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What is the fourth switch on a 620 trans for???

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I've seen two of these now... a 1978 620 long tail  trans, 31.5" long. It came out of a 1978 620 KC.

But it has 4 electric switches on the side????  What is the forth switch for?  :confused:





On my CA 79 620 4 speed it has three switches. See Pic below:

The rear switch by the shifter is a Neutral switch.

The one to the front is for Reverse

The third switch detects when I am in 4th gear and retards the timing 2degrees for smog emissions. It is called a Top switch on the four speeds, and it is called an Overdrive switch on the 5 speeds.

What the hell is the forth switch for on the forward part of the tail housing???  Haven't found it so far in a manual. Anybody have one who could tell me?




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It appears to be on the same level as the reverse light switch. As 5th and reverse are on the same shift rod, at a guess I would say it signals when in or out of 5th gear. Never seen 4 switches before.


The switch just behind and above the reverse switch is on the 3rd/4th shift rod and so signals in or out of 4th gear.

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I was thinking like Tendril, that maybe it was a dash indicator light for overdrive. But you'd think if that was the case they could have branched a wire off of the existing 5th gear switch (it it was a hot wire) and saved the cost.


Perhaps the 5th gear switch might be a ground wire vs. a hot wire... in order to activage the device that retards the timing when in 5th gear???   


Wish I had the trans and the wiring harness in order to check it out. Thought maybe someone currently with a 78 might know?

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Can't see needing to know if in top gear with a dash light. Likely emissions related.


The top gear switch is just that, 4th or 5th if a five speed. I checked and they both go to the vacuum cut solenoid. Still trying to find out what this does but only does it on top gear.


So that still leaves the extra switch.

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The CA guy who smogs my two Dats at a local smog shop has a couple old rides. He told me based on his smog training it was his understanding the top switch or overdrive switch (third switch) retards the timing at high speed, thus reducing high speed emissions. I'm not sure what the vacuum cut solenoid does? Wondering if it might reduce the overall engine vacuum, or hopefully reduce just the emission system vacuum enough for the dizzy vacuum advance not to work as much or partially work (if it does anything at high speed?)  thus not advancing the timing as much/at all at high speed? Or maybe the vac cut switch turns vacuum on to actually pull a dizzy diaphragm plate back a couple degrees? I haven't had a Dat distributor apart in a couple decades. Did they have the old mechanical advance weights? Will have to take one apart next time I'm in the yards to refresh my memory.


Then..as DZM states...there still lies the forth switch mystery!

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I know of this system. But I think it is OFF in the first 3 gears causing a controlled 'leak' in the vacuum advance and ON (closed) in 4th and 5th allowing full advance on the highway. This is how the earlier 521/510 and 620 systems work with dual points. Retard is in the lower gears (under certain conditions)

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  • 3 weeks later...

Hey DZM,


My curiosity about the 79 620 KC trans got the best of me, and for once I think it may pay off.


I found & a very battered & used up 1980 720KC. Initially, trans looked original and it did have 4 switches (although 2 were disconnected) but later found upper case did have a yellow X hand written on top of the bell housing like it may have come from a wrecking yard. Pulled trans and engine bay AC parts, cut off the drive shaft input shaft & sold the remains to Ecology auto wreckers.


Spent an afternoon cleaning the case. Was happy there is very little end play on either shaft and only slight fuzz on the drain plug. My only concern was the gear oil was an odd brown color like someone had just put a little creamer in a cup of coffee to turn if from black to a very dark brown. It didn't have any of the usual greenish blue or black color to it.  Right before dark I got my wife to come count output rotations.and confirmed 20 input turns = 5.5+ a smidge output rotations. Not even close to 6 output turns. I was elated and glad I had scored the correct CA trans replacement for my 620 KC with either three or four switches.


I typically use a down-loaded Nissan factory service manual (NFSM) for ease of searching and not to contribute to my physical book spine falling apart any sooner than need be. Sitting with my wife in front of the tube I reached for my original old fashioned book intending to trace the wiring diagram. Much to my surprise I flipped it open directly to page EC 26.


It confirms what you say: when the trans is in either 4th or 5th gear the top switch is ON and the Vacuum Switching Valve is ON thus providing full timing advance to the distributor. Other than 4th or 5th gear, the distributor is only partially advanced, thus retarding the timing in lower gears. That would be the exact opposite of what my state certified smog technician told me! lol  So I accepted the fact this trans has four switches instead of three, and  EC system dogs down the timing on both 4th and 5th gears.


This A.M. before work I rushed out and got out my trusty ohm meter and tested the switches using the shifter.

Reverse worked

Top switch (4th gear) worked

OD (5th) worked

Neutral was dead, will have to see if I can dean it as I've done with others.


Went to my downloaded Nissan Factory Service Manual to pull out the electronic page to send to you…and it wasn’t on that page! After a little head-scratching, I found the online service manual I downloaded was actually for a 78, not a 79. Didn’t have the time to look further thru the 78 to see the same info is there or not. Will let you know later. See photocopied page from my 79 Nissan Factory Service Manual about both Top and OD switches on same trans.



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Now I'm a bit befuddled. I cleaned up the trans without all the switches and put it back together to sell off at the Eagle Rock Swap meet tomorrow. I noticed it had a white speedo gear, just like my 4 speed out of my 79 620 KC.  Just for grins I pulled the speedo out of the 1980 720KC trans I just landed with all the switches on it...It has a blue speedo gear. And I noticed it only has one mounting tab instead of two behind the rear mount. I'm wondering if that trans has been switched out before with a mid range or close range Z trans? I marked the input, gave it the 20 turns, and my wife said she counted 5.5 turns, and we did it twice to be sure. Isn't that the correct ratio to match the 4:11's in the 79 620 KC? Now I'm spooked about selling the trans without all the switches. Lol

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One mounting tab is consistent with an '80 720 L series 5 speed. The '79 620 will have two.


20 input turns in 1st gear should turn the output 5.567 times yes. This should be the same wide ratio transmission as that in the '79 620 with the 4.11 differential. The different speedo cog could reflect a different tire diameter or simply that an '80 720 truck also used a 3.889 differential as well as the 4.11.

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