So, you reversed polarity on your engine battery and now the engine won't start. My steps to a good start.

[Bjohnsonmn]

We've all made mistakes in our owner maintenance of our rigs, or found things that just didn't seem right as we work to improve the RV's performance and improve longevity. Sometimes those mistakes have consequences beyond a blown lightbulb. This is my troubleshooting steps for a no-crank following an engine battery reverse polarity hook up.

Step 1. Assess.

After the sparks had stopped, I went around and assessed what was and was not working. For example, which of the annunciators stayed on.

In my case, the usual lights remained on, but I had failed to notice that the "check trans" light stayed on. More on that later.

Step 2. React and Trace.

Using the wiring diagram, I traced all of the things that were in line from the wire bundles that attaches to the battery. In our case, it is a negative wire with a secondary return line on the same battery terminal and the positive with the main positive and a secondary positive that feeds the fused 12v supplies to the ECM and other drive components.

This is where the carnage first showed itself. The reverse polarity had blown 4 fuses. Some 7.5 and 10amp.

Step 3. Fuses

The trace processes is repeated until you find all faults between the key switch and the starter solenoid. This means the fuse bundle that comes from the wire bundle directly attached to the battery's positive (red) wire. In the case of my vintage, it is several 7.5 and a few 10amp fuses located in my electrical bay in the back corner of the coach. I also inspected the large capacity solenoids in the engine bay. They include two grid heater solenoids and the starter solenoid. After 22 years, the contacts were quite worn with one 5" red 6awg cable that connected the two supply sides of the grid heater solenoids being melted through the insulation. (Replaced that with a high quality 6awg MTW and new lugs).

It was a good excuse to clean up all of these contacts in hopes of improving conditions in general.

NOTE: The simple act of cleaning the contacts, nuts, lock washers, and the cable terminations make a much higher quality contact and seems to have improved the starting process in general. The engine started with fewer cranks after sitting for 6+ months without a start and a battery that had just been very abused.

Step 4. Trace again.

With all of this work done, there still was not the satisfying click of the starter solenoid. This is when I noticed that the NSS wire is the wire that contacts the solenoid. This is the Neutral Safety Switch that actually drives the starter solenoid. This is a part of the transmission control system that ensures that the coach is not started in gear. There is a relay within the VIM (Allison box in the driver's side exterior electronics bay). There are 6 relays and two fuses in fact. The relay that drives the NSS is fed from one of those 10amp fuses and will not allow the starter to engage as it can't close. I checked both 10amp fuses and only one was blown. Replaced and BAM...

Step 5. Start Your Engines!

That's right, it started right up! The coach is running well, though I haven't attempted to move yet. There are still a few electrical gremlins in the works, though I believe those are unrelated to the original issue.

 

What's next:

It is possible that the alternator is not working as the engine battery was reading 12.08v at a slightly elevated neutral run level. I'll need to find out if a hidden fuse is blown in where the DUVAC controlled alternator lives. OR, the battery is so toast, it won't even allow for charging.

[Tom Cherry]

19 minutes ago, Bjohnsonmn said:

We've all made mistakes in our owner maintenance of our rigs, or found things that just didn't seem right as we work to improve the RV's performance and improve longevity. Sometimes those mistakes have consequences beyond a blown lightbulb. This is my troubleshooting steps for a no-crank following an engine battery reverse polarity hook up.

Step 1. Assess.

After the sparks had stopped, I went around and assessed what was and was not working. For example, which of the annunciators stayed on.

In my case, the usual lights remained on, but I had failed to notice that the "check trans" light stayed on. More on that later.

Step 2. React and Trace.

Using the wiring diagram, I traced all of the things that were in line from the wire bundles that attaches to the battery. In our case, it is a negative wire with a secondary return line on the same battery terminal and the positive with the main positive and a secondary positive that feeds the fused 12v supplies to the ECM and other drive components.

This is where the carnage first showed itself. The reverse polarity had blown 4 fuses. Some 7.5 and 10amp.

