What is that little oil pressure switch?

[Nonchalant1]

I discovered an oil leak on port engine (8V71TI on a 1978 53MY) that was coming from a little oil pressure switch screwed right into the RH side of the engine in the oil gallery next to the oil pressure sending unit. It's a little black 1" cone shape on a metal 1/8 pipe thread base with a single spade type electrical connection on the top and one wire attached to it. Anybody out there that knows what it is and what to replace it with?

I can't track down the wire easily because the wire has no number and there's nothing like it on my schemactics. Unfortunately, my alarm system doesn't work so I can't test that (another job for this winter!)

I called Steve McPherson at Sam's Marine since he's the best around, and he thought it was connected to a number of things like maybe turning on the alternator, turning off the battery charger and/or signaling the alarm system that oil pressure is OK. I bought a "similar switch" from him that has 2 terminals instead of one. He suggested connecting it to the "normally open" terminal on the new switch. However, I broke open the switch and looked inside and it's a simple switch which normally passes current through it ("closed"?) and when there's oil pressure it breaks the circuit ("Opens"?)

Any knowledge about this would help.

Thanks,

Doug Shuman

[BEC53]

I had one on a 43 Tiara that was essentially a starer lockout so that you couldn't accidently engage the starter while the engine was running. That was on a 6v92. You might try starting the engine with the circuit open and then with it closed to see if it is actually part of the starter circuit.

[Genesis]

That switch is likely for the alarm system, showing low/no oil pressure.

The other possibility is a starter relay interlock, but if you don't have TWO switches, my money is on the alarm system switch.

That switch is usually normally closed (no pressure) and opens under pressure.

[Nick]

If it is an additional switch to the alarm mentioned above, It may be an oil pressure switch used for an interlock to the battery charger. I recall seeing an electrical diagram for our 37 that showed the interlock. The purpose was to disconnect the battery charger and prevent it from charging the batteries, while the main engine alternators are charging the batteries and the genie is running. I'll look for the schematic.

[Genesis]

There is such an interlock with some of the chargers used, such as the Sentry ones.

The problem with a "hard interlock" like this is that if your alternator craps out your charger can't pick up the slack! That's not too terribly good, IMHO, and on a lot of Hatties which have split 12V systems (one side starts one main and the gen, the other starts the other main and sources house loads) you can wind up with a dead house side and no electronics offshore if you have an alternator fail! Been there, done that, and my next act was to modify the interlock!

If you're going to lock out when the main is running, you need to lock out only the engine that is running, and only when the alternator is producing an actual charge.

[Nick]

Karl,

You are right, I have the sentry charger.
How did you reconfigure the interlock to running engines?

Nick

[Genesis]

... the interlock, that is.

The Sentry charger that Hatt supplied on these boats has a shutdown circuit that senses the battery voltage of the "control" bank (make sure this is the HOUSE bank!) and turns it off when that bank is fully charged.

When the alternator is actually charging that bank, the voltage will be high enough to shut it down all on its own.

The ignition interlock is neither necessary nor, in my opinion, desireable. YMMV, of course.

There is nothing wrong with the charger running (assuming it has 110V power) when the mains are up, since it turns off when the batteries are fully charged. Until the batteries are fully charged, having it provide the amps it is capable of only restores your batteries quicker - it doesn't hurt anything.

The standard Sentry charger is only a 40 amp unit anyway. The alternators typically can provide much more current. But if the batteries are truly low, then I'll take all the recharge rate (within safety limits - not an issue in this case) I can get.

My Sentry got flaky (it was 20 years old!) and was replaced with a Charles 5000 series unit this year. Much lighter, still 40 amps, mounted in the same place the Sentry was. The only complaint I have with the Charles unit is that their battery terminals aren't bolted; rather, they are "cinch terminals" which I personally dislike. But Charles is known for making bulletproof stuff, so I'll deal. Its a modern 3-stage (bulkl/absorption/float) charger and will pick up DC loads up to its capacity without tripping back into "charge" mode (from float) as well, which I really like.

I have no "override" or "lockout" on the Charles, nor do I want one. It goes to float and produces essentially nothing when the mains are online, but remains up and operating in the event of an alternator failure.

I have reconfigured things a bit on my boat to stop a persistent battery boiling problem that I used to run into.

Specifically, my AC charger feeds ONLY the House/Port bank, since the STBD bank should NEVER be depleted (that bank is only used to start the starboard engine and genset.)

I then have a combiner between Port and Starboard banks which closes when the house bank is sufficiently charged to carry itself, or when the Starboard bank is charging (thus "shadow" charging the port side). This way if EITHER alternator works both banks get charged, but the one the alternator is connected to gets charged FIRST - which is as it should be.

If the AC charger is energized and the house bank needs a charge, it gets it. Once its nearly full, then the combiner insures the other bank gets floated. When the banks are restored, the charger shuts off. No more batteries eating water.

The simple way to do this is to remove the fuse in the main panel for the Starboard charger connection, and add the combiner. There were spare terminals in my main panels to connect the combiner to; I only needed to add the appropriate (80 amp) ANL fuses.

