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Overdrive override


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Has anyone ever condiered bypassing the 3rd/4th gear switch in the overdrive relay feed circuit so that overdirve can be used in all four gears?

Is this a wise thing to do?

It's a horrible bodge, so I'm sure that it's often been considered :lol:

 

I presume you have a late car with J-type overdrive, this can be modified for use in 2nd gear by removing the gearbox top cover and fitting a 2nd gear inhibitor switch. Some say that this is unwise as the overdrive was not designed for use in 2nd, some say it's OK, depends how you drive I think.

 

If the overdrive is used in 1st it will rapidly break because the torque multiplication will destroy it.

 

If the overdrive is used in reverse, unless you are lucky it will immediately break as the one-way clutch will fall apart.

And if it can be used in reverse it will be. You might think you will remember not to, but you won't. Or the MOT man will do it by switching the overdrive thinking it's the indicators then reversing onto the brake rollers. Byebye overdrive... :(

 

Ivor

Edited by 88V8
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It's a horrible bodge, so I'm sure that it's often been considered :lol:

 

I presume you have a late car with J-type overdrive, this can be modified for use in 2nd gear by removing the gearbox top cover and fitting a 2nd gear inhibitor switch. Some say that this is unwise as the overdrive was not designed for use in 2nd, some say it's OK, depends how you drive I think.

 

If the overdrive is used in 1st it will rapidly break because the torque multiplication will destroy it.

 

If the overdrive is used in reverse, unless you are lucky it will immediately break as the one-way clutch will fall apart.

And if it can be used in reverse it will be. You might think you will remember not to, but you won't. Or the MOT man will do it by switching the overdrive thinking it's the indicators then reversing onto the brake rollers. Byebye overdrive... :(

 

Ivor

All, true, but with respect to reversing, this is only true if the only if the relay is not working properly and doesn't disengage in any other gear than 3rd or 4th.

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I did this on an Austin-Healey 3000 in 1963. The advice from the Service Manager at the then local dealer was to increase the oil pressure in the O/D by packing the relief valve spring until a reading of (I think from memory) 500 psi was showing on the gauge he loaned me. This mod worked very well and was worth almost 1 second up the Harewood Hill Climb, the big benefit being the overcoming of the wide gap in ratios between 2nd & 3rd gears. I had to be very careful to disengage O/D before reversing and I was advised to limit the use of O/D in second gear to my competition outings. At the end of the season I returned everything back to standard. It does not seem a sensible option for road use as the life of the O/D will be shortened.

 

Jim

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All, true, but with respect to reversing, this is only true if the only if the relay is not working properly and doesn't disengage in any other gear than 3rd or 4th.

You've lost me there. (I'm easily lost :P ).

And the J-type doesn't have a relay..

 

Ivor

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what I meant was that normally the O/D does not (should not......) engage when in reverse gear due to the isolator switches on the top of the gearbox.

If the solenoid does not to drop out after activation it can be for a number of reasons, but most likely by the time you have figured that out, the OD will need overhaul being smashed anyway.

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what I meant was that normally the O/D does not (should not......) engage when in reverse gear due to the isolator switches on the top of the gearbox.

Ah, yes. But what the gentleman is mooting, is to bypass the switches - which of course are earthing switches - by just earthing the solenoid straight to the column switch so it can be operated in any gear.

I think we can agree it's not a stupendous idea. :P

 

Ivor

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Ah, yes. But what the gentleman is mooting, is to bypass the switches - which of course are earthing switches - by just earthing the solenoid straight to the column switch so it can be operated in any gear.

I think we can agree it's not a stupendous idea. :P

 

Ivor

 

I tend to agree with you statement here. Unless the gentlemen wants to have free thrills: will it blow up and if so when. Having the overdrive in all gears means in practice just 5 gears , since I don't see anyone going from 1 to OD, 2 to OD 3 to OD and finally 4 to OD, and when slowing down in reverse order. For touring OD on 3 and four is more than enough, and usually one goes form 3 to OD and then to 4. But that my 2 cents.

Edited by jobster
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I tend to agree with you statement here. Unless the gentlemen wants to have free thrills: will it blow up and if so when. Having the overdrive in all gears means in practice just 5 gears , since I don't see anyone going from 1 to OD, 2 to OD 3 to OD and finally 4 to OD, and when slowing down in reverse order. For touring OD on 3 and four is more than enough, and usually one goes form 3 to OD and then to 4. But that my 2 cents.

 

 

Hi guys

Just throwing my hat into the ring on what you can/would do with O/D 2nd

 

Being a '62 FOUR with an A type I have O/D on 2nd, 3rd & 4th. So its a 7 speed box and on road, the use pattern is

4th O/D 90%

3rd O/D 9%

2nd O/D 1% (bit of fast overtaking from low speed)

 

I always change up from direct third to forth as the O/D 3rd ratios are almost the same (O/D 3rd Vs direct 4th). Depending on the road you either hold 4th flicking in and out of O/D or on a slower more challeging road, hold 3rd and flick up to O/D third and back down for the next bend

 

Track use is different with 2nd O/D being used in slow corners, short shifting etc. The Works cars used it on hill climbs between hairpins to avoid having to grab 3rd as they ran out of revs and then get bogged down off cam as the hit the next hairpin. O/D 2nd comes into its own when really moving and trying to avoid the change to third for the sake of a few hundred feet to the next bend or your side of the road if passing a French Lorry :lol: .

 

However for road use its a marginal gear and its advantages on road do not, I believe, merit the risk to the J Type unit (and surely the six has no need of it due to its extra power and torque :lol: )

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Thanks for all the tips Gents. I don't think I will bypass the gear switches!

 

BTW my TR6 is a CP series H reg (1970). However, I am told the gearbox is from a Triumph 2000 which had the saem ratiosn and more or less the same dimensions. The last restorer (several owners before me) bothced it in somehow. The OD looks to be J type (as shown in the TR6 workshop manual). The wiring generally was not well done and crawls all over the car - some original, some new.

 

When I first took the car out this year I noticed for the first time that the OD was jumping in and out, and lately there was an arcing sound coming from under the car when the OD was on (see earlier topic). Hence the inspection and the disconnection of the solenoid feed.

 

Under the car the feed to the solenoid seems to be in the original coloured wire. The wires from the gear box switches confused me and did not appear to correspond with the wiring diagram with at least one connector being sealed off with insulating tape.

 

I will get to the top of the box and try to fix them back on correctly!

 

Paul Carey

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If the overdrive is used in reverse, unless you are lucky it will immediately break as the one-way clutch will fall apart.

And if it can be used in reverse it will be. You might think you will remember not to, but you won't. Or the MOT man will do it by switching the overdrive thinking it's the indicators then reversing onto the brake rollers. Byebye overdrive... :(

 

Ivor

 

 

Blimey, that doesn't sound good :(

 

I would wire in an in-line hidden switch to disable the O/D column switch specifically to prevent this from happening.

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