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fuse pull for FWD vs AWD?


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16 replies to this topic

#1 suprjohn

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Posted 11 April 2014 - 12:07 AM

So I was talking with a guy with a 2000??? Something outback. He said he pulled a fuse to disable the AWD, and run FWD in the summer netting him an extra seven MPG. 

 

Is this possible/plausible/advisable? Not applicable to my '89 DL wagon but my sister has a '10 Forrester that she says she's disappointed in the fuel economy with. 

 

Advice, opinions, info welcome!

 

John



#2 Bushwick

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Posted 11 April 2014 - 12:24 AM

No idea what they did in 2010, but on my 95' there's a little plastic fuse holder up against the firewall. If the car is to be towed for example, you can either stick a small 5 amp fuse (or jump with wire) in the holder and it'll disengage the AWD. On mine, a dash light comes on saying "FWD." Also, I noticed running it briefly the car behaved very different i.e. it really feels like a FWD car and takes turns differently, engine revs slightly quicker while driving, cars squats more in the rear while accelerating hard, etc. Some people are adamant there's no difference in MPG since they claim the added weight of the AWD is still being "pulled" as the car moves and the rear differential is still spinning. I haven't tested MPG, but the simple fact the engine revs differently and isn't actually powering anything beyond the front wheels it's gotta make a slight difference. Just like adding a 30 pound AC compressor means there's extra weight and you still spin the pulley with it off, but unless the AC is on you aren't getting the parasitic effect it creates with the engine's load.

 

As far as 7 MPG difference I'm calling BS on that. For example, a true FWD Legacy will only have about 1-2 mpg difference from it's identical AWD counterpart. That's going off the factory EPA MPG ratings. Also, it's said to be hard on the solenoid that controls the AWD engage/disengage function if left disengaged for long periods of time, BUT there are people here that run FWD all the time and leave the rear disengaged.


Edited by Bushwick, 11 April 2014 - 12:40 AM.


#3 suprjohn

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Posted 11 April 2014 - 07:13 AM

Thanks Bushwick! I'll pass that info on, and she can decide for herself if she wants to try it. :-)

 

John



#4 heartless

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Posted 11 April 2014 - 07:19 AM

...If the car is to be towed for example, you can either stick a small 5 amp fuse (or jump with wire) in the holder and it'll disengage the AWD....

 

the fuse in the fuse holder to disengage the AWD is only operational IF the key is in the ACC or ON positions! (according to my 95 owners manual) There must be power supplied to that circuit for it to work, and there is only power supplied when the key is either on or in ACC positions. (and this is only applicable to the automatic transmissions)

the fuse size doesnt really matter - it is completeing the circuit that makes the difference - most Subarus have spare 10, 15 & 20 amp fuses in the interior fusebox cover - use one of those.

 

7mpg gain? i dont think so. 7/10 of a mpg gain, maybe. Reason being the rolling mass is still all there, just not "driven" by the engine/transmission.

 

Winter fuel mileage suffers due to different formulas, longer warmup times, etc - I am betting that is what this other person is forgetting about when claiming higher mpgs in the summer. My 90 Legacy LS wagon would only get mid 20s in the winter - 24-26 mpg or so - but in the summer I could get 29 with no problem, broke 30 mpg a couple of times, but not often...havent had the lifted 95 long enough yet to know what summer mileage is with it, but winter has been pretty close to what the 90 got - mid 20s - and that is with snow tires on it.

 

Tires types also affect fuel mileage - a stickier tire compound (lower mileage rating) will reduce mpgs a fair amount. higher mileage rated tires will give higher mpgs

Example: had Cooper CS 4 touring tires (80,000 mile) on my 90 Legacy and got around 29 mpg overall...later put Kelly Explorer Plus tires on (40,000 mile) and overall fuel mileage dropped to around 27...



#5 grossgary

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Posted 11 April 2014 - 08:37 AM

 He said he pulled a fuse to disable the AWD, and run FWD in the summer netting him an extra seven MPG. 

 

that's not possible. Most importantly, others and myself have done it quantitatively and it makes no difference.

