Blower Motor and Clutch Inter-related?

88XJ, 4.0L, manual tranny. I've had an intermittant HVAC blower motor fpr some time now. The fan will intermittantly just die/fade down to nothing. After a while it will come back on, only to fade away again a little later. The problem does not appear to be tied to switch settings, speed, acceleration, or the like. Very annoying to say the least - especially when its 100+ degrees outside.

Anyway, the last few times I've had it on the highway, I've noticed that when the fan dies down, use of the clutch pedal re-engages the blower motor (albeight for a very short time). This seems to work every time the blower motor dies. Does anyone know if the clutch for a manual tranny shares an electrical circut with the blower motor? This seems very odd.

???

Nothin' electrical goes through the clutch in normal operation, so that's not the issue.

Next time you note your blower motor cuts out, feel up around the windscreen defogger vents and see if anything is coming out there. I'd be willing to bet that your blower motor is just ducky, but you've got a vacuum reservoir (should be pax side front bumper) with a leak, and the defog is the "fail-to" position when vacuum is lost.

Check it out, and let us know.

5-90

Pull your upper radiator hose up, and check the wiring that goes to the AC compressor. Bet ya there will be a bare spot allowing it to short. Wrap some elec tape around it.

When you engage/disengage the clutch you torque the motor in the mounts...allowing the hose to put pressure on the wire.

Dont laugh yet....check it first. :laugh:

Thanks for the input. I'll check both of these things out. As luck would have it whenever you want to troubleshoot a problem, the blower motor worked just fine today when I was driving around.

I knew the clutch would not affect the blower motor mechanically, but I wasn't sure if there might be a clutch interlock switch on a common circut that might affect the blower motor when the clutch was depressed (thereby activating any interlock switch).

I've used up a lot of cable ties on various XJ's up under the dash, near the brake linkage, I guess the clutch linkage is the same (I've never had one though). Troubleshooting shorts, chaffed wires and loose connectors under the dash is one of the less fun things to do on an XJ. If you go looking around under there be carefull, it is real easy to do more harm than good.

desertred said:

Thanks for the input. I'll check both of these things out. As luck would have it whenever you want to troubleshoot a problem, the blower motor worked just fine today when I was driving around.

I knew the clutch would not affect the blower motor mechanically, but I wasn't sure if there might be a clutch interlock switch on a common circut that might affect the blower motor when the clutch was depressed (thereby activating any interlock switch).

Click to expand...

The only "interlock" switch on the clutch is the one that disables the start motor unless you press the clutch while you turn the key to START, much like the NSS on automatics. It should not affect the HVAC/Blower motor at all.

However, when you press the clutch, the engine falls to idle (or close to it, anyhow) while you shift, which increases manifold vacuum and makes up for what you're probably losing from the vacuum reservoir in the bumper. Thus, my reasoning.

And, vacuum reservior failures are fairly common as they get older - it's only plastic. I've got to get around to designing one out of metal to replace it, one of these days....

5-90

I have yet to check the cannister behind the front bumper, but I have watched the fan a bit more. I have determined that my fan isn't really fading out, rather the air is being redirected. Everytime I thought the motor was dying out, the air was being redirected to the defrost vents on top of the dash. There is not much noise coming from the fan for some reason in this position, which is why I thought the motor was dying out (that and the loss of air from the forward facing vents). This is happening when I depress the clutch as well. The air is being redirected from the defrost vents back to the vents in the dash.

So, it appears I may have a problem with the air directional controls. Are these vacuum actuated? Anyone have a clue?

Yep, that's definetly a vaccum problem. From what I understand there is a vaccum line running under the battery tray to the vaccum canister behind the bumper that is very prone to breaking thereby creating a leak. I'm having a similar problem myself. In my case, the blower motor itself is shot (the fan won't blow air period) However when I get that fixed, I'll still have to deal with the vaccum issue, since the previous owner of my MJ apparently got in to an accident and the vaccum canister is non-existant. I'll probab;y just plug the lines on mine and hope for the best. I need to get it taken care of before winter though, I'm not really looking forward to sub-freezing temps with no heater

Hope this helps some.

Pat

Yep - it's a vacuum problem. The plenum controls are powered by manifold vacuum.

