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720 Diesel Cab Swap Info Request


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Long Story short this will be my first time posting in the forum. I have been lurking here for a couple years obtaining information concerning the Z24 engine. However, I have been around the 720 platform since birth. My grandparents helped move my family from Iowa back to Virginia in a 720. My grandfather road me around in that same 4x4 720 from 1994 to 2007 when the frame finally rotted through. This constant source of reliability ultimately led to my obsession with these trucks. Subsequently, three years after obtaining my drivers license I found my first 720 in late 2015 with a bad engine. My grandfather helped me swap the motor out of his old truck into it. I changed the oil in it every 2700 miles as I put a very hard 900 miles a week on that motor. I ended up with over 40k miles on it before that motor finally detonated

(currently rebuilding a motor for it). I have along the way also picked up four more project 720s three being sd22 diesels. 




Out of the three diesel trucks I have only one with a decent/repairable frame but bad cab and bed. However, the 1985 ST has a good cab and bed that I can switch over, I would like to know what needs to be removed in order to take the cab off. It seems like a straight forward project but was wondering if anyone had already done it and could give me some tips. Both trucks are also king cabs so they should swap without too much problem I'm hoping.


I know the A/C setup (condenser/drier/lines/evap core) will need to be swapped since it was deleted on the ST, rear harness, shifter, brake lines freed from cab, all electrical connections of harness to engine, possibly speedo cable, and throttle cable need to be loosened/removed is there anything else? 


As far as electrical goes, I have already re-wired a rear harness for the stop/tail/turn signals, as I had originally swapped in a sd22 harness to motor swap that truck. I just think it will be easier to swap cabs than redo the motor mounts, and fuel delivery system. I am going to have to remove the harness I installed as the truck with the good frame has A/C and I'd like to get that working again. I originally purchased the first diesel Oct '17 and the other two a few weeks ago. I also have the '81 sd22 fsm, a '82 combined z22/sd22 fsm, as well as, an '83 z24/sd25 fsm.


Thanks for looking!

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Well in no particular order...


Remove front bumper

Remove rad and or rad hoses to engine.

Disconnect vacuum lines to the charcoal canister on the rad support

Disconnect battery positive and negative cables from starter and intake manifold.

Disconnect any vacuum switches and wires from the right side inner fender to carb and intake.

Disconnect unplug (2) engine harness (oil pressure, transmission wires, start signal to starter, carb wires, alternator, temp sender etc are all on this) from the main harness on the right side.

On left side of engine disconnect the distributor and the two coil leads.

Disconnect the front and rear brake lines where they leave the cab mounts and mount onto the frame. I think there's a flex line for this.

Disconnect the steering column at the steering box or where ever.

Disconnect brake booster vacuum hose to intake.

Disconnect speedometer cable

Disconnect E brake cable

Disconnect throttle cable

Disconnect fuel line

Disconnect return line

Disconnect fuel tank vent line

Disconnect vacuum lines to the charcoal canister on the rad support

Disconnect clutch flex line at slave cylinder

Disconnect remove the shifter to make cab removal easier.

Disconnect 10 pin connector under passenger seat between cab and rear harness.(rear lights, fuel sender, fuel pump)

Disconnect 2 rad support and 4 cab mounting bolts holding cab to frame.


Probably missed more than a few.



Cab is lighter without seats, rad, battery and hood on

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Lots of good info! As far as running sd22 engines, I was also wondering about smoke color. The two engines I have ran both produce blue smoke to a degree. I know that is oil but is there an easy way to tell if it is from bad valve stem seals or worn rings. Both engines start right up but sat for a number of years and never touched. One of the trucks was actually sitting in a field when I bought it. The first engine smokes at acceleration (the last inspection sticker stated 1996) and the other only smokes at idle and is fine so far under load, this trucks last inspection was 1998.   


Basically, I want to take 2-3 trucks and make a decent running truck and yes I would like to keep the factory optioned A/C. 




*Will be an SD22 Engine*


Last edit:

Links to videos of the two engines running 

Truck one:

Truck two:


Both of these videos were taken immediately after they were first started.


Truck No. 3 I will attempt to start after inspecting it further and probably cutting out sections of the core support/fender

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Well you user name is correct SLO720, there is nothing fast about an SD22 engine, but they get great mileage if you keep them under 60mph.

As for smoke, if they sat for years the valve stem seals could have dried out, a good way to tell if it is the them is when you add a lot of oil to the engine does it smoke worse when you start it that time compared to when you just start it without adding oil, if it does smoke worse the valve stem seals likely need to be replaced, if the same then I don't know.

I over heated one on my SD series diesel engines one night(don't ask), after that night my diesel engine smoked like crazy until it completely warmed up, and it always smelled funny after that night, it was also hard to start, I gave up on it and put this rebuilt SD22 engine I have in it, got it running but was never happy with it, in the end I bought a Volvo wagon with a home made turbo setup on it and I put that into my 720, it took a few years but I finally figured it out and the engine runs great now.

Does the engine blowing blue smoke start easy or hard, does the smoke smell funny?

