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Old 12-01-2018, 11:38 PM   #1
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Why Did My Cummins ISC-350HP Motor Run Better After Changing The Key Ignition Switch

There were several things "going on" with my motor:

I picked up some bad fuel at a Walmart gas station in Destin, FL and my engine wouldn't start the next morning after the temperature dropped into the low 30's.

Then the mechanic told me Fault Codes 434 and 456 flashed to memory (not active). This I attributed to a clogged fuel filter and bad alternator that went out 2 weeks prior. However, I was also concerned about my key ignition switch that was "spinning" the just before these fault codes flashed.

Yes... I know these fault codes point to "low un-swiched voltage to the ECM" and "low fuel pressure", but when you can't start your motor... and are just about ready to call a tow truck... all sorts of things go through your mind.

That said, I decided to replace my key ignition switch as a "preventive maintenance" measure, and now I swear my engine runs smoother at idle and my gas mileage is slightly better. Most assuredly, I have better acceleration when accelerating up a hill and joining a freeway. BUT WHY?


Here's the video you should watch to replace your ignition switch:

You need to remover the tumbler cylinder before you unscrew the ignition switch.

Note: My 2004 Itasca Horizon uses a Freightliner Ignition Switch ($20) #CHS-95410. This part does not include the key or key cylinder, but you can re-use your old one. If you need the entire ignition and key assembly then you need FL#: A22-52688-000.

Note: Do NOT just pull the wire on the back of the ignition switch. We recommend you break-off the plastic sides of the switch and then pry the harness out using plastic (non-conductive) bicycle tire pry leavers.
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Old 12-01-2018, 11:47 PM   #2
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Strange indeed!
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Old 12-02-2018, 05:42 AM   #3
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I'll just take a wag and say the now smooth idle and power difference is because fuel filters were replaced after the yucky fuel deal
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Old 12-02-2018, 12:20 PM   #4
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I wonder if one or more components weren't getting a clean & consistent 12 volts?
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Old 12-02-2018, 05:11 PM   #5
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Right On That Point - Your ECM Needs A "Clean" Voltage For Optimum Efficiency

Talked with Cummins Care about all these subjects and they said it is possible the key ignition switch can affect the "switched" voltage to the ECM.

And they said, if your voltage is degraded somewhat, but not so bad that it would flash a fault code, then your engine performance could be affected, because a degraded voltage to the ECM would produce possible negative effects. This include idle, acceleration/performance, and even fuel efficiency (MPG).

FACTS: You can't tell if you have a degraded ECM condition since your engine will start and run. And things like engine sound at idle is rather subjective anyway, but most people can feel the positive effects on acceleration and MPG, which are real.

Cummins Care added that a loose or bad ground from an either an "unswitched" or "switched" voltage to the ECM can have similar affects and/or consequences.

Of course, looking for a bad ground can be very time consuming. But there are some obvious places to look; i.e., wherever you have an ECM ground or 12V power line held down by a bolt and/or nut... that might have vibrated loose.

For example, this might include inspecting the buss bar located behind or near your fuse box area. Just wiggle these connectors and tighten any loose bolt/nut you find.

Here's a YouTube video on this subject you might find interesting regarding an Unswitched Low Voltage (Fault Code 434).

Conclusion: Considering the cost of replacing the key ignition switch ($20-$30); the possibility of this part failing after 10 years of service; and the how easy it is to replace; and the upside of improved performance... I'm very glad I did replace it and I wish I replaced the ignition switch 3 years ago or 20,000 miles sooner!!! This preventive maintenance tip would have saved me a lot of aggravation and money in fuel savings too!

Yes I started this story off by talking about bad fuel. However, I did change my fuel filters out after that... drove for 300 miles on new filters... and then decided to replace my key ignition switch as a preventive measure.

So I should have never mentioned the bad fuel problem while talking about replacing my key ignition switch as I see these as two separate concerns.

In my case, one problem (bad fuel) which forced me to get service; lead me to find two (inactive) fault codes flashed; and that got me thinking about replacing my ignition switch.

...And after I changed the ignition switch I immediately noticed a smoother engine idle sound, better acceleration and MPG.
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Old 10-29-2019, 02:14 AM   #6
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"OP" follow-up.

About the time I replaced my ignition switch, I was also not sure why my engine would start and then quit about 1-2 minutes later.

A) I considered it to be an electrical switch problem. And this thread addresses this.

B) I considered I had clogged fuel filters, which I know know was not the case.

C) I considered I had a fuel delivery or restriction problem.

Well, about 3,000 miles after I replaced the ignition switch in Houston (and leaving my RV in storage for 6-months) the following summer I drove to Montana; and my engine problem reappeared.

The problem was in the "fuel delivery" area, but it was a leaking lift pump gasket that was sucking air into the fuel line. And all I had to do was tighten 3-lift pump bolts to fix the engine start then quit condition. (Just access the top of your engine from inside the bedroom. Caution: Don't tighten too much. You don't want to brake off one of those bolts!)

*** All this "stuff" and nightmares drove me to learn about the FASS lift pump upgrade for ISC/ISL engines. I wrote a thread about this upgrade and I highly recommend you do this to your engine to prevent fuel starvation problems like I was experiencing.

*** Ad for the key cylinder upgrade, I think is also something you should do if your coach has 75,000 miles or more.
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Old 10-31-2019, 02:37 PM   #7
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Where did you order switch? I am getting intermittent power faults and fuel pressure faults on my silverleaf and it has me baffled. So I am wondering if this is a simple part to just order and swap out while there is a no start condition. I would hate to have to sit somewhere for another two or three days and empty another case, ha.
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Old 11-02-2019, 02:46 AM   #8
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Freightliner Part #CHS-95410

In the 1st post:

You need to remover the tumbler cylinder before you unscrew the ignition switch.

Note: My 2004 Itasca Horizon uses a Freightliner Ignition Switch ($20) #CHS-95410. This part does not include the key or key cylinder, but you can re-use your old one. If you need the entire ignition and key assembly then you need FL#: A22-52688-000.

This is the cheapest part I ever picked up at FL. Not that I'm complaining. Without them I would loose my sense of security on the road, and I love that most locations work on a first-come, first-service business model. Just pull in that morning and more often than not you will be out by the end of the day.

Be sure to read my tips on how to install the switch... and don't be afraid of breaking your old switch apart. Trust me, breaking the old switch apart is better then pulling on the wire to separate the wire harness... then use a a plastic knife to pry apart since there are possibly "HOT LEADS" in that harness... Maybe even after you turn your battery disconnect switch off! (I didn't check, because I used a plastic knife to separate my harness from the switch.

Okay... you asked about fault codes. All I know is that I was getting Fault Codes 434 and 456 before I changed the switch and then after not a one. What codes are you getting?
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cummins, ignition, switch

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