TMCC Headlight voltage?

If this shorted closed then the short was not extended.  Look at the triacs itself, just a small change in the appearance, maybe a small blister or discoloration.  You probably will not see obvious damage to the board.  Unless you really smoked it.:-)

Now I would go back to what Bill said, did you reset the R2LC AUX code.  Steam engines leave the front light on even when in reverse.  A diesel code make the lights directional.

As this started, the problem was the headlight staying on.  With another R2LC you had light control, but were concerned that all the lights were flashing, correct?

G

I think I'd still do some tracing and testing outside the R2LC to make sure there is no shorts along the way to the light.  It's pretty rare for a triac to go unless it gets overloaded.

The triac is the small can like transistor.  If you can fine the one that controls your offending light and use the diode tester on the Volt meter check between the two outside pins. It should block and read infinity.  If it reads very low, the triac is shorted.  Do a google search on testing Triac and you can find instructions on how to test.  Since we believe this is a short, it would read that way.  I really think this is just a shorted TRIAC.  It can happen more easily than though specially if a bulb or bulb wire have a temporary short.  As far as your good one not changing lights is the AUX code set correctly for your CHessie.  G

Here's the mapping for the triac's on the R2LC, you can isolate the rear light one and be sure you're testing the right one.  You can also compare it to the front light to see if they're different.

Well you guys solved the riddle.

I checked the rear headlamp triac outer posts with my Fluke 16 on diode check and it read shorted.  I tested in both directions with same results.  None of the other triacs indicated shorted but instead read open.  We have our smoking gun.

Thanks everyone!

Now I guess I will be ordering a replacement R2LC board. 

Hi All,

I know this is a somewhat older thread but I found a nice description of TRIACS and SCRs from Allen Bradley and ways to test them.  The link is to a PDF which you can save to your computer by simply clicking FILE/SAVE AS in Internet Explorer or just right click the page and print it.

Here's the link:

http://www.ab.com/support/abdr...entation/fb/1012.pdf

Hope this is useful for everyone in the future!

Best,

Dave

You need a triac, what you're seeing on Radio Shack in the TO-92 package could be anything.  Thousands of parts are made in the same form-factor.  The ones that read NPN are doubtless transistors and not triacs.

I use the MAC97A8 as a replacement for that part.

The old board can be fixed, it's not a difficult repair for someone with the tools.