# 4 low



## vett (Feb 12, 2005)

what is the difference between 4 low and 4 low lock? :


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## BigRedBarn (Jan 4, 2003)

I think you need to be a bit more specific about what you mean.

(Translation: BigRedBarn is once again totally confused.)

Looking forward to more info. Thanks in advance.


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## vett (Feb 12, 2005)

*lo loc*

I'v got a 79 GMC 3/4 ton. On my transfer case shifter the knob has lo loc, lo, n, hi, hi loc. what's the difference between lo loc and lo. and whats the diff. between hi and hi loc.


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## derekbroerse (Mar 6, 2004)

What this means is that you have an NP203 transfer case--a fulltime 4x4 unit.

In order for a tcase to be full-time 4wd without the binding and hopping etc. when you turn corners, a differential (much like what is in an axle) was added to allow the wheels to turn at different speeds. This is active in the 'low' and 'high' ranges.

When you shift to 'low-lock' or 'high-lock' this differential is physically locked out, therefor 50% of the power goes directly to the front and 50% goes directly to the rear, no longer allowing the smooth cornering (it will jump and hop like any other 4x4).

Now, the monkey wrench is thrown into the mix when people add a part-time conversion kit to these. This changes the shift pattern, but I don't know what it ends up being.


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## vett (Feb 12, 2005)

*low loc*

Thank you for the info.,very helpful!


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## BigRedBarn (Jan 4, 2003)

I didn't know GM made such a transfer case for 3/4 tonners. 

It sounds like an ideal sort of tracsfer case. Let's face it, we all hate it when we have to stomp on it to get out trucks to turn when they're in 4HI. We all know what that's doing to the tires, driveline, etc.

I always assume that GM (and Ford & Dodge) made only fully-coupled transfer cases (i.e., w/o any sort of differential).

Our old '75 Chevy 3/4 ton didn't have any sort of transfer case like that, that I remember. (I still can't believe the crappy mileage and tire noise that beast made.)

A transfer case with a differential must've taken up quite a bit of room. 

Even today, when you have an "All Wheel Drive" system, the transfer case really doesn't have a differential but some sort of a device that acts much like a torque converter (which deosn't do much for mileage). 

How many guys on this site would like to see a true differential in their truck's transfer case like what's being described by Vette and Derek? Less tire scrubbing, less driveline stress, etc., etc.

Can we hear a little more about this transfer case? When was it made? What size trucks was it installed into? Was it bigger and heavier than the standard transfer cases? What was this "conversion kit" and why would someone install one? And so on.


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## festerw (Sep 25, 2003)

BigRedBarn said:


> How many guys on this site would like to see a true differential in their truck's transfer case like what's being described by Vette and Derek? Less tire scrubbing, less driveline stress, etc., etc.


I've got one in my Dakota it's awesome, not the same selections as his but the same deal. I've got 2 HI - 4 HI Part-Time - 4 HI Full-Time - N - 4 Lo Part-Time.


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## derekbroerse (Mar 6, 2004)

The NP203 was used throughout the 70's by GM, Ford, and Dodge. They came in half and 3/4 ton pickups, and K5 Jimmys/Blazers. They regularly are listed on ebay for around $25 etc. as they are kind of the redheaded stepchild of the transfercase offerings... not real popular.

They are chain driven, not as strong as the NP205 (gear driven) like your '75 would have had. Also full-time 4wd sucks up fuel unneccessarily and makes the front axle components wear even when they aren't needed. They came with the (often considered unreliable) automatic locking hubs on the front.

They are considered to possibly be the best transfer case available for snowplowing rigs, for the reasons you mentioned. They are also used by rockcrawlers as a gear-doubling setup for lower ratios in front of another trasfercase.


There are two conversion kits available for these tcases to make them act more like the NP205.... a cheap version and a more pricey version. Not sure what the difference is but I think the cheap version basically just locks the differential while the preferred version replaces it entirely with a new shaft. They also come with manual hubs for the front axle. The main idea was to increase fuel mileage and reduce unneccessary wear and tear.

I currently have three of them here in the yard (70's half tons), I know my friend's '75 K20 has one, and my dad's old '77 K5 had one too. They are nothing rare. Physically, they look almost identical to the NP205 but don't have a round cover on the back with the three bolts like the 205 does.