I don't know if the thing is in the aftermarket or not, but I can't see that a vacuum reservoir should be spendy anyhow. Chances are, that thing's leaking - it could also be a line going to it or coming from it, but I've not heard of that much (that doesn't mean that it doesn't happen - just that reservoir failures tend to be more common...)

5-90

Thanks guys. And I thought this was going to be a boring weekend. I now have a project!

5-90 said:

???

Nothin' electrical goes through the clutch in normal operation, so that's not the issue.

Next time you note your blower motor cuts out, feel up around the windscreen defogger vents and see if anything is coming out there. I'd be willing to bet that your blower motor is just ducky, but you've got a vacuum reservoir (should be pax side front bumper) with a leak, and the defog is the "fail-to" position when vacuum is lost.

Check it out, and let us know.

5-90

Click to expand...

how you say its the fail-to position. is that for safety issue. keeps the windscreen clear.

90xj06 said:

how you say its the fail-to position. is that for safety issue. keeps the windscreen clear.

Click to expand...

Exactly - in the event of a system failure, it helps to ensure your safety by keeping the windscreen clear.

Not everything in "failure mode" is designed badly...

5-90

Sounds like I am experiencing the same exact problem... I would like one of you knowledgable gents to confirm.

On my 4.0 with an Auto, it seems as if I lose vaccuam at 2500+ RPM. This is evident in the fact that heat stops blowing on my feet and goes to the defrost when I hit this RPM. Based on this thread, the likely cause seems to be bad vac canister.

But my question is really this - how/why does the RPM affect manifold vacuum?

-justin

Manifold vacuum is higher (lower pressure) when the throttle blade is closed - therefore, you'd have less vacuum (more pressure) at cruise and acceleration than you would have at idle.

The "vacuum reservoir" (a round cannister used to store nothing?) provides a vacuum source for maintaining power to accessories like HVAC controls and speed control when you're cruising/at speed and accelerating.

A failing vacuum cannister is easy to diagnose - if whatever is randomly shifting about goes right back where it's supposed to if you let the engine idle, then the controls are fine, and the cannister is shot. If it doesn't, check the controls.

5-90

Just a FYI. At idel (600 RPM) you'll get 14-16 inches of vacuum at the manifold. At the vacuum canister, generally you get 1 to 2 inches less.

5-90 said:

Manifold vacuum is higher (lower pressure) when the throttle blade is closed - therefore, you'd have less vacuum (more pressure) at cruise and acceleration than you would have at idle.

5-90

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Ahhh.... So it actually has more to do with the throttle (or position of the throttle blade, rather) than RPM. That pretty much cleas things up for me...

I also check my manifold vac at idle just for giggles...

-Justin

HTeK said:

Ahhh.... So it actually has more to do with the throttle (or position of the throttle blade, rather) than RPM. That pretty much cleas things up for me...

I also check my manifold vac at idle just for giggles...

-Justin

Click to expand...

Yep - most heat engines are little more than air pumps. They're going to try to move the same amount of air each time they turn - putting the throttle in there to restrict airflow creates resistance to turning by creating manifold vacuum (there's more to it than that, but that's the simplified version.) The reduced air pressure overtop the pistons on the intake stroke effectively slows down the piston's travel.

Diesels work differently - there's no throttle. Instead, they move (theoretically) the same amount of air each time they turn, and the fuel is metered into the cylinder. More fuel allows for more cylinder pressure to move the piston downwards, resulting in an increase in piston speed. Again, that's the simplifed version.

Make some sort of sense now?

5-90

5-90 said:

Manifold vacuum is higher (lower pressure) when the throttle blade is closed - therefore, you'd have less vacuum (more pressure) at cruise and acceleration than you would have at idle.

The "vacuum reservoir" (a round cannister used to store nothing?) provides a vacuum source for maintaining power to accessories like HVAC controls and speed control when you're cruising/at speed and accelerating.

A failing vacuum cannister is easy to diagnose - if whatever is randomly shifting about goes right back where it's supposed to if you let the engine idle, then the controls are fine, and the cannister is shot. If it doesn't, check the controls.

5-90

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I understand it a bit differently. While I have no disagreement with the statement that you have more vacuum with the throttle plate closed. Where I question your statement is that you'd have less vacuum at cruise vs. idle. Perhaps this is more of a driving style but when I'm crusing I generally have the throttle only very slightly open. Now I've never hooked up a vacuum gauge while I was driving, I suspect that the vacuum would be similar or perhaps slightly higher then at idle.