 

Technically speaking it could make a minor difference, but it would be nearly negligible in real world situations, though i'll concede some minor increase if someone was very perceptive and anal about driving habits - like maybe 1mpg.  7 is impossible with the FWD fuse alone.
 

2nd - it doesn't make sense.

a 4WD Subaru still has hundreds of additional pounds of 4WD components - rear bearings, hubs, axles, rear differential, rear driveshaft, carrier bearing, rear extension housing, 4WD clutches/center differential.

 

A FWD "car" does not have all that additional weight.  Putting in a fuse does not make that hundreds of pounds of components disappear.

 

Not only the weight - but the amount of additional rotational components induce drag and moments of inertia associated with every additional rotating part - let's work our way from the wheel to the transmission to see the increase of rotational mass, inertia, and resistance:

 

1. beefier rear bearings

2. rear outer CV joint

3. inner rear CV joint

4. rear differential stub shaft bearing

5. rear differential ring and pinion

6. rear differential front piniion bearing

7. rear driveshat ujoint

8.  center carrier bearing

9.  joint at center carrier bearing

10.  front driveshaft ujoint

11.  bearings, clutches, driven gears or center differential (manual trans).

 

running a 4WD vehicle in FWD does not magically make those things disappear unless he bought his car from Peter Pan.

 

More compelling information:  Average gas mileage for a 2000 Outback is 24mpg (http://www.fuelly.co...ru/outback/2000)

 

If he increased 7mpg - then he's getting 31mpg.  that's not possible without severe modifications to driving style or components.


Or alternately if he didn't get 31mpg - then his car probably wasn't running properly and getting less than average and some maintenance, repair, other issue was resolved.

 

and finally - i would highly suggest not putting much stake in "what some guy said" - i can't begin to tell you how many incorrect notions i hear about cars almost weekly.  people make improper conclusions, correlation doesn't mean causation, people change their oil and think they know everything mechanical, people drive one Subaru and think they know them all....

 

I could keep going - but the bottom line is this is completely off base and he's either whimsical in information or was honestly misled in some way.

 

my sister has a '10 Forrester that she says she's disappointed in the fuel economy with. 

n

 

not a good car for gas mileage conscious people. if gas milegae is a concern i would not recommend buying a high ground clearance, 4WD, automatic, about the largest 4 cylinder engine you can get, with roof rails.  that car has many items that detract from gas mileage, not a very efficiency friendly car for those interested in that.

 

it could also be:
driving:  letting the car warm up in the morning, short trips, heavy on the go pedal and break pedal.

location:  mountain driving, city driving, stop and go, frigid so the car is idling a lot while warming up or de-icing windshields.

 

what kind of vehicle did she have previously?

people coming from small FWD/RWD cars with good mileage are sometimes unimpressed with Subaru mileage.

people coming from big SUV/truck world will appreaciate the Subaru mileage and still having 4WD/automatic.

 

My in laws were entertaining Subaru and coming from american FWD cars and i told them not to get it, he would hate the gas mileage.  He bought it anyway but he was at least prepared and love the thing so much for it's snow prowess and other aspects he ended up buying another new Subaru Outback as well.  He values it for what it is.

 

if she had a dinky FWD american econobox then that may explain it.  they have good gas mileage and are cheap...but they're weak in about a dozen other ways.  can't have it all, gotta pick what's best for you.  if gas mileage is more important than safety, 4WD, ground clearance, aesthetics, etc - then gotta choose accordingly.


Edited by grossgary, 11 April 2014 - 09:32 AM.


#6 1 Lucky Texan

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Posted 11 April 2014 - 08:40 AM

yeah, with the fuse in, all the same parts turn and they have the same mass. hard to see where any fuel savings would come from. This comes up a lot and it's bogus unless parts are removed from the car - then you gain from weight savings, not FWD.

#7 grossgary

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Posted 11 April 2014 - 09:34 AM

I've even removed the rear axles, diff, driveshaft (or some of those) from Subarus with no net increase in gas mileage, even on all highway miles.

 

i'm sure some net gain would be possible but it would have to be very controlled and particular to driving style to see it.  otherwise the miniscule gains get washed out.


Edited by grossgary, 11 April 2014 - 09:35 AM.