Check your valve lash on the engine/engines, if they are tight it will smoke badly, but the engine tends to run very rough also, but generally that will cause white smoke.

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I think the white truck lived a pretty hard life before it ended up sitting in a field. It burned a good bit of oil until I changed the oil in it. The first oil change I did  the engine oil barely came out and didn't fully drain for 20 minutes. What was even worse was that the new oil added into the engine took 2.5 hours to reach the pan and read on the stick. The oil was nasty and I ended up adding 1/2 can of seafoam to the new oil over about a 15-20 hour run time. I changed the oil in it again and it drained a lot better still had some crud in it. i"m pretty sure I need to pop the oil pan on it and see if all that crud is now in the bottom of the pan. The oil usage had reduced to almost nothing before I pulled that motor. It also never had any problems starting except when I changed the fuel filter which was also really really nasty.


Truck two (w/ A/C) has been quite difficult to start over the last several days. On initial pre-check  of the motor I found that the fusible link was half burned so I wiggled them around until they started working. I ended up pulling the fusible links off one of the other motors harness and they now work as they should. I also had to swap out the DCM box from another truck to make the shut off arm work. It still has a high/weird idle leaving me to believe that the diaphragm in the pump may be torn. The high idle also seems to be preventing the truck from fully shutting down. As far as oil goes I have been checking it and it doesn't seem to have moved since I added half a quart to it when I had it towed home four days ago. The oil is dark but still clean looking and tacky so I haven't changed it quite yet.

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One thing that is easy to miss.


Under the back of the cab there is a steel strap bolted to the cab that loops through the frame. Two bolts secure it to the cab. Cab will seem free with this still attached as it allows the cab to lift a couple inches.

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So, the vacuum line going from the throttle body(carb looking thing) to the injection pump needs to be good with no cracks, when you look down the throttle body you will see a venturi that the butterfly valve/plate wraps around, that venturi hole needs to be clear as that is what creates vacuum to pull the diaphragm back to the idle position, critters could have made a nest in there.

The diaphragm would normally rest in the floored position if the injection pump control arm was not connected to the injection pump controller, there is a spring for that purpose, when idling the venturi creates enough vacuum to pull it to the idle position, if the hose or diaphragm are cracked it cannot create the vacuum needed to put it to the idle position, so then air getting past the butterfly valve sort of controls the idle, I have never actually seen a cracked diaphragm myself, but others have had cracked diaphragms.

If the diaphragm has dried out and gotten stiff it cannot move like it should move, so if the engine sat for years without running....................

As far as the engine not shutting down completely the IP controller could be stiff and not moving all that well, when you shut it down with the key, if it doesn't die then go to the injection pump and push the top of the arm on the side of the injection pump towards the firewall, does it move and does the engine die then?

Another thing to check is the idle screw, is the arm resting against it, can you push on it and force it to a normal idle, if it is not resting on the screw check to make sure that is nothing in the throttle body throat holding it open(critters nest?), you have to check everything after they have say a while.

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I did do some more checking, but won't have a full day to mess with it until about Thursday. I went ahead and replaced the vacuum hoses, and it still does not idle down right. if it is the controller itself sticking I should be able to check it be jumping pins at the dcm as illustrated in the fsm correct? Everything also looked clear of debris the few minutes I messed with it. I also have an electric vacuum pump that can be hooked into the system to manually create vacuum (originally purchased to check a brake booster). Also pushing the arm on the IP shuts it off, that is how I've been turning it off since I got it home. The last thing that I messed with the the clutch system today as the pedal felt "stuck" and the only way I could drive it was by leaving it in gear. I looked and found the guts of the slave cylinder rusted solid, I've never seen one on a Nissan do this. Once I get some free time I'll go ahead and do some more checking on the vacuum system. 


Again, Thanks for the help guys!

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The black box is called a DCP module, it controls the injection pump controller(thing with the rod going to the injection pump arm), it also controls other things.

If the injection pump arm was stiff or out of adjustment because it was stiff and moved out of adjustment it may not move the arm to the proper position to shut the engine down, there is an adjustment on the injection pump arm, but I would not mess with it too much unless you get everything else figured out, then play around with that.

Maybe it is idling high because the former owner had it idling high just to keep it running at some point, have you tried adjusting the idle screw on the throttle body?

Also there is this small devise with a spring in it that the throttle arm hits on the intake manifold, it appears to me that it is used to slowly let the engine drop to idle instead of the rpms dropping fast and having it die as these engines will idle very slowly, but it shakes the whole truck so I keep mine up to where it is smooth.

The diaphragms are hard to find, the Zexel part number is 9 421 610 043-753, there is also another number on the box, it is 155020-7920, there is another one for the Mercedes that appear to fit made by Flag which has several numbers on the box, they are in order/by line, 81 1382, then 1 420 503 903, then K00410019-1210, I have no idea which number is the correct number on either box, here are photos of the boxes, my camera doesn't like me anymore so it  either does't focus well or the flash hides print.

The Zexel part




The Flag part


Edited by wayno
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