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Overdrive Failure

Chet Zerlin

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Hi all!

Yesterday while driving my BN6 in overdrive it suddenly shifted out of overdrive and into third (regular) gear. From that point on the overdrive on/off switch had no effect on anything. When I got home I did the ignition on, engine off, in 4th gear and turn the overdrive switch on....no clicking of any type from the solenoid.

I'm thinking that since the overdrive just stopped working where just before it was working fine it might be as simple as one of the switches or relays failed and not the overdrive itself (at least that's what I'm hoping!). So for now I'm assuming that my problem is electrical. Reading through some prior posts (and online articles) the testing of the electrical system linked to the solenoid (after doing that simple "click" test that I did ) seems to assume that the solenoid is ok (i.e. each time it involves hearing or not hearing that clicking sound). But if the solenoid itself is bad seems to me that every test of those electrical components would result in a negative result even if they are all ok.

So should my first step be to see if the solenoid itself is ok and then tracing back? If so could someone detail the best (and easiest) way to test it?

On the other hand, it would certainly be easy to first check the continuity of the dash switch to see if it is working and then check the shift switch on the side of the gearbox to see if it is working. Or perhaps turn on the ignition, put the dash switch on, put it in 4th gear and see if there are 12 volts going to the solenoid. Thereafter I would appreciate some guidance on the easiest way to test the throttle switch and the overdrive relay.

What are the collective thoughts about where to begin?

All suggestions or advice are appreciated!

Thanks!
Chet
 
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Re: Overdrive stopped working - what to check first?

The shop manual has a comprehensive testing regimen for systematic, flow chart-type testing of the OD electricals--I diagnosed a similar problem in a hotel room once, and fixed the problem while my co-pilot did our laundry--but there are a couple quick checks you can do:

1) the dash-mounted switches--at least the newer ones--are pretty much junk (I've had a couple fail over the years, I now carry a spare). On a BN car, it should be pretty easy to jump the wires--in 3rd or 4th gear of course--or just install a new one (if the switch isn't the problem you now have a known-good spare), to see if the solenoid actuates

2) take the gearbox cover off and apply 12V directly to the solenoid with a proper ground; if it retracts then it's not the problem (if it doesn't, well, you've found the problem). You might be able to access the single wire to the solenoid--it's grounded to the engine--from underneath to save pulling the cover. You'll need a 12V source to test.

3) there is a relay in the circuit (probably because the initial current draw of the solenoid would let smoke out w/o it). In my experience; this causes intermittent operation; i.e. the OD will drop out, then can be reengaged after you cycle the dash switch (this is what I fixed in the hotel room; the relay can be carefully taken apart and the points filed and cleaned)

4) there is also a 'kickdown' switch, which is what keeps the OD engaged on downshifts until you blip the throttle (not used on other Brit cars with OD, I hear). It just has a cam with a set of points, which keeps current flowing until the throttles are opened-up a certain amount. I don't think this is your problem, as a problem with it would just cause the OD to disengage immediately when you throw the switch to downshift)

5) there is (yet) another switch that prevents OD engagement in 1st, 2nd and (most importantly) reverse. You'll need the gearbox cover off to test this--you might be able to reach it w/o pulling the cover--it should be an open circuit when not in 3rd or 4th, and should be a dead short when in those gears. This is a possibility, but I'd expect it to go intermittent first (but anything is possible)

If this problem started suddenly, it's likely an electrical issue; if it's gotten worse over time, it's likely a problem with the hydraulics. This can be established by testing the pressure at the operating valve (a plug with a 'nut' on top). If all the electrical tests fail, you'll have to address this possible cause--which might be fixable with a new pump or some other, simpler, remedies, otherwise it will probably require an OD rebuild--but try the simple/cheap fixes first.

Edit: I don't recall a fuse anywhere in the OD circuit--and don't have a schematic handy--but some owners install one. Obviously, if your car has one check that first.
 

John Turney

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The overdrive relay on the firewall is pretty easy to reach. When the panel switch is turned on, terminal W2 on the relay should energize. If the relay is working, terminal C2 should also energize. If those two things happen, the problem will be downstream, and you will have eliminated the power source to the relay, the panel switch and the relay. If those things don't happen, you don't need to remove the tunnel cover.
 

vette

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On my Bj7 the clicking sound, with engine off and moving the shifter from neutral to the right side of the gate, is the sound of the switch on the shifter which allows the OD to engage only in 3rd & 4th gear.
 

Jack T

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First see if holding the gear lever all the way to the right in 3rd and 4th has any effect. A worn gearbox switch needs extra pressure. Otherwise check for continuity through the circuit as Bob outlines above.
 