IMHO dual-output charger connections are unwise. Use a combiner if you want to restore more than one bank from a given charger, or install multiple independant chargers.

I've considered re-doing the entire mess and going to a big 6V/golf car bank for the house, rewiring both mains to run off one bank, and adding a small AGM battery for the genset. My thinking on this modification goes something like this:

1. The house bank is thus isolated from either engine, thus, if its flattened it has no effect on being able to start the mains. The AC charger would be connected only to this bank.

2. The mains bank could only be flattened by starting the mains. It would be charged by both main alternators. A combiner between it and (1) would dump all EXCESS charge, once engine starting restoration was complete, to the house bank, thus giving the house bank rapid recharge (VERY rapid!) once restart was assured.

3. As a tertiary fallback, the genset would have its OWN starting battery. It powers the AC charger, and the combiner between the house and starting bank insure that once the house bank is reasonably full it will restore the starting bank as well.

The manual parallel between (1) and (2) would be retained for emergencies.

The only thing that has kept me from doing this is the battery box mounting. My boat ('85 45C) has the batteries under the cockpit floor (a good location - easy to get to) in custom fiberglass boxes - 4 8Ds, two in each bank. Nothing else is going to fit in those boxes correctly, which means I have to rip them out (that will NOT be easy) and then fabricate appropriate replacements. The genset battery can go under the AC compressors (there's room where there used to be an oil tank which at some point was removed) but there's no other good place for batteries to go on the boat, and I really like the cockpit location - you can actually change a battery there without throwing out your back or being a contortionist.

[Nick]

I am with you on separate charger for the house side battery. I assume the combiner acts as, or is an isolator for a two aternator 4 battery configuration. Is that right? Otherwise I believe a dead cell in one of the p or s battery banks could take out the two batteries in the bank if they are not isolated?

[Genesis]

... is a bit different than an isolator.

Its a relay with some smarts in it.

Basically, here's how it works.

If EITHER bank is over about 13 volts, it closes and allows the two to be interconnected. The inrush and constant current is limited by the wiring used (#6); the continuous rating is in the 50A range, with inrush currents of up to 100A permitted. I use an ANL fuse in the panel for the connections to protect the wiring and prevent a short from causing a fire - there are probably situations under which that fuse could blow even though there's no malfunction, but I've not had trouble with it yet.

When charging ceases, the relay OPENS. This separates the two banks and prevents one from dragging the other down.

When charging re-commences, the bridge is left open until the battery comes up to a fairly well-charged state, at which point it is closed to allow both battery banks to be topped off. This is true regardless of which bank comes up first.

You can also force it either on or off with a control terminal if necessary, although using it as a parallel relay isn't too smart, since it can't sink enough current to really do that job.

Yandina makes them, and West sells 'em. There is a small and large model

Isolators have a fairly major problem in that they present a voltage drop. This requires a charger with a remote sense in order to compensate, or you will chronically undercharge the batteries and over a few years this will destroy them due to sulfation. The combiner does not suffer from this problem.

The big change is NOT trying to play "multi-bank" with the charger. This is a recipe for disaster when you have two banks with grossly disparate charge levels - you are virtually guaranteed to boil the full batteries doing this, or undercharge the lower bank. Either way you lose and your batteries are damaged.

Check out www.yandina.com/c150Info.htm (http://www.yandina.com/c150Info.htm)

Not the cheapest thing out there, but it works. I've had one on my boat now for a couple of years and like what it does for me in terms of managing battery charge states.

[Nick]

Thanks!!

[Nonchalant1]

The oil pressure switch will pass current when there's no pressure, but interrupts the current if there is oil pressure. Is that called "normally open" or "normally closed"? Also, my battery charger (Newmar ABC 32-30) has a sensor for positive current to turn it off when the alternators are available, It just senses the ignition key in the on position from current to the gauges, etc.

Genesis - Does the alarm system work through sensing a connection to negative ground as OK when the engine is running? That's all this switch would do.

[Genesis]

The original hatteras alarm system (which actually monitors TWO things per light - not too cool) and the aftermarket systems I've seen (and have on my boat, which monitor ONE thing per light) use switches which close to indicate an alarm condition.

For example, the oil pressure switch closes when there is NO (or low) oil pressure. The temperature switch closes when the temperature is too high, bilge water closes when the water level exceeds the safe limit, etc.

There is a normally open switch wired to the hobbs meter, but all the Hatties I've seen with Detroits have that connectd to the secondary fuel filter (that is, the hobbs meter runs when there is positive fuel pressure in the system - that is, the engine is running)

[Nonchalant1]

Steve McPherson and Genesis figured it out. It's for the old style (pre-1979) alarm system. When the engines are running with OK oil pressure, the switch is open. When they're off or low oil pressure, the switch is closed. Steve sold me two original hobbs switches so I can replace them both. Maybe the wire runs to another switch at the secondary fuel filter. I'll find that out when I repair the alarm system.

Thanks!

Doug Shuman
1978 53MY