258 BBD MCU concern re: OEM - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-17-2002, 11:04 AM
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258 BBD MCU concern re: OEM

258 carb BBD - I have learned that the engine control computer ( MCU )that was in my supposedly stock 88 YJ 258 has a Ford MCU part # E4-FF-12A651-AGA . Several " For Sale " sites have this part listed as appropriate for the 258 BBD setup . The Jeep dealer informs me that Jeep part # 5300-5523 is OEM replacement . It is my suspicion that the Ford MCU does not match the input/output terminals correctly . Hence , several sensor input systems and several output systems (carburetor / distributor / emission control) could not function . The 4 inch vacuum switch and the 10 inch vacuum switch do not receive juice in my Jeep. So, no signal is ever received by the MCU from these sensors. This would mislead the MCU as to what engine vacuum conditions exist. The circuits cannot provide electrical feedback to tell the MCU when a richer mix is needed for acceleration --pinging due to a lean condition upon acceleration would result . I also suspect that the juice generated from the antiknock sensor leads to either nowhere or to an inappropriate MCU terminal . Hence , no feedback to the MCU , thus no delay change of timing to the distributor even though the engine is knocking . Can anyone shed light onto this ? Is the Ford MCU correct or inappropriate? If the Ford MCU is inappropriate , it may explain why 258 BBD owners have experineced many headaches . For anyone willing to try , check the little yellow plastic Diagnostic Connector boxes for 12 v juice at terminals D-2-15 ( white 4 inch ported vacuum switch ), D-2-9 (green 10 inch Adaptive manifold manifold vacuum switch ). One of these should indicate 12v while the other is "off" . Does anyone have a Ford MCU with a completely operational engine control system ?
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post #2 of (permalink) Old 10-17-2002, 09:32 PM
 
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Re: 258 BBD MCU concern re: OEM

Just do a nutter bypass and loose that thing!
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post #3 of (permalink) Old 10-25-2002, 12:07 AM
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Re: 258 BBD MCU concern re: OEM

I replaced the Ford MCU with an MCU from a salvage yard 89 YJ , auto trans., 258 BBD . The MCU had correct PN 5300-5523 . I found that my Jeep would run , but I still lacked juice to several sensors / switches as evidenced by lack of juice to several terminals at the Diagnostic Connector Box #2 . A 12 v test light at the feed wire to each of the many switches / sensors demonstrated lack of feed power to the switch / sensor . I feel that I am running into a string of bad MCU 's. The dealer lists the MCU as $400 . Anyone know of sources for the MCU or a rebuild shop ? Now , I have 2 MCU 's that do not work 100% . Continuity testing on the MCU wiring harness revealed that the wiring harness is AOK . I have concluded that it must be that the replacement MCU is at fault. I am not yet ready to do Nutter bypass , though many folks have gone that way . I also feel that my original thought s about the Ford MCU not being appropriate may not be correct . I am inclined to believe that the MCU's out there have been failing at an incredibly large rate. Just check your Diag Box #2 with a 12v test light to check on your rig. Lack of juice to many of the D-2 terminals would point to a bad MCU .
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post #4 of (permalink) Old 10-25-2002, 04:37 PM
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Re: 258 BBD MCU concern re: OEM

I know that Carquest can get rebuilt MCU's. As for who rebuilds them, I have no idea.
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post #5 of (permalink) Old 10-25-2002, 06:03 PM
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Re: 258 BBD MCU concern re: OEM

Is your step motor moving your metering pins? If so I think the MCU is working.
post #6 of (permalink) Old 10-25-2002, 07:57 PM
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Re: 258 BBD MCU concern re: OEM

Thank you for the lead on MCU replacement .
Bchiker--In response to the question re metering pins -- yes , I can see them move and also can see the D-2 Diagnostic Connector Terminal pins 11 and 14 to oscillate corresponding to the metering pins moving . There is a chance that although I can get continuity from the MCU wiring harness terminals through to the sensors / switches , it may still be possible that a short to ground may co-exist in the wiring . I'll ohm that out in the next week . The wiring harness is almost impossible to move 1/2 inch as it is compressed behind ductwork . The A/C unit also limits accessibility big time .
post #7 of (permalink) Old 10-25-2002, 08:28 PM
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Re: 258 BBD MCU concern re: OEM

I don't get it why do you think it's not working?
post #8 of (permalink) Old 10-25-2002, 09:34 PM
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Re: 258 BBD MCU concern re: OEM

The yellow rubber splash boots contain two groups of easy to use Diagnostic Connector terminals . Per the Jeep shop manual , when a grounded 12 v test light probe is inserted into many of the terminal pins , the test light should glow . This would indicate that juice is originating from the MCU , travelling through the MCU wiring harness , splitting at a fork : one fork carries juice to a single Diagnostic Connector terminal , the other fork carries juice to a switch ( TAC TES , Engine Coolant Temperature Switch , Green Adaptive ten inch manifold vacuum switch , the creme colored 4 inch ported vacuum switch , the defunct PCV solenoid , the idle solenoid , the upstream and downstream solenoids , the WOT switch , 2 stepper motor metering pins ). Several of these end point devices are merely sensor type switches with calibrated trigger points to open /close their respective circuit according to either temperature or vacuum pressure acting upon the sensor switch . As the change in temp or pressure forces the sensor switch to either move to the open or closed circuit position , then the electrical signal either does not / does return through the sensor switch and back to the MCU . Then MCU gathers this electrical signal as input regarding operating conditions of the engine : temperature or manifold pressure . In response to this input data feeding into the MCU , the MCU then sends signals to either the stepper motor at the carb to change air/fuel mix or to the distrib to change timing or to pulse air solenoids to pulse open / closed . If the sensor switch circuits never get juice supplied to them ( an MCU issue or a short to ground issue or an open circuit issue ), then they can never send an electrical signal back to the MCU . Thus garbage in , garbage out . The TES and Engine coolant temp switches are key to operate open vs closed loop mode . The 4 inch ported vac switch / 10 inch manifold vac signal are key to telling MCU how far you are stepping on throttle and accelerating and decelerating . This leads to A/F changes and timing changes ( ??) . The WOT switch also tells MCU when you have floored the pedal. Lack of juice at the Diagnostic Connector Box causes me concern as it indicates that the systems are not functioning correctly . Those little discussed Diagnostic Connector boxes are great . Note that some terminals should not have juice : ground , idle solenoid after the relay if A/C not turned on .
post #9 of (permalink) Old 10-26-2002, 07:42 AM
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Re: 258 BBD MCU concern re: OEM

I just reread your post. I really think that you are going down the wrong road. BUT I'm very willing to help and to also learn what all that stuff does. So I'll help, we can use my jeep as a test bed. I have everything we need to do the testing. Just see what you get and I'll check mine. I have an 87 stock all but the palse air system. Mine's running good but let's check it out. You ask the question and do my best to check mine and give you the answer. Or write me @ [email protected]
post #10 of (permalink) Old 10-28-2002, 04:12 AM
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Re: 258 BBD MCU concern re: OEM

bchiker - Thanks for the support . I'd be happy to work with you to resolve this . I've got schematics , manuals, etc. Just can't identify why no juice through the wiring harness to the various switches nor to Diagnostic Connectors. As mentioned , the wiring harness is JAMMED against the firewall by the ductwork . Will hunt for short to ground .
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