The NP203 was phased out around 1980 or so. The NP205 remained and was joined by the aluminum part-time NP208.


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## tuna (Nov 2, 2002)

derekbroerse said:


> The NP203 was used throughout the 70's by GM, Ford, and Dodge. They came in half and 3/4 ton pickups, and K5 Jimmys/Blazers. They regularly are listed on ebay for around $25 etc. as they are kind of the redheaded stepchild of the transfercase offerings... not real popular.
> 
> They are chain driven, not as strong as the NP205 (gear driven) like your '75 would have had. Also full-time 4wd sucks up fuel unneccessarily and makes the front axle components wear even when they aren't needed. They came with the (often considered unreliable) automatic locking hubs on the front.
> 
> ...


They did not use auto locking hubs they used constantly locked hubs.They were found behind auto trans from 74-79 though there were a few mated to 4 spd`s in GM`s in `74 only.They were very popular though most people don`t even know how they work.Most remaining today have been converted to part time.


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## derekbroerse (Mar 6, 2004)

tuna said:


> They did not use auto locking hubs they used constantly locked hubs.They were found behind auto trans from 74-79 though there were a few mated to 4 spd`s in GM`s in `74 only.They were very popular though most people don`t even know how they work.Most remaining today have been converted to part time.


My mistake on the hubs.... brain was in the clouds for a minute there... locking hubs would be rather pointless, wouldn't they?

I have a 79 K10 with one behind a TH350. Another mis-and-match 70s vintage truck with the same combo. There is one loose one that came from behind a fourspeed (or so I was told, I didnt get the tranny) from a later truck than '74, but maybe it didn't belong there.

Speaking of the full-time locked hubs, one of my parts trucks is an odd duck. It's a K5 jimmy (chassis only) with TH350, NP205, but the Dana 44 up front has those hubs on it. Maybe someone's been mixing and matching again...?

Regardless, there are lots around but their popularity 'today' is quite low compared to the 205. Is that a better way of putting it?

Run a search on ebay.


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## jrm123180 (Aug 23, 2004)

I have a 79 K10 With a 350sb, and a th350 w/ NP203 Both the tranny and transfer case have well over 225,000mi on them and have out lasted 3 engines. I've plowed with them and gone off roading. It's a great setup....just really sux on gas.


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## BigRedBarn (Jan 4, 2003)

derekbroerse said:


> The NP203 was used throughout the 70's by GM, Ford, and Dodge. They came in half and 3/4 ton pickups, and K5 Jimmys/Blazers. They regularly are listed on ebay for around $25 etc. as they are kind of the redheaded stepchild of the transfercase offerings... not real popular.
> 
> They are chain driven, not as strong as the NP205 (gear driven) like your '75 would have had. Also full-time 4wd sucks up fuel unneccessarily and makes the front axle components wear even when they aren't needed. They came with the (often considered unreliable) automatic locking hubs on the front.
> 
> ...


WOW!!! Thanks for all that info.

I find this is very interesting... probably because I'm sort of a Chevy guy (although my first three cars, back when I first started driving, were Fords, and all our tractors are old Fords)... and probably because I'm a mechanical engineer. I used to work on gearbox designs (not too complex ones, though) for heavy equipment back in my college co-op job days in the late 1970's.

Our old '75 3/4 ton had a manual tranny, so I guess that's why it didn't have that locking center diffy/transfer-case. I can't imagine that old truck having an automatic tranny plus that center diffy... that is, I see no way for that truck to have gotten even worse gas mileage. I guess we'd be talking GPM - gallons per mile. Wow, no wonder people did conversions to up the mileage.

We sold that old 75 to a guy who owned a construction firm. He added a plow and towed a work trailer with it after he bought it (we never had a plow on it). When I last knew, the next owner had over 300K on it w/o any engine work. We always change oil on a regular basis in all our vehicles as well as our tractors, riding mowers, etc. I think that's the one thing you can't skimp on with any vehicle or equipment.

I'm guessing I'm not the only one who likes to read about this sort of thing. Keep it coming whenever you get a chance. I know I appreciate it.

Again, thanks for the excellent info.


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