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oxygen sensor heater circuit

I am having trouble with the oxygen sensor circuit on my 88 325 ISC with 4
wire oxygen sensor. Any help from the net would be appreciated.

Symptoms: "check engine" light is on and idle pulsates.

The problem first occurred in Roswell (mysterious ?) when the "check engine"
light came on. Upon removing the oxygen sensor and cleaning with soap,
water, toilet paper (you know, all the things that are available in a motel
shop) and replacing the sensor, the check light when out. Many miles later,
the symptoms reappeared.

I replaced the oxygen sensor (ouch $) only to have the same symptoms. While
driving, the check light went out and symptoms disappeared for a while.

Upon returning home, with shop manual in hand, I found that the oxygen
sensor heater circuit is not receiving 12 volts at terminal 3 & 4 at the
sensor connector. The manual says to check the heater relay or the circuit.
I switched the heater relay with the fuel pump relay and no difference. The
manual does not give a wiring diagram for the oxygen sensor circuit. I guess
that only "authorized repair shops" are privy to that info.

Is there a fuse or circuit breaker that I should check before I go to the
repair ($) shop.

Any help is appreciated, Thank You in advance.

Wayne Nelson                 
NRaD, Code 4123          
53140 Systems Street     
San Diego CA 92152-7565    
(619)553.4099           
fax  553.4808