Step 3. Fuses

The trace processes is repeated until you find all faults between the key switch and the starter solenoid. This means the fuse bundle that comes from the wire bundle directly attached to the battery's positive (red) wire. In the case of my vintage, it is several 7.5 and a few 10amp fuses located in my electrical bay in the back corner of the coach. I also inspected the large capacity solenoids in the engine bay. They include two grid heater solenoids and the starter solenoid. After 22 years, the contacts were quite worn with one 5" red 6awg cable that connected the two supply sides of the grid heater solenoids being melted through the insulation. (Replaced that with a high quality 6awg MTW and new lugs).

It was a good excuse to clean up all of these contacts in hopes of improving conditions in general.

NOTE: The simple act of cleaning the contacts, nuts, lock washers, and the cable terminations make a much higher quality contact and seems to have improved the starting process in general. The engine started with fewer cranks after sitting for 6+ months without a start and a battery that had just been very abused.

Step 4. Trace again.

With all of this work done, there still was not the satisfying click of the starter solenoid. This is when I noticed that the NSS wire is the wire that contacts the solenoid. This is the Neutral Safety Switch that actually drives the starter solenoid. This is a part of the transmission control system that ensures that the coach is not started in gear. There is a relay within the VIM (Allison box in the driver's side exterior electronics bay). There are 6 relays and two fuses in fact. The relay that drives the NSS is fed from one of those 10amp fuses and will not allow the starter to engage as it can't close. I checked both 10amp fuses and only one was blown. Replaced and BAM...

Step 5. Start Your Engines!

That's right, it started right up! The coach is running well, though I haven't attempted to move yet. There are still a few electrical gremlins in the works, though I believe those are unrelated to the original issue.

 

What's next:

It is possible that the alternator is not working as the engine battery was reading 12.08v at a slightly elevated neutral run level. I'll need to find out if a hidden fuse is blown in where the DUVAC controlled alternator lives. OR, the battery is so toast, it won't even allow for charging.

You sound way too advanced for me.  My only comment on your alternator is that It is, I suppose, possible to damage the diodes.  A good alternator shop should be able field that question.  If you have to rebuild the alternator, make sure that the shop orders genuine Leece Neville parts….NOT altermarket.  The diodes and voltage regulator are the parts that I would replace, just on general principles.  
 

You knowledge and perseverance is top drawer.  Good job.  Thanks.

[Rocketman3]

Good Luck getting everything fixed!

Your description brought an unrelated thought to my mind…

We are full-timers and boondock a lot!  This last year I heard of two different occasions where a MotorHome was stolen when the owners were away. I was thinking of a adding a switch somewhere that would prevent the rig from starting. (Even if the switch up front was hot wired).
 

From your account- it I add a switch onto the wire between the neutral safety switch and the starting solenoid - the rig cannot be started without that switch being on. 
 

Is there any downside to doing that?

 

Thanks!

[Bjohnsonmn]

1 minute ago, Rocketman3 said:

Good Luck getting everything fixed!

Your description brought an unrelated thought to my mind…

We are full-timers and boondock a lot!  This last year I heard of two different occasions where a MotorHome was stolen when the owners were away. I was thinking of a adding a switch somewhere that would prevent the rig from starting. (Even if the switch up front was hot wired).
 

From your account- it I add a switch onto the wire between the neutral safety switch and the starting solenoid - the rig cannot be started without that switch being on. 
 

Is there any downside to doing that?

 

Thanks!

That's a good question. I am not sure that specific location would cause issues, but it could add resistance?

One other option you may have is using the remote start switch in the engine bay if you have one. We have a small black box in the engine bay near the outside big hatch that has two rocker switches. One is a start button that is held similar to holding a key in the start position. The other is a three position selector. Up is front start. Middle is NO start, Down is rear start. I believe I inadvertently figured out that it has a similar effect to a disconnect in that it is like a selector for the ignition wire. I also don't think the rear start switch works without the key in the run position up front, but I will test that for my own purposes as well.

Super simple to test on your rig if it has one without needing to do any wiring, cutting, or switch installing!

[cbr046]

Could always pull a fuse or two while in storage . . . . Better yet install a blown fuse.  What thief is going to troubleshoot a non-starting rig?  Nope, he'll just move on to the next one.

- bob

 

[Dr4Film]

16 hours ago, Bjohnsonmn said:

 I also don't think the rear start switch works without the key in the run position up front, but I will test that for my own purposes as well.

That is correct. The ignition key has to be in the run position for the coach to start from the rear AND run.