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TR2/3/3A How do I test overdrive relay diode jumper?

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TuffTR250

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In a forum thread from 2013 that I found in a Search, Randall suggested making a diode jumper to help protect overdrive relay. I made one with a IN4004 diode at end of the jumper toward the 87 terminal of the overdrive relay. I would like to test the jumper to make sure all my solder/crimp joints are good. When I tried to do a continuity test on the jumper, I got no reading. I don't think I have bad solder joints, but how do I test it to make sure?

Here is Randall's recommendation from the 2013 forum entry:
Your diagram looks good to me; but I'll throw in one more thing. In my experience, although the original relays would stand up to the inductive kickback from the solenoid, modern ones won't. Once the original relay failed on TS39781LO, I went through several replacements (including an expensive reproduction) in just a few months. The cure I found was to add a diode to help absorb the kickback and keep the relay contacts from arcing each time. That was in about 1995 and I'm still using the same cheap junk relay today. Any of the 1N400X series diodes will work fine. (Radio Shack sells a 25 pack "assortment" of 1N400X diodes for $3.50, and I used whatever fell out of the package.) Connect the diode from the load terminal on the relay (87) in your diagram to ground, being sure to match the diode polarity to be opposite of your batter polarity. The diode will have a white stripe on one end, which is the cathode. If your car is the original positive ground, the white stripe goes towards ground. If your car has been converted to negative ground, the white stripe goes towards the relay terminal.
Thanks for the help!
Regards, Bob
 
Did you try your continuity test both ways? What were you using for your continuity test?

Some meters put out enough voltage to overcome the diode drop, some don't. Also, some offer a "diode test" function that puts out a bit more voltage, again to overcome the diode drop. (A typical silicon diode, like the 1N400x series, will not conduct until the anode is roughly 1 volt more positive than the cathode. This voltage is what I call "diode drop", the data sheets call it "forward voltage" or "Vf".)

If your meter won't do it, and you don't have a powered test light type continuity tester (which almost always uses more than 1 volt); then you'll have to rig something up. Possibly the easiest thing would be to disconnect the diode lead from ground and connect the solenoid lead to the solenoid. Then use a voltmeter or unpowered test light to check between the disconnected diode lead and your battery hot terminal (with the battery ground hooked up). If the light lights, or the voltmeter reads a bit less than battery voltage, the diode is conducting.

Hopefully that's clear; I'm not quite awake yet :smile:
 
Great!! Thanks Randall! I did not know it, but checking the manual for my Harbor Freight Cen-Tech multi-tester it had directions on how to test a diode. i.e. set dial to >+ and connect red to anode end of diode and connect black to cathode end (which is end with stripe on these diodes). The meter will show the approximate "Forward voltage" drop of the diode. Connecting my jumper as the manual says I get a reading of .596. So it appears that the diode is functioning and my jumper has the required continuity. Thanks again!!
p.s. I tested a single IN4004 diode out of the Radio Shack bag and got a reading of .604.
Regards, Bob
 
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