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Old 04-27-2021, 11:28 AM   #1
Hootbot
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Default Early m46 overdrive solution

Hey, I'm trying to figure out a solution to the overdrive on my early 240. This had the sliding switch originally that simply supplies B+ to the OD solenoid and dash light, the 4th gear switch is in series with the solenoid and after the switch.

http://pierce.smugmug.com/photos/i-s.../i-sHcMRdM.jpg

My car was missing the original shift knob and switch when I got it, I put a later m46 shift knob on it with the push button switch but of course this only operates the OD if you continuously press the button. I ended up having to put a toggle switch next to the shifter but this look stupid and I want to make the push button work.

I don't have any of the original Volvo relays or wiring and was trying to find a solution involving a normal 5 pin relays, but I can't find a solution that works. I'm not even sure if the later OD solenoid is different from the early one.

http://cleanflametrap.com/85wdm114.jpg

If I just purchased the od relay for this diagram and wired it up to my early gearbox would it work? My 4th gear switch is wired in series with the solenoid so 85 would just go to ground instead.

I also found this https://i.imgur.com/2GCSwaCh.jpg

But I don't see how it would behave any differently than the current way the car is wired, I actually tried it anyway and it of course didn't work because the momentary switch is just activating the relay and then it's not activated when you release the switch.
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Old 04-27-2021, 11:34 AM   #2
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240turbo.com sells harnesses and relays
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Old 04-27-2021, 11:48 AM   #3
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I have a good sliding switch top half off the knob I might be persuaded to sell
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Old 04-27-2021, 12:08 PM   #4
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Quote:
Originally Posted by Hootbot View Post

If I just purchased the od relay for this diagram and wired it up to my early gearbox would it work?
Yes

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My 4th gear switch is wired in series with the solenoid so 85 would just go to ground instead.
You would be better off to wire it according to that diagram. Then it will disengage the relay when you shift out of 4th. Otherwise, it will engage O/D when you go back to 4th, having likely forgotten that it was already engaged.
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Old 04-27-2021, 12:39 PM   #5
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I recommend re-wiring for the blue O/D latching relay.

We've installed this relay in several 122, 140 and 1800 Volvos usually while doing O/D conversions.
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Old 04-27-2021, 05:12 PM   #6
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To use the newer relay and wiring harness with the pushbutton. You have to ground one side of the older fourth gear switch. Then it will act as ground for the relay. Otherwise you can also change to the newer fourth gear switch so it works properly. I would upgrade to this setup as planetman suggests.

If you do end up keeping the old style setup. You should put a reverse polarity diode across the solenoid for flashback protection.
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Old 04-28-2021, 05:13 PM   #7
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Yes



You would be better off to wire it according to that diagram. Then it will disengage the relay when you shift out of 4th. Otherwise, it will engage O/D when you go back to 4th, having likely forgotten that it was already engaged.
Yes, well that's the way it is stock with the sliding switch. I can see why they changed it so it "resets"
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Old 04-28-2021, 06:03 PM   #8
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Yes, well that's the way it is stock with the sliding switch. I can see why they changed it so it "resets"
When you have the sliding switch it does not disengage when you shift out of overdrive. If you shift into third and then went directly back in to fourth with the switch on. Then you'll be in overdrive. If you have the relay as Harry mentions. Then overdrive is always off when you shift out of fourth and then back into fourth. The relay resets the circuit as mentioned.
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Old 04-28-2021, 07:47 PM   #9
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When you have the sliding switch it does not disengage when you shift out of overdrive. If you shift into third and then went directly back in to fourth with the switch on. Then you'll be in overdrive. If you have the relay as Harry mentions. Then overdrive is always off when you shift out of fourth and then back into fourth. The relay resets the circuit as mentioned.
I am guessing to do this you need a momentary switch?
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Old 04-28-2021, 08:03 PM   #10
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The sliding switch is an on/off fixed switch. The relay setup uses the momentary switch as you surmise. It pulses the relay with 12v to change output on/off. The ground for the relay is the fourth gear switch. The relay switches cleaner and removed the need for the flashback diode.
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Old 04-29-2021, 08:47 PM   #11
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Yes, but the momentary switch has to be able to pass + power.

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I am guessing to do this you need a momentary switch?
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Old 05-05-2021, 02:02 PM   #12
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You should put a reverse polarity diode across the solenoid for flashback protection.
I remember reading this suggestion from you years ago, and it has been in the back of my mind since. Nobody else to my knowledge has ever mentioned it. I am running a slide switch setup without that device, and have not detected any problems so far.

I would like to know more about why this is needed and some details of the correct diode to do the job.

If you have a minute. Thanks
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Old 05-05-2021, 02:32 PM   #13
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I first noticed the diode on an m46 when I got one from a salvage yard to give my 79 242dl another gear. The diode is used to protect from the current and voltage surge when the overdrive is shut off and turned on. The solenoid is a coil and when a circuit is suddenly shut down like when you turn off the overdrive. The coil acts like an inductor and has a current and voltage surge that can make a big enough surge to theoretically jump across the solenoid terminals and possibly damage the solenoid. I admit I've never had an issue with an old style m46 without the diode but it was on my tranny and when I read up about it seemed like a good idea. A diode you could use would be a 5amp rated diode. It is installed reverse polarity so the cathode goes to the 12v feed and the anode to ground. The diode on the trans was a round ball type which don't think you can buy anymore.

There is also a surge of current to activate the solenoid and then there is the stable operating current. The relay setup takes care of the potential flashback issue. I usually added relays to the british cars I've worked on that were wired without a relay like the slide switch m46 setup.
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Old 05-05-2021, 02:38 PM   #14
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Thanks, Dave. That was quick. I think I have a solenoid harness with the diode in it, somewhere, in a box...
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