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[Datsun 1200 encyclopedia]

Automatic Transmission

From Datsun 1200 Club

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Categories: Automatic Transmission | Transmission

Datsun 1200s came with a traditional 3-speed fully automatic transmission. This was the JATCO 3N71B, as used in most Japanese cars in the 1970s. It is a very reliable transmission, typically not needing any service for 10 years or 100,000 miles. The car body is different in the tunnel area: Automatic-equipped cars had a significantly larger front tunnel area. The rear of the tunnel is the same.



Early Datsun 1000 was fitted with 3N71, and later 1000s were fitted with the revised 3N71A. Datsun 1200 was only fitted with 3N71B.

29144.jpgAlbum 29144

Cars with automatic weighed 75 pounds more than standard cars. All automatic 1200s have a small shift Console box.

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Identification by ID plate: B110...A means an originally Automatic equipped car. Only sedans and coupes were fitted with autos. Wagons and Trucks (B120) never got them.

Engine Compartment differences
* No clutch cable
* Automatic dipstick/filler tube
* Automatic wiring harness

25782.jpgAlbum 25782

A side effect of this larger tunnel means an auto-equipped 1200 is the perfect donor for engine transplants. Larger transmissions fit more easily e.g. 63a B210 5-speed, L-series engine transmissions or CA18, SR20 transmissions.

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See: Manual transmission conversion

JATCO stands for Japanese Automatic Transmission Company. JATCO was a joint venture between Nissan and Mazda, both owning 50%. The JATCO unit is very similar to a Ford C4 and almost bulletproof, the only thing that seems to go wrong with them is that (rarely) vacuum modulator diaphram ruptures and ATF gets sucked into the intake manifold, causing them to not shift right and the engine to billow clouds of oil smoke.

Apparently there never was a 4-speed auto available for the A series. 4 speed autos didn't show up until the mid 1980's, by then Nissan had stopped using the A series in any great numbers to make it profitable to do the design and engineering. See L4N71B Swap for more information.

The 3-speed automatic is less efficient than the 4-speed manual for two main reasons:

  • Some loss of power due to the fluid coupling (Torque Convertor). This means top speed is a few mph less. Newer transmissions starting in the 80s overcome with with "lockup" gears
  • One less gear. 1st gear in the auto is fine, but there is a big RPM drop going into second. Just as with the manual Three Speed Transmission

3N71B introduction April 1971 for Datsun 1200
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Gas Station Information

The rotating torque converter drives the front pump in the trans & circulates oil through the trans & converter. Keep checking the oil level while idling & top up as per the book.

Lubricant: DEXRON ATF (automatic transmission fluid).

IMPORTANT: Check with engine idling in PARK, transmission HOT after driving 25km (15 miles). Do Not check cold -- this fluid expands a lot with heat.



1972 USA Owners manual

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[Use "D"] for most city and highway driving.


Automatic T/M Fluid: Type DEXRON (3N71B A/T)

From the Daily Care page of the Owners manual, these were recommended:

  • ESSO A.T.F.
  • MOBIL A.T.F. 220
  • SUNOCO Transmission Fluid DEXRON Type
  • TEXACO Texamatic Fluid DEXRON

Any DEXRON compatible will work, including:

  • DEXRON 6

The bottle should say something like "Suitable for use in applications calling for DEXRON". Back in 1971 there was only one type. Now it has been superseded by Dexron VI (Dexron 6).


Auto-equipped 1200s came with a 'Nissan Full Automatic' or 'Nissan Automatic' badge:

174_5d6f4c3942ebf.jpg 26202.jpg


3N71B is a tough gearbox. It is not little, it is a 71-series (large) transmission and it weighs 70kg more than the manual transmission. Versions of the 3N71B are available for L28 turbo.

B110 version
* less clutch plates
* air-cooled
* small torque converter

Other versions have more clutch plates to handle more torque. The oil cooler lines are already there, just add a cooler or a radiator that includes built-in cooler.


You can use the same methods as with any automatic transmissions:

  • A specialty transmission shop can modify the converter to a higher stall, for faster acceleration. NOTE: if it's a khanacross/motorkhana car, you won't want ANY STALL at all. The stall means it won't move until the engine revs up, which is good for drag racing.
  • If you want to favor low-speed operation, fit 4.10 gears from the 1200 wagon or ute to your coupe or sedan
  • No lockup converters are available. This generally takes a different transmission design. It would only help high-speed running anyways, it wouldn't help to make the car accelerate faster

Discussion: Race cam 1 with an automatic?


Identification by Chassis ID plate: B110...A means an originally Automatic equipped car.

Identification by transmission ID tag: is bolted to the right side of the transmission, next to the shifter linkage.
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First digit: Year 0=1970, 1=1971, etc.
Second digit: Month 1-9, X=October, Y=November, Z=December
Last five digits: Serial number for given month

Long-tail automatic (B310) vs 4-speed automatic
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1200s with automatic transmissions have slightly different wiring than manual transmission cars.

