Telltale tale

[Lin]

I spent the best part of the last two days trying to revitalize my telltales.  I've had some reasonable success.  Low Oil Pressure is working, Stop light is working, Hot Engine is connected but can't tell if it works without draining the coolant and running it for a while (which I will wait till its summer in the desert to try).  The Low Air Pressure is working somewhat; it cuts out at 30 psi and cuts back in even lower.  That one was a bear to fix since there were at least 3 separate problems (missing wire that was broken when found, bad flasher, and finally bad low pressure air switch.  It was filled with grease!  I disassembled, cleaned and reassemble it to use while I get a new one.  I am not sure if I have the Not Gen one right.  Should it come on when the ignition is one without the engine running, or does it only come on when the engine is running and there is a malfunction.  Also, is the light energized and grounded through the sensor, as some of the others are, or is it energized directly when there is a problem.  This work is only possible from having the wiring info from the book.  It would have been ridiculous otherwise.

[JackConrad]

Hot Engine is connected but can't tell if it works without draining the coolant and running it for a while (which I will wait till its summer in the desert to try). 

Lin,
   You can check the HOT ENGINE light by turning on the master "ignition" switch and grounding the hot engine wire at the sensor(s) on the engine.  Jack

[Stan]

Lin: Your Hot Engine light will not come on by running the engine without coolant. It needs coolant in contact with the sensor. Jack's suggestion will test the circuitry and to test the sensor you have to remove it and test it in a pan of water on the stove.

The Not Gen light does not have a sensor. It should be on when you turn on the Master Switch and stay on until the alternator is charging. The R terminal on the alternator picks up a relay which is the switching circuit for the Not Gen light and the HVAC system lockout.

[Sammy]

  Remember that this circuit works in conjuction with the low air switch and the field relay.
The "low air" pressure switch controls the coil side of the field relay.
When you have 100 psi of air pressure,the field relay contacts close,carrying voltage to the the "POS" terminal of the voltage regulator.This now lets voltage regulator send a controlled field voltage to the alternator from "F" terminal of voltage regulator to F1 stud on alternator.
At this point (air pressure at 100 psi, bus running) the alternator should now be charging, and producing a 12 v output on the "R" stud on alternator.
This output voltage energizes a coil in the Discharge Relay, which controls the telltale light - as Stan has already said. This discharge relay is located in the rear electrical box.
Hope this may help. Good luck with your repair 

[Lin]

Thanks, this is really good info.  I will have to absorb it for a bit.  Is the not gen light supposed to be energized full time so, as with the low oil pressure light, the discharge relay would shut it off by interrupting the ground, or does the current come from the discharge relay to the light which should be grounded in the cluster like the stop light telltale?

[Sammy]

Lin,
I'm using a diagram from a 1978 MC-5C.
It shows telltale lights grounded on one side all the time, power to lights controlled by discharge relay.
It's the closest diagram I have to your bus. Might be different, could be the same. Something to lead you in right direction. Good luck. 

[Stan]

Lin: So you don't get too confused, air pressure is not involved in a 5A like the later model MCI buses. The alternator belts are always tight and the voltage regulator gets power as soon as you turn on the master switch (unless the overvoltage relay is energized - which could only happen if you have a battery charger hooked up with a stopped engine). With field voltage applied, the alternator will start to generate voltage as soon as the engine starts. When the R terminal reaches sufficient voltage to pull in the Discharge Relay the Not Gen light will go out.

The circuitry is on the Charging System Wiring Diagram. When you turn on the master switch, voltage goes to terminal 14, then to a 6 amp breaker and then to the Not Gen bulb. The ground side of the bulb goes to terminal 11 in the front panel then to terminal 20 in the rear panel and wire #5 on the Discharge Relay which makes the ground.

The Not Gen light is completely separate from the other Telltales and is not fed voltage through the diodes but directly from the front panel. HTH Stan

[Sammy]

Stan, thanks for the updated info on the early MC-5's. I only have info for the 5C's we ran. 

[Stan]

Sammy: If it is any help, MCI used the same wiring system from the early 50s up to and including the MC-7. In the very early buses they had generators and no A/C but the same wire terminal and wire number did the same job. They just added more terminals and wire numbers when they added something different or new. With the introduction of the MC-8 they made a lot of changes and those were incorporated  into the MC-5B and then with the MC-9 they added the same changes to the MC5-C. After the 9, I have no idea what they did, as I haven't worked on them.

[Lin]

Thanks, I think that I've got it now.  The bulb can be energized from any source then since it grounds through the #5 wire.  I connected it, and the Not Gen light now comes on with the Main switch.  I can't start the bus because it is indoors, but will get a chance on the weekend. The only ones left are BU lite telltale, but I do not have backup lights, and Emerg Brake telltale which may need some looking into.

[Sammy]

Stan, that info is very helpful. Always interested in more info for my personal library.Much appreciated. 

[Stan]

Lin: On your previous post about the low air pressure switch - There is nothing special about it. All it does is complete the ground for the buzzer and the Low Air Telltale. The switch should close about 65 PSI.

The Backup Light Telltale comes from terminal 41 through the flasher to the bulb. Terminal 41 also feeds the backup lights. Depending on your serial number, there may be a diode between the 6 amp breaker and terminal 41.

One of the most important Telltale lights which you haven't mentioned is the blower compartment doors. There should be a switch (like a car door dome light switch) on both sides that turn on the Blower Door Telltale. If the blower door is not positively latched,  the engine overheats.  HTH Stan

[Lin]

Stan,
     I have found the source of one of my confusions.  The coach does not have backup lights.  There is a telltale light on the cluster though.  I had inspected the rear several times to find the back up lights but there were none.  Now it appears to me that the reflectors on the bumper, which are just like the ones on the side bumper, are covering up the holes where the backup once were.  I do not have a blower door tell tale on the dash, not do I find wiring for it listed in my book.  Was that for a later model? 

[Stan]

An interesting point on the blower doors. Looking back at old serial numbers, there is no alarm on them.  I don't know when they started but they are on buses after SN 5493 (1965). If you don't have the alarms, make sure the rubber seals are good and the doors are latched tight,