The Triumph Dolomite Club - Discussion Forum

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 Post subject: O/D wiring issues
PostPosted: Mon May 23, 2022 8:47 pm 
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Posts: 37
Having sorted out the brakes on my Sprint (see previous post ) I find that the O/D seems to have blown the bottom fuse on the holder when testing the car. Stripped the tunnel out. ordered new g/stick switch because it seemed to be working back to front; Ie when it was IN it had no continuity and therefore no power to solenoid. So after redoing some of the connections it was was working but still back to front ( made sure Ig. SW was on and in 3rd gear !!) I had no idea how to reverse the switch. Luckily the guy who had his TR6 in for a respray used to work on overdrive units on Triumphs.
He diagnosed a faulty inhibitor switch on top of G/box. Part ordered from C. Wittor. I will keep you posted. No Idea why the fuse went but then when replaced was OK. perhaps that was when the inhibitor switch decided to give up the ghost.

Bob

Round 2. Fitted new inhibitor switch. No change.

I still have a live solenoid when the gearstick switch is in the OUT position. I don't think that the solenoid allows the O/D to work when there is no power to it. This would be counter intuitive and not supported by the overdrive specialist I spoke to.
Any ideas?

Bob


Last edited by 10yearsafter on Tue May 24, 2022 6:30 pm, edited 1 time in total.

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 Post subject: Re: O/D wiring issues
PostPosted: Tue May 24, 2022 5:35 am 
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Location: Auckland, New Zealand
When fitting a replacement inhibitor switch to the top of the gearbox it is worth getting the operating height setting just right by adding / subtracting or changing the thickness of washers. Good idea to use a test light. Otherwise it is possible that once the gearbox is at operating temperature, expansion will not allow the switch to function correctly.

Also, worth checking the wire that runs up the gearlever to the switch, both the insulation of the wire and connectors can short out.

All the above have caught me out in the past!

Geoff


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 Post subject: Re: O/D wiring issues
PostPosted: Tue May 31, 2022 5:36 pm 
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Back again. Despite the edit to the original post. Have had no success. Tried jacking up the rear wheels and running the car in 3rd at an indicated approx 35 MPH nothing happens when the G/stick switch is operated. I am still not sure that having no power to the solenoid when the G/S switch is IN, is how things should be.

There has been a suggestion that the O/D comes out automatically if the revs drop below a certain figure. I am not sure that this is what I remember but it was over 20 years ago that I last drove the car!!

All comments gratefully received.

Bob


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 Post subject: Re: O/D wiring issues
PostPosted: Tue May 31, 2022 8:10 pm 
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It seems surprisingly difficult to find the wiring diagram for the electrical connections to the solenoid in the ROM.
However, my memory from when mine was blowing fuses all the time is that the switch in the stick is in the earth side. If that's true for the inhibiter switch as well (and I have no recollection either way), then one side of the solenoid would indeed be permanently live, and the other side only earthed when the switches all make.

I seem to remember thinking it was an odd way around, since if the feed down to the solenoid shorts to earth, the fuse blows whether you use the overdrive or not. So it's fix it, or do without all the other things that run off that fuse.

Graham

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The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


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 Post subject: Re: O/D wiring issues
PostPosted: Tue May 31, 2022 9:32 pm 
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Joined: Thu Sep 28, 2006 10:06 pm
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Location: Bristol
My assumption is that the Sprint will be the same as my 1500Hl Dolomites and in them the 12v positive supply that is used to feed the reversing lights is used to supply 12v positive for the O/D solenoid operation. The 12volt positive goes via the gearbox inhibit switch and then to the gear stick O/D switch. It then goes to one of the solenoid connections. The other solenoid connection has a short earth connection to complete the circuit when O/D is required.

I will attached a copy of a drawing that I made of the arrangement when I was providing the details needed to make up the loom.

It is quite common for the connections in the gearstick to short together or to earth. When I had a problem I replaced the wires through the gearstick with a length of house lighting pendant cable as this is heat resisting and as it is sheathed it does not get damaged if you ensure the sheathing just comes through holes for protection.