Also see: Manual transmission conversion

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Inhibitor Switch

The Inhibitor Switch:

  • prevents the starter from engaging unless the shifter is in Park or Neutral.
  • engages backup light circuit when shifter is moved to Reverse

The Inhibitor Switch is located on the exterior of the transmission linkage.
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Nissan 31918-X0185 Switch Inhibitor $76.67 USD
fits many including:
* Datsun B110 "1200"
* Datsun B210
* Datsun B310 "210"
* Datsun 510
* Datsun 720 Truck
* Datsun 910 "810/Maxima"
* Datsun S110 "200SX"
* Datsun S130 "280ZX"
AFTERMARKET Neutral Safety Switch
Duralast JA4084 $95 at
Airtex/Wells 1S5497 replaces:
* 31918-X0101/31918-X0103, 31918-X0185/31918-X0187

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Reverse Lamp Wires
Red or Red/Black (R or RB)
Park/Neutral Starter Inhibit Wires
Black with Yellow stripe or Black/Red (BY or BR)
Inhibitor Wires generic
* Starter circuit: two BY wires (black with yellow stripe)
  Should have continuity only in P and N position
* Reverse lamp circuit: two RB wires (red with black stripe)
  Should have continuity only in REV position
Inhibitor Wires 1972 USA
* Starter circuit: two BY wires (black with yellow stripe)
  Should have continuity only in P and N position
* Reverse lamp circuit: two R wires (Red)
  Should have continuity only in REV position
Inhibitor Wires 1973 USA With Neutral Relay
* Starter circuit: two BR wires (black with red stripe)
  Should have continuity only in P and N position
* Reverse lamp circuit: two R wires (Red)
  Should have continuity only in REV position

Kickdown Switch

Kick-down Switch on the Accelerator Pedal, activates the Downshift Solenoid on the transmission. The latter is located on the left rear of the transmission case. This causes a shift down to the next lower gear (within certain vehicle speeds) when the pedal is push almost to the floor (7/8 or more pedal travel).

NOTE: this means Datsun 1200 does not have a mechanical kick-down rod from the carburetor like many cars.

Kickdown switch and striker plate in Red
21635.jpgAlbum 21635

  • 3rd-2nd Kickdown is possible below 80-90 kph (50-56 mph)
  • 3rd-1st or 2nd-1st Kickdown is possible below 40-50 kph (25-31 mph)
  • Nissan 31941-X0102 ASSY-SOLENOID,DOWN SHIFT $98.83 USD
    • replaced 31941-X0101

NOTE: The speed is sensed by RPM of the input shaft, and is internally set to match a Datsun 1200 with stock gearing and tire diameter.

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Shift Speeds

The speed is sensed by RPM of the input shaft, and is internally set to match a Datsun 1200 with stock gearing and tire diameter.

Shift specs are indicated below. RPM is of the propeller shaft (driveshaft): Kick Down (0 mm vacuum)

  • D1->D2 @ 1840-2340 RPM
  • D2->D3 @ 3340-3840 RPM
  • D3->D2 @ 3460-2960 RPM
  • D2->D1 @ 1790-1290 RPM

Half Throttle (200mg vacuum)

  • D1->D2 @ 330-830 RPM
  • D2->D3 @ 1720-2220 RPM
  • D3->D2 @ 1350-850 RPM
  • D3->D1 @ 1350-850 RPM
  • D2->D1 @ 700 Maximum RPM

Full Throttle

  • D1->L1 @ 1860-1360 RPM
  • Reduce speed by shifing from "D" position to "1" position (output shaft 2000 rpm)

Minimum Throttle (450 mm vacuum)

  • D1->L1 @ 1860-1360 RPM
  • Reduce speed by shifing from "D" position to "1" position (output shaft 2000 rpm)


Unlike the Datsun manual transmission, the automatics have a removable bell housing.

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Vacuum Diaphragm

The vacuum diaphragm (a.k.a modulator) is on the left center portion of the transmission case. Engine vacuum from the manifold is connected to this, and enables the JATCO to shift harder or softer depending on the engine load.

Connection to intake manifold:
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Be sure this is connected, and the hose is in good condition (still flexible). Inspect the ends of the hose carefully because this is the first place it will crack, thus creating a vacuum leak.

21632.jpgAlbum 21632

If it is not connected, shifts will always take place as if the throttle is floored (hard shifts).

If the diaphgram is leaking (won't hold vacuum) it will probably leak transmission fluid into the engine, creating smoke.

