-
Interesting Alternator Story
Just wanted to share a recent occurrence; I thought it was interesting and possibly worth sharing. The rig is a 95 Marathon XL45 and new to me by about 2 months.
The 12v electrical current throughout the coach was fluctuating between 11.7v-14.2v in a rhythmic fashion. This was with engine idling, disconnected from shore power and generator off, inverters on. There was a visible pulsation in ceiling puck lights and dash lights. The pulses sped up with increased engine speed but never stopped completely.
I had been chasing a problem with the MAC II air system (Marathon version of OTR air) in which the electronic controls for the system would alternately work/not work/malfunction. I finally identified the problem (with help from Red Dot who makes the a/c components installed) as fluctuating voltage rendering the controls either erratic or non-functional. The controls are fussy about steady current.
In chasing down fluctuating current, I checked the 50DN 24v alternator at the alternator posts and received the same fluctuating readings. The dash charge always showed 28 volts, and everything did work including inverters so I didn’t think that it was a failing alternator though this was a distinct possibility for a rig of this age. I pulled the external voltage regulator and had it tested, and it came back 27.7v which I am told is within specs. The leads to and from the regulator also showed the same variability and pulsation. The house and chassis battery equalizers all tested ok with .2v-.3v maximum difference between the 24v and 12v. So…..puzzling.
I cleaned and tightened all of the grounds that I could find, and also connections to each battery with no success. The system was charging and the batteries are new and held charge just fine.
A check of resistance between the ground post on the alternator and the large chassis ground bolt about a foot away showed between 600-800ohms. AHA. Should be close to zero.
Jumpering the alternator ground to chassis ground solved the issue. So what was going on?
The alternator was grounded to it’s case, which was grounded via connection to the engine. The alternator is held in a sort of cradle by 4 bolts, and apparently corrosion/dirt/age was interfering with the ground connection. The moving alternator provided the ‘pulsations’ as the alternator shifted in the cradle. Situation fixed by installing a battery cable between the alternator ground and the chassis ground bolt nearby.
At any rate – I have a friend to thank for suggesting that I check resistance on the ground side only. I could easily see where I could have pulled the alternator and taken it in for testing (my next planned step), at which point the alternator would test fine. After re-installing the alternator (and cleaning everything, of course) the problem would have been resolved with me never figuring out what the actual problem was.
I just wanted to share what I thought was an interesting situation with a simple fix, with the actual cause not being completely intuitive. I knew to check grounds, and in my own way did so without expanding my thinking, causing me to overlook a problem which seems obvious in retrospect.
John
-
7 Replies
-
Good info thank you for sharing.
-
Good job, a ground can cause all kinds of weird problems.
-
As our coaches age checking and cleaning and tightening grounds returns huge dividends.
I chased an issue that looked like an inverter problem, and it turned out to be white corrosion in the stack of cable terminals bolted to the frame. Externally the connections looked clean, tight, and perfect. Separating the cable terminals showed corrosion between almost all terminals.
-
Regularly spraying or dripping an anti-corrosion liquid onto every wire termination and component mounting point you can find is helpful in rooting out these seemingly mystery electrical issues. Corrosion-X found at marine stores I have found to be good at creeping into multiple wire terminations. It also works wonders as a lubricant between dissimilar materials and even plastics. Strangest of all uses is for mounting bicycle tires and allowing the tire to tube movement required to prevent pinching on the small 35-349 size tires on my Brompton bikes – the Folding bikes designed and made in the UK for multi-modal commuters that fit so well in the bays of our buses. I spray the Corrosion-X inside the new tire and on both beads before insertion of the tube and stretching of the tire onto the rim. Woks way better than talc and no downside I have found yet.
Jerry
-
Excellent sleuthing. Intermittent ground issues can be hard to find. Does it look like the bus was exposed to road salt in the past?
-
Hi –
No road salt; the rig has ‘lived’ most of its life in the South (Alabama, Texas). Having previously owned a rig which had been in the north for some time, I was exceedingly careful to check that aspect of things for this purchase and knew a lot more about what to look for.
There is no evidence of corrosion, salt, rust, etc. The rubber parts are uniformly degraded (!) as would be expected of a 25 y/o coach. The years take their toll. Otherwise, the rig is in very, very good condition and suffering more from lack of use versus outright neglect, etc.
That said – when not used, there are no opportunities for breakdowns and the resultant inevitable ongoing repairs so well known to us all. I purchased the rig with the understanding that 10-20k would be required to get the it where I want it to be.
John
-
Yes. Regular use and exercise of all systems is THE secret for our older coaches.
Log in to reply.