the knowing happy, and the others status quo.) When suggesting resistor values I pondered "what is the current rating of an in-build diode"? I doubt that it would be even 1A?? FYI - I only ever use "raw" relays. If I ever had a diode protected relay, I'd probably try to blow it open or remove it. And that reminds me....

Dang #2 - I was going to re-reply with a specific "Topic" heading. Alas this forum doesn't have that..... Anyhow, it was going to be "A Brief about Relays and Circuit Transients" (with optional <Ramble> after Brief).    It was spawned by this thread, my avoidance of relays with inbuilt protection, and something I read somewhere recently - maybe on 12Volt...? The issue had to do with NOT having a protection diode (or quenching circuit) across a particular relay. The comment was something like "although that relay may be ok, I don't know about the effect on other relays...." etc. The simple answer is all the relays (solenoids) will be subjected to the same voltage spikes etc. The way it was written, it made it sound as if the unprotected relay could inject a spike into another relay and wreck it, or trigger it etc. No. Even if they are in parallel, they both see the same spike and react the same. A 200V spike from the 1st relay has that same effect on itself as it does on the 2nd relay etc. Not that they would be in parallel, but that is probably the worst case. If they are in series, then the other relays will fight (suppress!) the spike caused by the first. But other relays are usually in some semi-parallel arrangement with each other (eg, common ground; various solenoid voltage sources). The general reason for quenching is certainly NOT the protection of other solenoids (inductors), nor other circuits, but noise suppression (EFI/EMC) where the philosophy is quench as close to the source as possible.    IMO it is not for circuit protection as any relay switching circuit should have its own protection. And any equipment that is intended for vehicle operation should have its own protection for outside of the common 8-16V operational range, and the common 200V or higher spikes expected in a vehicle's environment. If not for EMI/EMC, relays would be unprotected and a single diode or snubber placed across equipment inputs/outputs are required. Oh well, what started as a correction to an incorrect "but logical" circuit analysis is now getting TEMPESTuous. (That relates to EMC...) At least is is brief.... I could have gone into detail....