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Chet Zerlin

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Still learning about how to use a multimeter.
Checked continuity by placing one probe to W2 and the other to ground when dashboard switch is turned on W2 is energized (multimeter sounding). Placing one probe to the same ground and the other probe to C2 it is not energized (no sound from multimeter).
Am I checking both W2 and C2 correctly?

Thanks!
 
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Chet Zerlin

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First see if holding the gear lever all the way to the right in 3rd and 4th has any effect. A worn gearbox switch needs extra pressure. Otherwise check for continuity through the circuit as Bob outlines above.
Jack - tried as you suggested but no effect.

Thanks!
 

British_Recovery

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If taking the tunnel cover off is not an issue, start with the mechanical engagement of the overdrive. On the right side, there is an operating lever with a hole in the end. Pushing it forward allows the oil inside to get where it needs to go to engage overdrive. You would have to drive the car and carefully reach to push the lever forward. If overdrive kicks in, then it leads to why the lever doesn't move electrically enough, or at all, to engage overdrive. At least you will know what you have inside the 'box.
 

Bob McElwee

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If you do take the tranny cover off and drive it for a test:
1. do NOT wear loose clothing.
2. flip the od switch and listen for the click of the solenoid.
3. pull up (i think) on the lever to see if the od engages. Before you start see which way the lever moves with car not running.
4. when engaged, a drill bit goes through the hole in the lever and into an indentation in the od case.
There is a very detailed procedure for setting the lever movement in the shop manual.
Good Luck.
 

John Turney

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Still learning about how to use a multimeter.
Checked continuity by placing one probe to W2 and the other to ground when dashboard switch is turned on W2 is energized (multimeter sounding). Placing one probe to the same ground and the other probe to C2 it is not energized (no sound from multimeter).
Am I checking both W2 and C2 correctly?

Thanks!
That tells you that the dash switch is good. To check C2, disconnect the wires from C2, then check C2 for voltage, not continuity, by turning on power and turning the dash switch on. If you get no voltage at C2, the relay is bad.

If you get voltage at C2, as above, turn off the power and reconnect the wires you removed from C2. Turn the power back on, and check voltage at C2 again. If you still get voltage, and you don't hear the solenoid click, it's time to take the tunnel off and check the 3 & 4 gear switch.

If you hear the solenoid click, then check the solenoid as described by British Recovery and Bob.
 
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,,,
4. when engaged, a drill bit goes through the hole in the lever and into an indentation in the od case.
....

FWIW, I was told by a very experienced and well-known Healey restorer, maintainer and shop owner that the 'drill bit' method is not usually the best way to set the solenoid lever. The best way is to measure the lift of the operating valve; IIRC 1/32" lift is correct (best to use a dial micrometer). I did it that way and, indeed, a drill bit would not have aligned with the hole in the case.
 
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Chet Zerlin

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That tells you that the dash switch is good. To check C2, disconnect the wires from C2, then check C2 for voltage, not continuity, by turning on power and turning the dash switch on. If you get no voltage at C2, the relay is bad.

If you get voltage at C2, as above, turn off the power and reconnect the wires you removed from C2. Turn the power back on, and check voltage at C2 again. If you still get voltage, and you don't hear the solenoid click, it's time to take the tunnel off and check the 3 & 4 gear switch.

If you hear the solenoid click, then check the solenoid as described by British Recovery and Bob.

Thanks John!
 
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Chet Zerlin

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Went through the process as outlined and found the culprit. It was a bad 3 & 4 gear switch.
Thanks everyone for your help with this!

Chet
 
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Note the 3/4 'isolator' switch requires zero or more fiber spacer washers; Moss# 324-145 (why fiber? I have no idea). These set the depth of the switch plunger; not enough depth and the switch may not operate properly, too much and I think you could damage the switch and possibly interfere with the gear lever. It's been a long time since I replaced my switch, but a new switch may take more or less spacers as the vendors sometimes substitute similar but not OEM parts.

Edit: It just occurred to me that your switch may be good, but the depth may not be set correct (easy to test with a multimeter or circuit test lamp).

https://mossmotors.com/washer-fiber-4


 

vette

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Good for you Chet.
 

Jack T

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What I found was that my old switch, using the fiber washer, would not complete the circuit unless the gear lever was held to the right. I removed the washer and used just a smear of gasket maker instead. It's still working fine today.
 
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Chet Zerlin

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Thanks everyone for the followup info on the switch. Waiting for one to arrive now and I'll keep this good advice in mind when mounting.

Chet
 
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