When O/D is not selected I have found the hydraulic pressure within the O/D is between 15 - 20 psi and when O/D is selected, O/D solenoid energised and in 3rd or 4th is rises to about 300 psi. It is quite easy to measure if you have the relevant pressure gauge as it is just a matter of taking a plug out and fitting the pressure gauge.
Image


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 Post subject: Re: O/D wiring issues
PostPosted: Tue May 31, 2022 9:44 pm 
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Location: Bristol
I will also add a photo of my gearstick fitted with house hold lighting pendant cable.
Image


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 Post subject: Re: O/D wiring issues
PostPosted: Wed Jun 01, 2022 8:21 am 
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Location: Auckland, New Zealand
You definitely should have power to the solenoid when the gearlever switch is 'IN'. Try following back, using a test light or voltmeter to see how far the current reaches with the switch in. Hopefully you should then locate the fault. As long as the reversing light work, that will show if power is getting to that area as they both share the same green wire as the power source.

When I last rewired an overdrive gearbox, I used contrasting colours so thought the photo might be useful.

Green supplies the power to the reverse light switch and brown takes it back to the wiring harness and eventually the reversing lights.
Green also goes directly to the gear lever in/out switch. Once IN, power flows to the switch on the gearbox top (yellow wire) so when in the correct gear, power flows to the solenoid (blue wire). Solenoid is then double earthed (black). You should hear an audible click when it actuates. G'box in the photo has two switches, one operates on both 3rd and 4th, the other on 2nd. Just ignore the one on the left for your g'box.

Issues with non engagement are usually due to malfunctions with 1) the gearlever switch 2) the switch on top of the gearbox, or 3) the solenoid. Only way to really determine which is to remove the g'box tunnel and methodically work through finding out where power reaches. Remember that the switch on top of the gearbox itself needs the number of washers between it and the g'box casing to be adjusted so that it actually functions when g'box is hot and components have expanded.

Hope this helps


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 Post subject: Re: O/D wiring issues
PostPosted: Wed Jun 01, 2022 11:59 pm 
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Note on the pic in the post above, this car has been improved by the addition of a second inhibit switch to permit the use of overdrive in 2nd gear as well as 3rd and top.

Just pointing it out to avoid confusion!

Steve

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 Post subject: Re: O/D wiring issues
PostPosted: Fri Jun 03, 2022 3:09 pm 
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thanks for the response. Let's put it this way; brain fog and not driving the car for over 25 years had a lot to do with the problem as well as some poor wiring.

Sad to say the O/D does not work. So what next? The solenoid is live.

Bob


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 Post subject: Re: O/D wiring issues
PostPosted: Fri Jun 03, 2022 3:31 pm 
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Quote:
thanks for the response. Let's put it this way; brain fog and not driving the car for over 25 years had a lot to do with the problem as well as some poor wiring.

Sad to say the O/D does not work. So what next? The solenoid is live.

Bob
So the solenoid gets a live feed when the switch is "IN" ond zero Volts when "OUT"?
If so, is it clicking? Check teh earth wire, I have had 2 where the earth wire looks OK, but had fractured internally. One on the way to the start of the RBRR, and it was sheer luck I found it.

After that, if an OD has been sat they can stick, and sometimes a good thrashing, switching in and out and getting good and hot will sort it out.

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 Post subject: Re: O/D wiring issues
PostPosted: Fri Jun 03, 2022 9:19 pm 
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From the TR6 factory manual, rudimentary, but useful:


Image


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 Post subject: Re: O/D wiring issues
PostPosted: Thu Jul 21, 2022 6:42 pm 
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Quote:
thanks for the response. Let's put it this way; brain fog and not driving the car for over 25 years had a lot to do with the problem as well as some poor wiring.

Sad to say the O/D does not work. So what next? The solenoid is live.

Bob
"Life in the slow lane" ; Having established that the wiring for the O/D is correct and working. Test drive showed that the unit was not engaging.
Solenoid removed and cleaned and checked to be working. The plan was/is to reassemble G/box tunnel and get the car on a ramp to clean filters if not then try some "Italian tuning" that is to say get it out on a run to warm everything up. When I started putting the gearstick back everything seemed fine until I tried changing gear with the ignition ON, the O/D IN but the engine not running, this showed a short at the base of the gear stick and blew the 35 amp fuse. To cut to the chase it was obvious that the O/D switch wiring was sound but the switch was shorting onto the interior of the gear knob; caused by the gear knob being too far down the gearstick which allows the switch to contact the top of the stick. The skinny nut that fits inside the Knob should
be just flush with the top of the threaded gearstick. Then tighten the bottom collar. Obvious really!!

Should the above not work. Can the O/D unit be taken out by removing the prop and cross member??

Bob


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