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Swapped engine What was it in?

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I just purchased a 521 pickup and discovered that at some point the engine was replaced with a rebuilt L-16 with a LaserLite rebuild sticker on the valve cover.  New engine number is L16-  Not sure where the engine came from, but when replacing my headlight switch I found that the switch for a 1971 521 has a six prong connector while this one has a 4 prong connector.  Maybe the wiring harness was changed out with the new engine, but why the dash?? I'm confused but thought someone may point me to an engine number chart so I can see what year I have.  Scratching my head here.

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If it has a rebuilt engine it may be the original one that came with the 521. Look on the cowl above and behind the valve cover. On it, it should have the engine number. See if it matches the one on the engine. After 50 years a lot of things might have been swapped to keep it on the road including the valve cover with the sticker and the headlight switch. 


A rebuilt engine could be from any of these vehicles...


 '70-'72 521

'68-'73 510

'73 620 truck.


The electrical wiring would not need any changes if a different L16 were installed. The sender for the temperature gauge, the oil pressure warning light and the plug for the alternator would all connect to the 'new' engine. 

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On the dash switch and honestly alot of the parts sold for the 521 are not 100% correct .. 

I have had this issue with a bunch of parts.... 

Couple off the top of my head...

Motor mounts

Fuel sender

Differential seal

Front inner wheel bearing....

Sometimes the wheel cylinders too.... I got metric verses the sae the 521 should be....


Best is to remove the part first before purchasing to make sure.....


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Thanks for the quick reply.  The numbers do not match so it is a replacement engine, but the issue is really the headlight switch connector.  My switch is bad, and has been "worked on" in the past.  The plastic case is broken and the slider is likely shorting out.  Blows a fuse each time it is used.  I tried the new relay fix on this site before just pulling the switch and seeing it is broken.  Looked on ebay and the replacement for a 521 is a larger switch with 6 pins.  This one has 4 pins, and looks like the one on ebay for a 620 J15, but the 4 pins on mine are tighter together than the 620 connector pins which look to be the wide 6-pin connector with only 4 pins.  This is confusing, and I am still looking around.  May should post this under "electrical".  Where is the best place to check for electrical parts?

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The engine has nothing do with the dash harness. You might have an early build 71  that could have probably retained the previous years dash wiring. So check the '70 harness schematic and see if is more like  your old one. 


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 Lets start with this.



This diagram is not 100% accurate, but it is close. It does not have any of the of the wiring for the crude emission controls on USA trucks involving several switches on the transmission, carburetor, firewall, or gas and clutch pedals. 

Sometime between 1970 and 1971 model year 521 trucks the headlght wiring was changed.  There was also a factory option fog light setup, but I do not think that made it into very many USA 521 trucks, but it may have been a dealer installed option.

Earlier and later 521 trucks had a six fuse fuse box.  The earlier truck has one solid red hot wire to the light switch, and had three fuses always hot, three fuses ignition on hot.  Later 521 trucks split out the headlights from all the other lights, and had two always hot wires to the light switch.  The fog light option had a third hot wire from the fuse box, with a  additional in line fuse.  The wiring diagram above shows the four hot fuse box, with the fog light option. 

A "normal" non fog light option light switch has a four pin connector.  Two contacts are power from the fuse box through the switch, to the headlight relay.  The other two contacts are for all the other lights.   I think that the wiring harness between the fuse box to the light switch on the three hot fuse trucks has a "Y" in it to feed the two circuits to the light switch.

The OEM 521 light switch has two positions.  Pull out one click, everything except headlights come on.  Pull out second click adds headlights to everything else.  If you have the fog light option, when the light switch is pulled to first or second click, the switch will rotate clockwise, similar to the washer squirt switch on the wipers.


About a third of the way down this page,


Is a headlight switch that I put in one of my 521 trucks, the six wire option.  The four pin plug fits the dashboard wiring harness, and the lights work normally.  The knob on the 52136 switch is not the OEM knob, but I was able to put the original knob on the six connector switch by tapping the hole in the switch shaft to fit the screw thread from the original 521 light switch.


If you go to a hobby store, a company called Du-Bro sells a tap and drill set, 10-32, 8-32, 6-32, 4-40, and 2-56.  All these tap sizes are used on a 521.   While on the subject of thread sizes, 1/4-28, 5/16-24, 3/8-24, and 7/16-20 are also used on the body of the 521.


Headlight wiring is pretty simple on a 521.  A red wire from the fuse box goes to the light switch.  Headlight power from the light switch to the headlight relay us a red wire with a yellow stripe.  The headlight relay has four 1/4 spade terminals, power in, low beam out, high beam out, and relay coil ground.  The relay coil is internally connected to the power in terminal.  The low beam out terminal is normally closed to the power in terminal, the high beam out terminal is normally open.  The relay coil is grounded by the dimmer switch on the turn signal.   Wires from the headlight relay are red with a black stripe to the low beams, red with a white stripe for the high beams.   The headlights are grounded by a black wire in the wiring harness, that goes back to a lug under one of the screws holding the voltage regulator to the inner fender.  This wire MUST be still grounded to the inner fender, even if you are now running an internally regulated alternator.

There MUST also be a ground wire from the inner fender back to the alternator frame, and a wire from the alternator frame to the battery ground connection on the cylinder head, just behind the fuel pump on a L-16.

These ground connections between the body and engine are critical.  If these grounds are removed, or go away, the cab will ground through the throttle cable, and that will melt the throttle cable.


Now I hope you will understand why I did not want to type this on my cell phone.

I have two threads on 521 trucks.



lots of info, some boring, in those threads.




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Thank you for the great and insightful info.  I have several Datsun Roadsters but this is my first truck and like the roadsters it appears that things changed during model years as the parts bins depleated, so you may not be true to a model year.  That said, I think I can now rewire the six pin switch that I came across to work with my four pin connector.  I would love to see a close-up of the 52136 switch on the site to see which 2 wires are not connected to the 4-pin connector, but I think I can figure that out.  Will go back to the garage today for some trial & error, but you sent me in the right direction.  I'll post the outcome.

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