#8 1 Lucky Texan

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Posted 11 April 2014 - 09:42 AM

I've even removed the rear axles, diff, driveshaft (or some of those) from Subarus with no net increase in gas mileage, even on all highway miles.
 
i'm sure some net gain would be possible but it would have to be very controlled and particular to driving style to see it.  otherwise the miniscule gains get washed out.

weight savings probably figure more in stop&go (but, slight - like leaving a pre-teen/small adult ??? out of the car) , I agree, very slight/no help on the highway.

Edited by 1 Lucky Texan, 11 April 2014 - 09:45 AM.


#9 johnceggleston

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Posted 11 April 2014 - 09:54 AM

the fuse in the fuse holder to disengage the AWD is only operational IF the key is in the ACC or ON positions!
(according to my 95 owners manual) There must be power supplied to that
circuit for it to work, and there is only power supplied when the key
is either on or in ACC positions. (and this is only applicable to the
automatic transmissions)

 

a couple of thoughts,

when the engine is not running there is no fluid pressure on the transfer clutch,

therefore no power / connection to the rear wheels.

so regardless if the circuit is completed with the key in ACC position,

the fuse does not improve anything unless the engine is running.

 

this means, with the engine off, you can jack up one rear wheel and spin the wheel freely, (90 - 04)

so the fuse is useless unless the engine is running.

 

also if the engine is not running there is NO AT fluid to lube the bearings in the transfer clutch / extension housing.

so fuse in, engine off, creates a situation where you could damage the trans.

this is why you CANNOT tow an auto trans car with the fuse in , engine off, and front wheels up, rear wheels down.

 

and lastly, about the improved gas mileage with the fuse in,

if in fact you could get 7 mpg improvement with the fuse in,

subaru would be advertising this through every possible media outlet all over the world.

 

did the push button 4wd GL / DL cars of the 80s get much better gas mileage when in 2wd?


Edited by johnceggleston, 11 April 2014 - 10:00 AM.


#10 heartless

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Posted 12 April 2014 - 06:09 AM

John C - the information I posted on that came directly from the owners manual of my 95 Legacy in the towing section for towing with all four wheels on the ground, and it says basically the same thing for towing with a tow dolly or similar with front wheels off the ground. I can scan & post the pages if you like - I thought the car had to be running for it to be operational as well, but the owners manual suggests otherwise...

 

Personally, if my car has to be towed, it goes on a flatbed truck or full trailer.



#11 CNY_Dave

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Posted 12 April 2014 - 08:50 AM

4EAT no VTD:

If the engine is not running there is no hydraulic pressure to press the AWD clutch plates together.

Towing with just 2 on the ground does not cause damage because the AWD is engaged, the AWD is as disengaged with the engine off as it can/will ever be.

 

The damage results because the plates are rubbing together (not pressed together but still touching) with no fluid flow to lubricate or cool the rubbing surfaces.

The plates overheat, the friction material degrades, bad and expensive things happen.

 

If you HAD TO tow with two on the ground you'd do it with the fuse in and the engine running. If you can do that though you probably don't need a tow, though.



#12 johnceggleston

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Posted 12 April 2014 - 09:57 AM

i never knew the manual allowed towing with the fuse in, wheels down.

but i have not read all the manuals either.

 

and towing short distances, 2 mi. to the shop,

well almost anything is possible.

 

but i sure would not do it long distances or at high speed.

 

and i'm pretty sure by 97 they changed the recommendations,

but again, i have not read the manual cover to cover.

 

i would be willing to bet that no dealer service department would suggest towing with the fuse in and wheels down.

 

fyi, the fuse completes a circuit to ground.

it grounds out a pin on the TCU.

this energizes the duty c all the time.

which dumps any fluid pressure .

no pressure, no connection to the rear.

(just like with the engine off.)

 

i'm not sure what difference having the circuit complete makes when the engine is off.

but this assumes there are no movement inside the auto trans when the engine is off.

i guess there could be parts turning inside the trans if the front wheels are turning.

but would the oil pump turn? would fluid move?

idk, i'm not really familiar with the inner workings of the auto trans.



#13 MilesFox

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Posted 12 April 2014 - 10:38 AM

running the FWD fuse is not recommended as it puts unduy stress n the duty c solenoid.

