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Unread 07-12-2002, 04:02 PM   #1
CJ8DV8
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BBD Carb Stepper Motor Question

Hi Gang-

Educate me, please.

I've recently done some troubleshooting with the diagnostic connector on my CJ. Specifically, my multimeter's voltage reading is supposed to pulse on and off with the in and out moton of the metering pins in the carb. This diagnostic connection pin is basically a remote test location for the carb's stepper motor signal.

Here's my question:

I noticed that the stepper motor voltage doesn't move when the engine is at idle. I have to crack the throttle up to about 800 or 900 rpm before the voltage starts to pulse, indicating stepper motor activity. Below this speed the stepper motor is "dead".

Is this normal?

I'm having drivability problems (hesitates just off idle and just after the auto upshifts, each gear) and started poking around when I discovered this stepper motor phenomenon.

Any thoughts?

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Unread 07-12-2002, 04:48 PM   #2
johny1i
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When the engine is warmed up and idling, it is in what is called a "closed loop" operation. You can read all about the wonderful world of stepper motors in a service manual, but basically there are about 4 "open loop" modes in which the stepper motor will be adjusting the carb--a cold engine (or at least cold outside air), Wide-open-throttle (flooring it), drops in manifold vacuum (stomping the gas, but not quite flooring it), and another one I can't remember--something about fluctuations in timing or something.

The easiest way to make sure it's working is to look down the carb when the engines warmed up and idling, then punch the gas lever and the little rod inside the carb should slide back in the direction of the rear of the vehicle. It will then slowly slide forward as the condition disappears.
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Unread 07-12-2002, 04:52 PM   #3
johny1i
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Sorry I didn't answer your exact question, I don't know much about automatic transmissions. Are you sure that the problem is in the engine--e.g. could the transmission be low in fluid and not shifting strongly?

A "flat spot" in the carb can usually be corrected by adjusting the accelerator pump and/or the idle mixture. You'll need to manual for that, though, because I don't think I can illustrate with words how you adjust the accelerator pump--it's easier with pictures.
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Unread 07-13-2002, 11:00 AM   #4
CJ8DV8
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Thanks Johny1i-

I know that the stepper motor is controlled by the computer and that the computer "listens" to all kinds of input from the various sensors - inluding rpm's. I just thought it strange that the computer "gives up" adjusting the fuel mixture when the vehicle is at idle.

As far as the drivability problem:

The tranny shifts fine and solid. The problem is when accellerating and the trans shifts, the engine stumbles or misses (hesitates) for a second before the rev's start to pull again.

My thought was the accelerator pump also, but I just had my mechanic do some work and he replaced the BBD carb with a new (rebuilt) one. He explained at the time that it needed replacing because the accelerator pump was bad in that one! Same problem, new carb, lighter wallet!!

ARGHHHH!!!!!
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Unread 07-17-2002, 09:10 PM   #5
Bob K
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The computer does not need to adjust the stepper motor at idle because it sets it right for that rpm (and other info it is receiving) then just leaves it there until something changes ie engine rpm/throttle setting.
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