Cleaning Up a Nutter Bypass - JeepForum.com
 
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post #1 of 7 Old 06-29-2020, 09:19 PM Thread Starter
kek
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Cleaning Up a Nutter Bypass

I have a new-to-me 85 CJ7 with the 258. It is a well kept stock setup. I did notice that the vacuum lines had been simplified but it's always ran well. I took up the project of replacing some damaged vacuum lines and understanding and tweaking the routing. But as I've dove down that rabbit hole I realized it does have a bypass, but I'm hesitant to call it a 'Nutter'. Whoever did the work did a solid job. Every is put back into a wiring harness, taped, and the splices are all heat shrunk and clean. The thing that threw me for a loop is that the orange splice on the distributor side is a 3-way splice. Meaning The orange wire from the ECM is was never cut. The stepper motor is also in place and wired in, which I wouldn't expect if someone spent the time and care to bypass it. Now I want to reset the needles on the stepper motor and double check the timing, but it clearly starts, idles and runs well as is.

Has anyone seen a bypass like this or can explain what I'm looking at? Or am I better off just cutting the orange ECM wire and resetting the stepper pins and timing?

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post #2 of 7 Old 06-30-2020, 02:11 AM
BagusJeep
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hold your horses!

your stepper pins are not moving, right? when bypassed they stop working. if yours are moving.....

this is the ignition diagram. the aim f nutter is to have three wires from module to distributor direct. two are sensor wires, purple and orange, the black is a ground. You cannot have two signals on the purple and orange, the computer must be inert. if your pins move, then this is not the case
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1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
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post #3 of 7 Old 06-30-2020, 08:54 PM Thread Starter
kek
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Originally Posted by BagusJeep View Post
hold your horses!

your stepper pins are not moving, right? when bypassed they stop working. if yours are moving.....

this is the ignition diagram. the aim f nutter is to have three wires from module to distributor direct. two are sensor wires, purple and orange, the black is a ground. You cannot have two signals on the purple and orange, the computer must be inert. if your pins move, then this is not the case
Great question. You ask good questions. The stepper pin ARE moving, that's a good detail. I'm definitely saving this diagram. This is super helpful so thank you. I've marked it up with how this thing is wired. I've checked it out everywhere else and I don't notice anything else that has been messed with. The Vacuum lines on the other hand are completely simplified down so the MCU isn't getting any vacuum information.
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post #4 of 7 Old 06-30-2020, 11:32 PM
BagusJeep
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Good idea by a PO, it may have overcome some timing issue.

The wires from the distributor were used to feed engine speed data to the MCU. This drives some of the logic in the MCU. The diagrams are confused, more detailed diagrams show the orange and purple going to the MCU, others only the orange.

The orange output from the MCU to the ICM has been disconnected. This was used by the MCU to advance and retard ignition based on engine conditions. The output is not entirely directly related to the input, it is governed by logic concerning a number of factors of engine conditions.

And then the PO changed it.

The orange and purple are now connected directly between distributor and ICM with a shunt to the input of the MCU. The ICM is most likely now operating just on raw timing data from the distributor and is not being advanced or retarded by the computer. The MCU is almost certainly still receiving engine speed data.

Worth checking what is in the distributor, how much mechanical advance that distributor has. The mechanical advance with the MCU computer maxed out at 5.75 degrees, the MCU governed the rest so Jeep put in this small advance head.

You can swap (or even file down) the advance head fairly easily to 13 - 18 degrees to liven it up, but only if you have no computer controlling the rest of the timing. It is a key feature to getting the benefit of the Nutter bypass to change this advance head.

Therefore the ignition timing aspect of the MCU system has been disabled, but the PO may have thought he needed to keep the mixture control, hence this complicated approach.

This MAY be what the PO tried to do.




There are seven inputs to the MCU:

- Distributor (that orange wire or orange and purple feeding engine speed data)
- 2 vaccuum switches
- Wide Open Throttle switch
- Coolant Temperature Switch
- Thermal Electric Switch
- Knock Sensor
- Oxygen sensor

The distributor is supplying engine speed data (engine speed data but not timing, this is not the monitored input). This is being maintained.

However it sounds like you are missing at least two other inputs as the vacuum switches are disconnected. Vacuum switches and the WOT switch on the carb are concerned with throttle position and engine demand.

It is not clear if the temperature switches are working.They tell the MCU about outside air temperature and coolant temperature i.e. when the engine is warmed up. Before the engine warms up it is in an open loop mode, closed loop when warmed up in cruise.

The knock sensor is not relevant any more as it modifies the ignition timing when the engine pings or knocks.

As the stepper pins are moving, you must have oxygen sensor data. There is a narrowband O2 sensor in the exhaust, it tells the MCU when it is rich or lean either side of 14.6:1 mixture. That the stepper pins are moving indicates the MCU is operational and changing mixture but it will not know the true operating condition of the engine, that comes in part from the vacuum switches. This may solely be operating with reference to the O2 sensor. In open mode the O2 sensor is not used, this is whilst in warm up mode.

The chance of the MCU performing correctly at all times in mixture control by adjusting the stepper pins is zero.

It would appear on a casual examination if you lifted the air cleaner that mixture control was in place. However it is possible the MCU may actually be stuck in part throttle, cruise, closed loop and warm weather conditions, a default setting caused by missing inputs.

For instance when you put your foot down and need richness the MCU should not be striving to give the stochiometric mixture of 14.6:1 by leaning out the mixture, which it does in part throttle cruise. Have you noticed any reluctance to accelerate hard?



You could improve mixture by disconnecting the stepper motor and setting the pins full rich before tuning the carb.

I would also check that mechanical advance. Plenty on the Forum about the ideal timing curve.

