4WD light off, under full throttle

[Gary E]

I am sure Rev knows this but for others, It effecitvely gives you 2wd low range IF you have an open differential. Your driveshaft will still turn. and if you have a locker the left wheel will have power going to it. True 2wd low range does not apply power to the front DS.

my light goes off, stays in 4wd...'88 4.0l 5-speed



Q: if this vaccume lock was replaced with a solid mechanical link, could I run it in 2LOW with the front axle disconnected?

Thanks.

 

[Rev Den]

I am sure Rev knows this but for others, It effecitvely gives you 2wd low range IF you have an open differential. Your driveshaft will still turn. and if you have a locker the left wheel will have power going to it. True 2wd low range does not apply power to the front DS.

Yup.



Rev

 

[SV1CEC]

While the axle shift is done under vacuum, it apparently (by consensus - I've not checked personally) stays put until shifted the other way.

The light, however, must always have vacuum to stay on. At WOT, vacuum is minimal (there is little to no obstruction to the intake air flow,) so there is little manifold vacuum. Highest vacuum occurs at idle, when the throttle valve is nearly closed and the displacement of the engine is unchanged (it is the reduced airflow that causes the low turning speed of the crankshaft - the engine isn't moving as much air as it could.) Therefore, if your vacuum reservior is losing its integrity, you will lose vacuum to the switch if at WOT for more than a few moments - like at cruising speed. Therefore, it is possible that the 4WD indicator will go out - but you will still be in 4WD - remember that...

5-90

Now folks, let's see. The vacuum canister is there to do just that, provide consistent vacuum under circumstances where the intake manifold is not. Couldn't we just move that vacuum pipe, from the intake manifold to the vacuum vanister(s) and thus avoid the light going off under WOT? This (I suppose) comes down to "is the vacuum canister able to provide enough vacuum to move the disconnector?". I'll have to get some vacuum pipe and do some tests, but has anyone done that?

Rgds