• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

TR4/4A TR4 Overdrive Switches--Top Cover

Share this page

KVH

KVH

Darth Vader
Silver
Country flag
Offline
Assuming I have the two pairs for each switch marked correctly (for each pair there is a positive and negative), does it make any difference which wire goes to which side of the switch?

I can't see any designation on the switch, and the manual gives no clue.

Thanks
 
Nope, makes no difference.
 
Randall, how do we ever return the favor? You've just about rebuilt my car for me. I'll just keep asking questions and hope I can share more of what I know one day.
 
Follow the wiring diagrahm. There is not a positive and negative at each switch. You are only controling a ground to the relay via the top cover switches.
 
JerryVV said:
Follow the wiring diagrahm. There is not a positive and negative at each switch. You are only controling a ground to the relay via the top cover switches.
Click to expand...

That daisy chain of switches:

OD%20Wiring.JPG


I added an indicator light:

OD%20Wiring%202.JPG


I also added a diode to prolong relay life per you-can-guess-who's suggestion:

OD%20Diode.JPG


This is the diode I used:

ODDiode-1.jpg


Note -- these images are for POSITIVE GROUND. The diode in particular would have to be reversed for negative ground.
 
I don't have any mechanics or engineers in the family. I'm it, and I owe a lot to everyone on this Forum.

I need to find a good article about auto electrics. I'd enjoy reading it.

So, what do those OD Cover switches do when you drag the shifter from 4th to 3rd? Or over to 1st or 2nd?

How do they correlate to the workings of the solenoid?

What's happening when you're in neutral?
 
Dan Masters "Electrical Maintenance Handbook for TR 250 & 6" from the usual suspects is a great resource.
 
Geo Hahn said:
I also added a diode to prolong relay life per you-can-guess-who's suggestion:
Click to expand...

What is the purpose of the diode?
 
When you go through neutral, both switch plungers drop down, so both switches are open and the solenoid is disengaged. There is a sentence in some manual (perhaps the A-type service manual, don't recall offhand) that talks about the timing of that event being important relative to the synchro operation, which is why you are supposed to shim the switches. (The shafts have a ramp where the switch plungers ride, so the height of the switch affects the timing of the switch closure relative to the motion of the shaft.)

My original OD relay on TS39781LO lasted a long, long time; but I was having trouble with replacements becoming erratic within a year or so of installation (including a fairly expensive reproduction relay). So I added a diode to act as a snubber, to hopefully keep the kickback voltage from the solenoid from causing an arc at the relay contacts as they open. The result seems to have worked out very well, even with a cheap relay (under $3 in bulk).

I just grabbed a diode from the parts bin, but I believe it was rated for around 1 amp and 50 volts. Anyway, I think any diode with those ratings (or higher) would work fine. The one Geo shows above would work, or here's another example:
https://www.radioshack.com/product/index.jsp?productId=2036270
 
Back
Top