Unless you have a state regulation that requires working emissions control, you would be better off without the computer. The mixture control will not be accurate without those missing inputs and this is one of the reasons owners abandon the system rather than try to maintain it and find rare parts.

BagusJeep lives in Bali.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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post #5 of 7 Old 07-01-2020, 03:40 PM Thread Starter
kek
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Quote:
Originally Posted by BagusJeep View Post
Good idea by a PO, it may have overcome some timing issue.

The wires from the distributor were used to feed engine speed data to the MCU. This drives some of the logic in the MCU. The diagrams are confused, more detailed diagrams show the orange and purple going to the MCU, others only the orange.

The orange output from the MCU to the ICM has been disconnected. This was used by the MCU to advance and retard ignition based on engine conditions. The output is not entirely directly related to the input, it is governed by logic concerning a number of factors of engine conditions.

And then the PO changed it.

The orange and purple are now connected directly between distributor and ICM with a shunt to the input of the MCU. The ICM is most likely now operating just on raw timing data from the distributor and is not being advanced or retarded by the computer. The MCU is almost certainly still receiving engine speed data.

Worth checking what is in the distributor, how much mechanical advance that distributor has. The mechanical advance with the MCU computer maxed out at 5.75 degrees, the MCU governed the rest so Jeep put in this small advance head.

You can swap (or even file down) the advance head fairly easily to 13 - 18 degrees to liven it up, but only if you have no computer controlling the rest of the timing. It is a key feature to getting the benefit of the Nutter bypass to change this advance head.

Therefore the ignition timing aspect of the MCU system has been disabled, but the PO may have thought he needed to keep the mixture control, hence this complicated approach.

This MAY be what the PO tried to do.




There are seven inputs to the MCU:

- Distributor (that orange wire or orange and purple feeding engine speed data)
- 2 vaccuum switches
- Wide Open Throttle switch
- Coolant Temperature Switch
- Thermal Electric Switch
- Knock Sensor
- Oxygen sensor

The distributor is supplying engine speed data (engine speed data but not timing, this is not the monitored input). This is being maintained.

However it sounds like you are missing at least two other inputs as the vacuum switches are disconnected. Vacuum switches and the WOT switch on the carb are concerned with throttle position and engine demand.

It is not clear if the temperature switches are working. They tell the MCU about outside air temperature and coolant temperature i.e. when the engine is warmed up. Before the engine warms up it is in an open loop mode, closed loop when warmed up in cruise.

The knock sensor is not relevant any more as it modifies the ignition timing when the engine pings or knocks.

As the stepper pins are moving, you must have oxygen sensor data. There is a narrowband O2 sensor in the exhaust, it tells the MCU when it is rich or lean either side of 14.6:1 mixture. That the stepper pins are moving indicates the MCU is operational and changing mixture but it will not know the true operating condition of the engine, that comes in part from the vacuum switches. This may solely be operating with reference to the O2 sensor. In open mode the O2 sensor is not used, this is whilst in warm up mode.

The chance of the MCU performing correctly at all times in mixture control by adjusting the stepper pins is zero.

It would appear on a casual examination if you lifted the air cleaner that mixture control was in place. However it is possible the MCU may actually be stuck in part throttle, cruise, closed loop and warm weather conditions, a default setting caused by missing inputs.

For instance when you put your foot down and need richness the MCU should not be striving to give the stochiometric mixture of 14.6:1 by leaning out the mixture, which it does in part throttle cruise. Have you noticed any reluctance to accelerate hard?



You could improve mixture by disconnecting the stepper motor and setting the pins full rich before tuning the carb.

I would also check that mechanical advance. Plenty on the Forum about the ideal timing curve.

Unless you have a state regulation that requires working emissions control, you would be better off without the computer. The mixture control will not be accurate without those missing inputs and this is one of the reasons owners abandon the system rather than try to maintain it and find rare parts.
This is unbelievably helpful. Thank you so much! This really helped wrap my head around it and actually understand what I am looking at.

I'm not noticing any power problems. It responds quick and doesn't lag. I'll check the timing advance but the timing generally seems ok. I do have minor rough idle problems. It seems to idle too lean and occasionally will stall at idle, but rarely. And that seems to happen once it's warm. It idles well cold, but sometimes needing a few turns to start. Now that I write this down, it could be a timing issue as well.

The O2 sensor looks new. At least it's been replaced at some point.

Would it make sense if the stepper motor was making the mixture too lean at idle when warm? I need to walk through your explanation in my head a few times. It does seem backwards from your explanation.

I am fortunate that i don't have emissions requirements so the better bet might just be to eliminate it and get the timing advance right.
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post #6 of 7 Old 07-01-2020, 04:34 PM
LumpyGrits
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Idle issues....
Ck the 2 idle mix screws on the front of the carb.
You want them both, turned out the exact same amount.

Have'n you along, is like loose'n 2 good men
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post #7 of 7 Old 07-01-2020, 06:46 PM
BagusJeep
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Your timing is independent now. You need to know the amount of timing at idle with the vac advance disconnected, the total advance at about 2000 rpm with the vac advance disconnected and the total advance at idle with the vac connected (if you are hooking up to manifold vacuum, which will be different to how it was hooked up from the factory)

This will tell you if the distributor has been altered.

As to idle, that is effected by the idle relay and solevac throttle positioner. they are supposed to raise the idle slightly when coming to a stop etc. This is another feature of the MCU. Have you checked all these bits work?

What is really annoying is that a 1980 has none of this and will idle just fine. I suggest you follow LG advice and get the curb idle, idle air bleed screws and choke mechanism set up as per the factory settings.

BagusJeep lives in Bali.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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