Jumpy serp belt tensioner

[seu318]

Picked up a '95 x-cab 5 speed 2.3L, great truck and my second Ranger. (Had an '89 x-cab 5 speed years ago). 185+K miles (odometer stopped - going to replace the gear soon).

I replaced plugs/wires, fuel filter, oil change, trans fluid change, serp belt and belt tensioner. (going to do the timing belt in the next two months). I have a quick question. My A/C seems to work great - nice and cool - but the belt tensioner gets noticeably 'jumpy' (for lack of a better term) when the A/C is on, and the idle dips real low - almost like it wants to stall but doesn't. When A/C is off - belt tensioner is nice and steady, and idle is steady. Is this a problem ? My A/C on my 1989 never worked, so I never had a chance to look at the tensioner when it was on - largely in part because I never turned the A/C on.
Anyone else have a jumpy tensioner with A/C on ?

Thanks in advance -

 

[tomw]

I would check the IAC - Idle Air Control - which sets the running idle speed, and compensates for the added load of the A/C compressor. It also should provide a fast idle when a cold engine is first started, and then slow it down as the engine gets warmed up.
They can go bad or get gummy, and fail to perform, and you may get a slow idle, and I think a jumpy belt as the engine works to maintain speed with minimal fuel/air.
tom

 

[seu318]

thanks for the response - I will check that out !

 

[seu318]

Well - I removed and cleaned out the IAC and replaced. (it was QUITE dirty in there !) After a couple days - not any improvement, so I went ahead and ordered a new Motorcraft IAC from Rockauto. At least its a small $$ part to replace. Thanks.

 

[bobbywalter]

pretty normal on a 4 whizzer to do that with a well worn tensioner and ac load. unfortunately...some new out of box tensioners are crap....brand new junk...

the ac system may need freshening as well.

 

[enjr44]

I think you may have two problems. One, the IAC/computer should keep the idle rpms steady under differing loads (lights, A/C, blower motor, etc.). You need to fix that first. I think it should be 750 to 800 for a 4cyl.

And, two, maybe a worn out belt tensioner. The A/C compressor does have/makes an uneven load at low rpm (like any compressor, it has intake stroke and compression stroke, not actually; but, simiular).

 

[tomw]

If the idle is smooth when the A/C is off, check the rpm with and without the A/C load. It should remain constant, or close. If the idle falls with the load, then either the IAC if bogus, or the computer is not seeing the input from the A/C control that tells it the compressor is engaged. On my old one, there was supposed to be a lead from the compressor back to the factory wiring loom. It has, should have, and embedded diode to protect the computer from voltage developed as the clutch dis-engages. If the diode dies, the computer won't know the compressor is on, and won't bump the idle.
If you get good 'fast idle' when cold, and it walks its way back to normal as it warms up, your IAC is working. IMO. I'd be sure the conductor to the computer is intact, and that the diode is not wide open or shorted. I think one end of the diode goes to ground, the other to the power lead to the clutch. It's been a while.
tom