MCI 102c3 Overcharging

[rgrimard]

Ok guys see what you think. I have a 1992 MCI 102c3 with a Cat 3176 engine. Recently on a trip in South Carolina I noticed that the voltage gauge began to clime to over 30volts. When I pulled over I noticed that the batteries were quickly becoming over charged. I disconnected the field wire from the voltage regulator and ran the rest of the way on home to Florida on just the batteries.

I figured it was a bad regulator...but... just before I replaced it, I checked and found a field relay with a fused contact. I replaced the 24 volt relay and installed the new regulator. Now, when I fired up the bus, the alternator does not seem to be charging and I am only running off of the batteries. Just to make sure I put the old relay in to see if the voltage would climb instantly ...and it did...so I know the alternator is generating voltage. I only ran it for less than a few seconds.

On the 50DN alternator there is a F1 (field) terminal and a Relay terminal. Does anyone one think that there may be a bad relay in the alternator or how does the field relay (in the rear elecrical connection box) Is there an oil pressure signal  (or something else) that causes the alternator to come on line? Since the field relay was originally installed to help facilitate all of the heavy factory equiplent draws (ie..factory A/C) and I don't use the AC can this relay be bypassed? Thoughts?
Regards,
Ray Grimard

[gumpy]

Prettyr sure the field relay on your bus is controlled by the air pressure switch. It's a holdover from the belt driven, air adjusted tensioner days of old.

Check the voltage at the field terminal, and at the Positive terminal. Also check your ground. When I replaced my regulator with a solid state unit, I had to run a wire directly from the batteries to the batteries to get a stable voltage to the regulator.

Now, you said you replaced the field relay and it didn't work and you put the old one back in and it worked again, although iit was over voltage. That tells me the second relay is likely bad and the first is good, but the regulator may be sensing low voltage. Try hooking it directly to the battery and see if it stabilizes.

[rgrimard]

Thanks Gumpy. When you say hooking it directly to the battery, do you mean hooking it directly from the alternator (F) terminal to the regulator and bypassing the relay? (I am also replacing the old large regulator with a smaller solid state unit) which I will take you counsel and run a dedicated ground from the regulator to the battery ground.
Ray

[gumpy]

No, the F1 terminal goes to the field terminal on the alternator. It should have around 12+ volts on it (or half the voltage on the batteries). The field relay controls the sense voltage at the A1 terminal of the regulator. The regulator A1 is where you would connect the wire to the battery 24 volt terminal. Check your bus schematic. Pg 177 in the copy I have.

[rgrimard]

Awesome thank you very much. Ill let you know how I make out.
Regards,
Ray

[luvrbus]

Sounds like he has the classic MCI ground problem between the alternator and the voltage regulator if the regulator doesn't sense an ground they over charge 

[gumpy]

It's possible. I don't recall if the ground is local to the regulator, or if it goes back through the harness to the rear junction box.

[rgrimard]

Thanks Gumpy,
I extended the regulator wires so I can at least work on the regulator given the fact that the rear luggage compartment now houses the fluid tanks. Like I mentioned, I replaced the field relay and I have a solid state regulator now in place of the large Delco. I ran an external wire to the battery ground from the regulator ground and still no charging. Luvrbus mentioned a possible missing ground or faulty ground in the classic MCI style. Man it sure seems like the alternator is just not coming on line. I checked for a pressure switch and there doesn't seem to be any...am I missing something?

[gumpy]

Did you run a new 24v sense wire (POS) to the batteries? 

Did you check the output from the field terminal of the regulator, and also at the field post of the alternator?

You alternator is gear driven. The pressure switch is not part of your system. Your alternator starts charging as soon as the bus is started, even though the dash light may indicate otherwise.

[rgrimard]

Gumpy,
where do I run the new 24v wire?...from the regulator to the batteries ? Also,I did check the field terminal at the regulator and the on switch (in on the bus drivers console is on) it reads battery voltage 24v. However, when the bus is running there does not seem to be any voltage reading at the regulator. The filed terminal at the alternator while running is 24v. The alternator is belt driven on this bus. I apologize in advance for my obvious ignorance. I must be missing something.
Ray

[gumpy]

Well, it appears I may have given you incorrect information when I told you the field terminal should be 12 volts or half the coach voltage. Looking at the manual was of little help, but there is one reference which seems to indicate the field input into the alternator be 24 volts.  I have not had to mess with this stuff for many years and was trying to regurgitate from memories which evidently have long since failed me!

So, lets assume that's correct since that's what you're reading.

First of all, if you have a belt driven alternator, rather than a gear driven alternator, then the Penn air switch will come into play and will activate the field relay in the Rear Junction Box (RJB) when pressure get to something like 60 psi. Until then, the Not Charging light will be on, and the low air buzzer will be on, and your voltage at the batteries will be something less than 27.2. The output of this relay should be attached to the regulator POS terminal.

Note that some of this is speculative based on the MC8 and MC9 models because my 102 manual doesn't show a belt driven alternator; only gear driven.


The NEG terminal on the regulator should be attached to ground, though I can't tell where by the schematic in the manual I have.

The F1 terminal of the regulator should go to the field terminal on the alternator.

So, my suggestions:

1. Connect a line from the 24V Positive post of the battery directly to the regulator POS terminal.  Make sure you tape up the original wire you removed from the old regulator as it should be hot and you don't want to short it against the chassis.
2. Connect a line from the 24V Negative  post of the battery directly to the regulator NEG terminal.
3. Air the bus up to 120 psi using shop air.
4.Take a voltage reading across your battery 24V POS and NEG posts. Should be around 25-16V. Maybe a bit lower.
5. Start the bus.1
6. Take a voltage reading across your battery 24V POS and NEG terminal. Should be around 27.2V. If so, it's working.
7. Take voltage readings across the POS and NEG terminals on the regulator. Should equal battery voltage from above.
8. Take a voltage reading across the regulator F1 terminal and the regulator NEG terminal. Should be somewhere above 24v.
9. You can also take a reading from the alternator field post to ground. Should be the same as the regulator F1 reading above.

[bevans6]

The voltage at the field coil F terminal is a variable switched voltage, not a constant voltage.  The regulator switches that voltage on and off very quickly to control the average current in the field coil and the output of the alternator.  The switching happens anywhere from hundreds of times per second to thousands of times per second depending on the regulator, so a DC meter may well read a full 24 volts.  You might try a meter set for AC.  If a true solid 24 volts (or really whatever the batteries are at) is applied to the alternator it goes full on and outputs its maximum voltage that it can produce, anywhere from 30 to 90 volts depending on the load.  In this case the battery load is limiting the output voltage to around 30 volts

In the interests of science, and not related to this particular problem, here is a patent wherein Delco Remy has patented a switching regulator that uses inverter technology to improve the efficiency of the regulator.  The basis of inverter tech is pulse width modulation controlling MosFet output switches, and that is what this new regulator is doing, variable frequency and pulse width signal to the field coil.  http://www.google.sr/patents/US20100123439

Brian