I'm not trying to nit pick here. It's more that I just started an automotive technology program this term, and we're discussing vacuum and how to use it to get an idea on how the compression is for the engine, along with the basic theroies behind vacuum in an automotive setting, so I'm sort of trying to verify my understanding on this. I also believe that you will see higher vacuum readings when an engine is at higher RPM given of course the the throttle plate is closed.

-Pat

You will see a higher gage reading (lower vacuum/higher absolute pressure) with the throttle valve open than you will with it closed.

The manifold absolute pressure is less related to crankshaft speed than throttle valve opening, I think you'll find. Granted, the engine will move more air per unit time when running faster, but it is the throttle valve that proves the ultimate bottleneck when it comes to moving air (and the only one you can adjust easily. There are, of course, others - most notably the intake opening (throttle body) and the cylinder head ports/runners.)

You'll see "higher" readings in the sense that you're going to see higher manifold absolute pressure (measuring from 0m/m Hg, or psia - pounds per square inch, absolute) yet the manifold pressure is still going to be "below atmospheric) (measuring from ~760m/m Hg:1bar:14.7psia:760 Torr, or psig - pounds per squar inch, gage.)

The principal difference between "psia" and "psig" is where you start counting - with psig, you're starting at ~14.7psi, or atmospheric pressure (14.69... psi @ STP, as I recall.) "High vacuum" is a low gage reading on the absolute scale, or a greater difference between atmospheric/normal pressure and pressure as observed. "Low vacuum" is a high gage reading on the absolute scale - with "no vacuum" being one atmosphere or 760m/m Hg.

So, it seems counter-intuitive at first, until you get used to the two scales. Check your automotive programme (I'm assuming you're dealing with secondary education?) for a "sidebar" course in something like "Automotive Mechanisms" - you'll pick up a fair bit taking that course. Pressure/vacuum, mechanical advantage of various lever and pully systems, elementary fluid power - all the sort of stuff we covered in high school phyzzies 'way back when. (Pity they don't do that in primary education anymore. I had to take "Automotive Mechanisms" at De Anza - I could have taught the course off of what I'd learned in high school.)

Anyhow, you're probably not going to really dig into pressure/vacuum in an intro automotive course - just get into what causes manifold vacuum (and why!) and what it's used to control. More's the pity.

If you don't have that "sidebar" course I was talking about, you'll want to take a basic Physics course (which you probably need anyhow.) Everyone should be at least "grounded" in physics, chemistry (at least inorganic,) and maths - then you can branch off into things like organic chemistry, petrochemistry, geology, optics (what fun!), aerospace, metallurgy, and such. But, if you don't know the basic "whats" and "whys" of how the world works, you're going to be well behind the curve...

Meanwhile, tell you what - see if you can explain what I'd just said about vacuum back to me (without looking!) and I'll see if you've got it. Assume, when writing, that I don't know anything about the subject, and you're trying to explain it to me.

5-90

Phager said:

I also believe that you will see higher vacuum readings when an engine is at higher RPM given of course the the throttle plate is closed.
-Pat

Click to expand...

From a perspective of vacuum only, not all that neat stuff 5-90 was amusing himself with and making our eyes rattle, the vacuum actually decreases at higher RPMs. Now if you could get on the straight and flat to where you could back off the throttle in order to keep the same RPM, the vacuum might start to rise.

I have a vacuum guage on my dash and when I'm towing, I use it as an indicator as to when the transmission is going to down shift.

As long as there is throttle pressure (plate opening), the vacuum decreases toward zero. In order to increase RPM, you have to open the throttle plate.

If you take your foot off the accelerator, the decompression of the cylinders momentarily increases (around 22 inches) the vacuum past the normal idle vacuum. For sake of argument, mine normally idles at 16 inches (I use the old school term).

Real world example. I'm in fourth gear going up hill. In order to maintain my speed, I increase the throttle and I watch my vacuum gauge decrease toward zero. The angle of the hill continues to increase and I am still applying throttle to try and keep the speed. The speed begins to fall and when the vacuum hits zero, my transmission down shifts into third. My foot is still in the same positon on the throttle so third gear RPMs are high (over 3000 RPM) and my vacuum is still close to zero.

Did I help or add to the confusion?