TR2/3/3A Wrong OD Relay?

[CJD]

Alright... I may have made (another) Ebay feaux peaux.

Several years ago I bought an "Overdrive Relay" off Ebay. The issue is the wiring diagram for the OD shows terminals C1 and C2. This relay has terminals C2 and C3, with no C1. I figured this was just a difference in labeling, but then I started trying to find an internal diagram for the relay.

I now believe that a relay with C1 and C2 terminals would be normally open...which is what you need for our Overdrive circuit. BUT, a relay with only C2 and C3 would be a normally CLOSED relay...that would leave the OD engaged until you flip the switch, and then it would disengage.

So, can anyone confirm...is a C2/C3 relay opposite, and unusable?

 

[TR3driver]

I'm afraid so. Here are some diagrams from a Lucas data sheet

 

[CJD]

Thanks, Randall. Well, glad I caught it before trying to back out of the garage!

Unfortunately the normally open relays are NLA. I'll have to see if the original can be brought back to life.

 

[TR3driver]

Hmm, might be worth double-checking that. I did a little poking around and came up with this https://www.holden.co.uk/displayproduct.asp?pCode=33135/SB40

The photo shows C2/C3 like you have, but the text clearly says "normally open". I suppose it's possible someone copied the wrong pattern when making the reproductions.

Just put an ohmmeter across C2 & C3. If you get continuity with no power on W1/2, it's normally closed.

 

[CJD]

I cracked open the case and here is what I found...



That looks open to me! So I apparently got the relay you linked to, and it is normally open. Go figure. It must be another case of that drifting British terminology!

Anyway, thanks again, Randall.

 

[TR3driver]

Just a thought, since you've already got it apart; you might want to add a suppression diode inside to help protect the contacts. Last time I used one of those repro relays for the OD, it only lasted about a year of daily driving (which may also be why none of the reputable suppliers list it as an OD relay). I suspect the original Lucas 33157 (which is specifically called out as an overdrive relay in the Lucas catalog) had hardened or special material contacts to better withstand the arcing from inductive kickback. At any rate, it seems like there must've been some reason the original lasted for many decades while the repro only lasted a year. Adding a diode made the el-cheapo replacement last a long time (it's still working over 20 years later).

Extra hassle to put it inside the case, but that way it wouldn't show from the outside.

 

[CJD]

Good idea! What amperage rating would you recommend?

 

[TR3driver]

I used a 1N4004, which ISTR is rated 1 amp average, 30 amps surge. But any of the 1N400x series would work fine, the last digit just indicates the voltage rating. I got an "assortment" from Radio Shack, that turned out to all be 1N4004. Looks like they don't carry the 25 pack any more, but there are lots of options on flea-bay
https://www.ebay.com/itm/25-x-1N400...095096?hash=item3cb99cbe78:g:ml0AAOxyFiRRzFEx

 

[DavidApp]

Or at All Electronics. Electrical parts store.

https://www.allelectronics.com/search/diode/results/2.html

 

[Geo Hahn]

Looks like TRS may still sell singles --

https://www.radioshack.com/collections/diodes-rectifiers/products/1n4004-micro-1-amp-diode

Of course the trick may be finding a Radio Shack.

The one I used was a 6amp which they may or may not still have in a 4-pack:

https://www.radioshack.com/products/4-rectifier-diodes

 

[CJD]

Thanks to all!

Now I just have to remember to think backwards for the positive ground when I install it.

 

[DavidApp]

Going to stay with Positive ground?

David

 

[sp53]

I bought an aftermarket relay from Moss that I have not tried yet, and you guys gave me a head ache, so please post the final results

 

[CJD]

Going to stay with Positive ground?

David

Yes. Every restoration has a goal, and this one is originality. It will never be driven that much...at least by me, so the dynamo will fit my needs. If I planned to drive it daily, I would go with the alternator conversion and negative ground.

 

[CJD]

I bought an aftermarket relay from Moss that I have not tried yet, and you guys gave me a head ache, so please post the final results

Sorry about that! Moss still has the relay, but with the newer style spade connectors, so if that is what you have you should be good.

 

[TR3driver]

Here's my final result. The relay came from a "fog light installation kit" from Chief Auto parts (who used to be open 24/7), all the other bits were just what I had on hand. Strictly quick and dirty (it was after 10pm and I wanted to drive the car to work in the morning), but it's just kept working all these years so I didn't change it even when moving the OD from my wrecked 3A to the current TR3. The diode is inside the black heat shrink that loops under the relay, from the output terminal to the mounting screw.

Just remember that you don't want the diode to conduct all the time, so the stripe has to point to the positive side. (It conducts when the stripe is negative.) Ground for positive ground, or the relay terminal for negative ground.

 

[Geo Hahn]

I fitted connectors on each end of the diode and attached it to C2 and a ground. Positive ground TR4: