Bad battery isolator ??

[John Kaehms]

I have been following this thread and I am truly impressed by the technical knowledge of “twinboat”. Therefore, I would like to ask some additional questions.

My earlier post spoke about the problems I had with the Trombetta relay in our Monaco Cayman that was solved by changing to a Blue Sea relay. This problem is only one of many electrical gremlins.

We bought the Cayman from a private party in 2012. On our first long trip I noticed the volt meter was gradually beginning to rise from 13.2 volts going up past 16 volts and then would drop back to about 13 volts. Thinking there was a problem with the alternator voltage regulator, I had the alternator checked and a new regulator installed. The alternator is a Leece Neville. I also installed an additional volt meter in the dash to cross check the factory dash gauge. The problem with the dash gauge continued but I could see from the secondary gauge that the voltage was remaining in the normal operating range. If I stopped and shut off the coach the gauges would re-set back to normal. The same type of problem has on occasion plagued the engine temp gauge, the odometer and speedometer. The gauges would malfunction but could be re-set by stopping and shutting off the coach.

Because of repeated gauge problems I installed a Scan Gauge connected to the OBD port. The readings from the Scan Gauge stay normal even when the factory gauges act up. Following other threads I installed a dedicated chassis ground wire to a dash gauge wire marked ground. Problem still exists. Took the dash panel apart looking for a MMDC. The wiring harness to the dash gauges goes through the firewall but I could not find a MMDC. Traced the harness to the motherboard below the driver. Found several loose white plugs. I checked these plugs and siliconed them in place. Problems still happen but are not as frequent or dramatic. I suspect that somewhere there is a a corroded relay that will re-set when the coach is turned off. Does anyone have any ideas what could be causing these gremlins?

[YC1]

John, if there is a voltage regulator somewhere inside the gauges they could be getting sick. Sometimes components actually change value after they run for a time. Or heat and cold for example can cause components to act strange.

Without some precise measurements at the input to any of the gauges this doesn't help you but I have seen hundreds of component failures over the years and capacitors are more common than resistors changing values but temperature compensating components can lose their minds.

Some components are designed to change value under temperature changes. The remote sensors for air conditioners are an example.

[Migkiller]

The coach is at the rv dealer for repairs !

Since we live remote its very hard to get tow companies to come up here. After some thought i elected to drive it the 60 miles to the dealer. No issues at all ( other than the V gauge at 16 V ). Purred right down the road at 65 !!!

Explained everything to the service rep...he quit writing after about 10 minutes. They will do a complete check of the charging system first off. I told him if they need some explanation on things i can come and explain them...he said that might help if they get stuck !!! Also it looked to him that i pretty much found the culprit in it being the gauge, but they want to double check everything.

The reason they don't work on rv's over 10 years old is that parts are to hard to get.

The going hourly rate is $159 an hour...ouch !!

[oceansailor]

Originally Posted by twinboat

I believe voltage drop across a diode depends on current flow.

No. Diodes are junction devices, which means they have a constant voltage drop. You may be thinking of resistance across it... which is non-linear and does depend on the applied voltage.

Back in the old days, we used germanium diodes for power circuits, which have a 0.3v drop (rather than the typical 0.7v for silicone.) However, the latter is more reliable - so it's taken over the market. The right answer for power circuits these days is either a sense line from the batteries - which provides feedback to the alternator so that it can increase the voltage appropriately (but could fry any sensitive electronics that are connected before the isolator), or (a much better option) a combiner, which uses either a relay or a high-power junction device with essentially zero voltage drop.

[oceansailor]

Originally Posted by John Kaehms

On our first long trip I noticed the volt meter was gradually beginning to rise from 13.2 volts going up past 16 volts and then would drop back to about 13 volts. Thinking there was a problem with the alternator voltage regulator, I had the alternator checked and a new regulator installed. The alternator is a Leece Neville. I also installed an additional volt meter in the dash to cross check the factory dash gauge. The problem with the dash gauge continued but I could see from the secondary gauge that the voltage was remaining in the normal operating range. If I stopped and shut off the coach the gauges would re-set back to normal. The same type of problem has on occasion plagued the engine temp gauge, the odometer and speedometer. The gauges would malfunction but could be re-set by stopping and shutting off the coach.

Because of repeated gauge problems I installed a Scan Gauge connected to the OBD port. The readings from the Scan Gauge stay normal even when the factory gauges act up. Following other threads I installed a dedicated chassis ground wire to a dash gauge wire marked ground. Problem still exists. Took the dash panel apart looking for a MMDC. The wiring harness to the dash gauges goes through the firewall but I could not find a MMDC. Traced the harness to the motherboard below the driver. Found several loose white plugs. I checked these plugs and siliconed them in place. Problems still happen but are not as frequent or dramatic. I suspect that somewhere there is a a corroded relay that will re-set when the coach is turned off. Does anyone have any ideas what could be causing these gremlins?

John, I would say that YC1 has it right in a number of ways: analog gauges get old. But a consistent rise, then drop in voltage indicates some kind of a regulation problem, possibly a temperature-related one... a gauge that's off would be off consistently, not intermittently - and certainly not in a varying pattern. Making sure that you have a solid ground at the dash is a very good idea, since yours might have been temp-related (loose connection that gets better due to heat-based expansion, then gets worse as it cools) - but since that wasn't it, I'd suspect some wonky control circuitry at the alternator. Very, very unlikely to be related to the MMDC.

One of the cheapest solutions to getting a reliable voltage reading - I seem to recall someone (twinboat?) mentioning it here - is by plugging in a cigarette-lighter type volt meter. I use one that I'm very satisfied with; it has a couple of high-power USB charging ports, which I use to power my SilverLeaf monitor and my tablet/GPS, and it also tells me the current going to each of those. A little pricey as compared to similar ones, but I had several others that blew out...

https://www.amazon.com/gp/product/B08F25BP4X

If you're still having trouble, I'd run a temporary wire directly from the back of the alternator to the driver's position and hook up a voltmeter to it. If you're still seeing that kind of voltage fluctuations, you're pretty much down to a bad regulator (often internal to alternators but replaceable) or a bad sense wire to the battery (which may or may not be there, as we saw in this thread.) Good luck!


For myself, I've actually come to the point where I don't trust the dash gauges at all; the ones in my 1999 Monaco Windsor have lied so much and so randomly that I bypassed the whole thing and installed a SilverLeaf monitor. Love that thing, and it gives me a lot of peace of mind when driving down the road. Might be a solution worth keeping in mind.

[John Kaehms]

Thanks to all who replied to my questions about Monaco Cayman gauges. Here is a little more detail.

The gauges are Medalion Gauges. Apparently Monaco daisy chained all the grounds. Consequently they tend to act like a string of Xmas lights. A problem with one impacts several.

Does anyone know what makes the gauges “cycle” prior to starting the vehicle?
Is the ECM module controlling this function ...or is there another device? I cannot seem to find a MMDC (multiple module data computer). Gauge harness goes through the firewall to the mother board. Mother board is mostly fuses but has a number of small plug in relays. I have not been able to locate a reliable electrical schematic to help diagnose the issues.

Additionally the coach has a Banks IDash Tuner. The dash problems were present before the tuner was installed. The tuner has gauge functions in the display which coincide with the Scan Gauge readings. However, the tuner can be finicky. The tuner and the ECM need time at start up to boot up properly together. It may take several start attempts starting and stoping the engine before the coach and the tuner will work together. Once the coach and tuner are in sync the tuner provides amazing performance. Banks says that the current draw during the start process will throw an error code into the tuner if the start voltage falls below 10 volts. I have two each group 31, 850 cca batteries that are only 6 months old. The Scan gauge shows 12.6 volts prior to start but will drop down into the low 10 volt range while cranking the starter. Does this sound like there is a starter or starting solenoid problem? I will gladly change and up grade the starter if there is a better aftermarket product than the stock starter.

Once again thank you for all your comments.
JK

[YC1]

The gauges cycling like they are testing may be the medallion system itself. I can go into a test mode on mine and it will move the gauges to full scale and back.

[oceansailor]

Originally Posted by John Kaehms

Thanks to all who replied to my questions about Monaco Cayman gauges. Here is a little more detail.

The gauges are Medalion Gauges. Apparently Monaco daisy chained all the grounds. Consequently they tend to act like a string of Xmas lights. A problem with one impacts several.

Ugh. The problem with daisy-chaining is that the resistance of every single wire and connection is additive... I spent many miserable lifetimes crawling under houses with a jackhammer when I worked in the alarm industry, mostly due to old, daisy-chained systems of alarm sensors.

You'd probably be better off spending the hour or two that it would take to home-run all of the gauges. If you were looking for a project, that is.

I have two each group 31, 850 cca batteries that are only 6 months old. The Scan gauge shows 12.6 volts prior to start but will drop down into the low 10 volt range while cranking the starter. Does this sound like there is a starter or starting solenoid problem? I will gladly change and up grade the starter if there is a better aftermarket product than the stock starter.

In all of my experience - and I spent a number of years troubleshooting boat power systems in the Caribbean - it's poor connections much more often than it is the batteries or the starter/solenoid. Make sure your batteries are fully charged, and maintaining that charge; check them directly at the terminals with a good meter after they've been charged and then rested without any load for at least an hour. If they drop much below 12.7, check the cells with a hydrometer and replace the battery if it has dead ones. Next, put a decent digital voltmeter between the positive battery terminal and the positive terminal on the starter and have someone crank for a few seconds; the voltage drop should not exceed 0.25V (and it should be less - a really good connection should be about 0.1V.) Repeat it for the negative terminal/starter body. Clean and tighten - and I mean REALLY crank down, with star washers to bite into the metal if needed - any connections between them, including the battery terminals themselves. (I once discovered a battery terminal clamp that was creating a 0.3V drop all by itself - and limited the charging current going into that battery. That was a bear to find in a large bank of batteries...)

Overall, if your system is healthy and your batteries are properly sized, even cranking a big diesel for half a minute should not take them below 11V when fully charged - at least not for more than the initial fraction of a second. But the above procedure should get it as good as it can be. You'd be surprised how many starting problems I've made disappear with just that simple set of tests and fixes.