Nissan 31931-H0101 ASSY-DIAPHRAGM,VACUUM $59.75 USD
AIRTEX 7R1015 {#TM45} Automatic trans.; without Stripe
AIRTEX/WELLS 7R1031 Automatic trans.; with Green Stripe

Fram-2346.jpg BWD-VM503a.jpg BWD-VM503.jpg


Datsun B10 & earlier B110 used the JATCO 3N71A trans while the later B110 & B210s used the Jatco 3N71B. This JATCO is a shared design used by Nissan, Mazda, Subaru, Isuzu, Suzuki, BMW, Volkswagen, and Land Rover.

Larger Datsuns earlier than the 1200 used Borg Warner 2-speed automatic transmission. For the 1971 model year, across the Datsun range a switch to the better JATCO 3-speed automatic was made.

A12 JATCOs are air-cooled and have a grille on the bottom of the bell housing. A14 and larger autos are water cooled, and require the radiator to have transmission fluid cooler fittings.

Differences includes:

  • Converter housing (bell housing) to suit the engine
  • Torque converter spec. to suit the torque curve of the engine
  • Output shaft
  • B310 auto is 5-3/8 inch longer than B210 auto

Nearly every year something about the JATCO changed, sometimes a small change, sometimes a more significant change. However, generally you can swap transmissions.


  • L: Light duty
  • 3: Three forward speeds
  • N71: Series
  • A: First revision

Hollander USA

Strict Interchange: Hollander #932 is for Datsun 1200 1971-1973.

  • quote: Our #931 fits. The different parts are the converter & converter hsg & output shaft length.
    • Auto Trans Assy 31020-X2402. Datsun 71-76 510 & 610. 73-79 620 pickup. 74-77 710. 77-79 200SX.
  • Torque Converter: 927
    • Nissan PN 31100-X0100
      Datsun 71-73 B110
      Datsun to 2/74 B210
  • Converter Housing: 928
    • Nissan PN 31300-X0104
    • 71-73 B110
  • Front Pump: 926
    • Nissan PN 31340-X0101
    • Datsun 71-79 (and probably newer) most (if not all) models

Part Numbers

Input Shaft

Extension Housing Seal  
* Timken 710324 $4.05
* National 710324 $5.83
** Oil Seal: Nitrile
** Shaft Size=1.378"
** Housing Bore=1.969"
** Outer Diameter=1.976"
    • Width=0.433"
Filter Kit
* ATP B61 

Master Repair Kit

  • ATP NM1 (includes Contains Friction and Steel Clutch Plates, Gaskets, Seals, O-Rings and Sealing Rings) $108.99
* Standard Motor Product TM34 $34.79
* Airtex 7R1015 or TM45
Pan Gasket
* ATP NG2 or D37Z7A191A (14-bolt gasket) $1.68
* Fel-Pro TOS18644 $3.37
Oil pump Seal
* SKF 16489 (0338-19-223A, 31344X0102, 90311A0005)
Rear Seal
* SKF 13907
* SKF 99139 $24.89
Torque Converter Seal
* Timkin 2025 (Viton) $7.67
* National 2025 $10.99
** Oil Seal: Fluoroelastomers 
** Shaft Size=1.654" 
** Housing Bore=2.362"
** Outer Diameter=2.379"
** Width=0.276


Shifter came with a console.

See Main Article: Automatic_Shifter


The automatic has a larger spline. The spline is the same as what the larger Datsuns used.

See main article: Propeller Shaft

Rear Engine Mount

The rear engine mount, also called the "transmission crossmember" is mounted about six inches further back.

See: Gearbox Mount

Engine Differences


For emission controls, the carb differed. However you can use any carb with an automatic.


Auto cars had a different distributor curve for emission control. However you can use any distributor with an automatic.

Flex Plate

Automatics don't use a flywheel, as the torque convertor has enough mass to act as the flywheel. Instead they use a thin "flex plate" to carry the ring gear teeth for the starter.

Note the flexplate adapter spacer
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Crankshaft with and without Flange adapter (POST discussion)
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PILOT-CONVERTER AUTOMATIC [between crank and flexplate]

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IMG_0892.jpg IMG_0894.jpg

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Clutch Fittings

Automatic cars don't have:

  • Clutch pedal
  • Clutch master cylinder (LHD models)
  • Clutch cable (RHD models)

Drag Racing

Automatics are used in the quickest cars. Favored by drag racers for consistent shifts and minimal friction.


JATCO is the stock Datsun 3-speed automatic. They can be modified for drag racing.

CA18DET: JATCO with trans brake and scatter shield
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KA24DE: GM Powerglide 2-speed
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Fitting Manual

See main article: Manual transmission conversion

Fitting Automatic

See main article: Automatic transmission conversion

4-speed Auto Swap

A Jatco_4N71_transmission was made during the 1980s. The top gear is an overdrive ratio, for reduced revs at highway speeds. It also has a lockup system (3rd and fourth gears are direct) for fuel-economy approaching that of a manual transmission.

オートマ ミッション オートマチック トランスミッション 

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