 

The analogy to an ac compressor being extra weight but no load is analagous to the engine running with the trans in neutral (no load)

 

The rolling resistance of the AWD components are still engaged with the wheels while driving.

 

The fuel economy of a forester is based on its final drive gearing, not the awd



#14 Bushwick

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Posted 12 April 2014 - 11:07 AM

I used the AC analogy as the engine has to have the idle kicked up in order to keep running. It creates an additional load that parasites some power and the engine has to work harder to overcome. Go on loose pavement/gravel/snow and trying spinning all 4 tires. The engine has to work HARDER to spin all 4. Force FWD and it can easily spin the fronts (at least mine can but it's still running the skinny tires) since the extra effort isn't wasted trying to power the rear drive line and the extra weight doesn't matter in that scenario. People talk about the weight of the AWD cars like it's an AWD Taurus or something. My wagon with auto and AWD still somehow weighs LESS than my 4 door 5-speed Saab hatch back did stock. Sorry for stirring the pot in the previous post as that wasn't my intent. I just was trying to comment more that there is a difference driving AWD vs. FWD only, as experienced in turns and accelerating. Whether that relates to a fractional MPG difference or not, I haven't tested and not trying to say otherwise.

 

 

If you really want to increase MPG (pretty much any vehicle), run "approved" full synthetic oil, fluids, etc. Remove any intake restrictions and a non restrictive air cleaner (turbo's don't matter as much before the turbo, after the compressor is another story), get tires that offer better fuel economy, LOWER the center of gravity i.e. lowering springs, and keep the cruise control on constantly at high way speeds. My old 96' Mark VIII with a DOHC 4.6L was rated at 28 highway factory. After lowering the air bags via custom switch brackets, full synthetics oil & trans, removing air horn, K&N, touring tires, and full tune-up every 15k miles (oil every 3k as I ran the car) I was able to squeeze out 33 highway on flat surfaces with cruise on. 32 was norm on mild inclines. It had a cluster gauge that monitored and was surprisingly accurate. Also note the 33 MPG highway came with extra weight in the rear pushing curb weight to 4000+. I had to constantly drive long distance with that car and it beat most 4 bangers of it's time (both in speed and MPG). It's funny having to top off the Legacy almost 1.5 times more often with a fraction of the miles LOL.



#15 suprjohn

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Posted 12 April 2014 - 07:43 PM

Thanks for all the input guys! I'm glad I asked here before I suggested it to her. I guess if something sounds too good to be true, it probably is!

 

Don't get me wrong, my sister loves her Forrester. And you're right, she did get it after someone side swiped her 2000-something Malibu, so her mileage expectations may have been a little high. 

 

At any rate, thanks!

 

John



#16 angelram10466

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Posted 11 August 2014 - 10:06 AM

All these mis-information.... I did a test on my Subaru Impreza 2002 TS.. manual.... by the way a lot manuals say pulling that the fuse doesn't work on manual transmissions...well I had forgot that the fuse was there... and I kept complaining about performance. but then I found weird that every time I filled my tank I was getting 21.8mpg, 22mpg. I do mostly city driving. after I realize what was holding back the performance I removed the fuse. Now I get 16.7 or 17mpg.. In the summer



#17 1 Lucky Texan

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Posted 11 August 2014 - 10:30 AM

All these mis-information.... I did a test on my Subaru Impreza 2002 TS.. manual.... by the way a lot manuals say pulling that the fuse doesn't work on manual transmissions...well I had forgot that the fuse was there... and I kept complaining about performance. but then I found weird that every time I filled my tank I was getting 21.8mpg, 22mpg. I do mostly city driving. after I realize what was holding back the performance I removed the fuse. Now I get 16.7 or 17mpg.. In the summer

 um - can you tell us to what fuse you are referring?

 

In a 5 spd manual, there is no control over the center diff short of removing mechanical parts. And, in the pre-2004 or so autos I KNOW, the fuse is INSTALLED in a FWD slot - not 'removed'. I THINK the same might be true for later 4EATs and the 5EATs but - I have no direct experience with them.

 

so, w'ever fuse you are handling, it is unrelated to your 5 spd manual transmission's AWD/FWD operation.






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