Starter relay

[jhaggerty]

Hi Folks,

I am working on my 83 MC9. The bus gets used quite regularly but now I have a problem with starting. The start relay is located in the rear junction box. I have  toggle switch with the push button start. The one large terminal is energized at all times. When you push the start button the smaller two terminals are energized. However the other large terminal is not energized at all. According to my wiring schematic, that wire runs to the solenoid to start the engine. This is a brand new relay that I just put on because the other one was doing the same thing. Is there something else that I am overlooking? Also, can I jump between the two larger terminals and see if it will start or will I screw something else up? Thanks for any and all advice.

Jim

[Jriddle]

Not sure this will help any but I had problem and found fuel pressure switch wiring to be bad.
http://www.busconversions.com/bbs/index.php?topic=17237.0

John

[jhaggerty]

John,
I saw that in the schematic. The smaller wires go thru the fuel pressure switch. I figured if they had power , it would be alright. But... maybe not. Mine is an 8v71. Where is the fuel pressure switch located?

Thanks.

Jim

[thomasinnv]

fuel pressure switch is located on the fuel filter. (that be the second one)

[Chopper Scott]

I fought something similar before. The switch grounds out with fuel pressure and disables the starter. When you are running down the road and the engine shut down happens due to something like overheating you can hit the start button and over ride it and get to a better stopping place.

[jhaggerty]

Scott

That makes sense. If both those small terminals are energized it would mean the current is passing right through to ground. Right? Or is it just passing through to the press. switch? Schematic says fuel pressure switch opens above 8 psi. "Opens" in this case means breaks the circuit. Doesn't it?

Jim

[gus]

If both terminals are energized does that mean they are both the same voltage? There should be a voltage drop between the two if they are both ends of the primary coil which operates the relay.

Sounds to me as if you are not measuring the relay operating coil ends, but one end of it and one end of the secondary coil.

If you jump the two primary coil terminals the relay won't operate, you're then bypassing the primary coil and it won't work. You must ground one end of it or jump battery voltage to one terminal, depending on how it is wired. Some operate by being grounded and some are fed voltage. If you ground one terminal make double sure you have the correct terminal, if you ground the voltage side you will have some fireworks!

You can jump the two secondary terminals and the starter will operate unless there is another switch in series (fuel press?) which will keep it from working.

You can jump the starter solenoid directly from the battery lead to the solenoid terminal to check if it is the problem. Starter relays are infamous for failing since the terminals seem to corrode or loosen, I've had numerous problems with them, but the solenoid seems to go on forever.

[Jriddle]

Where is the fuel pressure switch located?

Here is link to picture of pressure switch. Mine is located on the left side of the engine as you look at it.
http://www.busconversions.com/bbs/index.php?topic=14563.0

John

[jhaggerty]

I think I have solved my problem. Definitely something to do with the fuel pressure switch. Having found it , thanks to your advice, I cleaned it off. Guess what. There was some oil on it! After cleaning it off, I took my heat gun and warmed it up a bit. Not hot, just warm. Went to the front of the bus pushed the button and away it went! I am assuming there was some moisture or condensation in the pressure switch. Our temps have been right around freezing, above and below, almost all winter, so I am assuming it was frozen. There could be either moisture from condensation or from the fuel. Either way, I am putting some diesel conditioner in the tank. The fuel is fresh, but there still could be moisture in the tank. Thanks for everyone's help and now I have learned a little more about this  MC9.

Jim

[gus]

Did you unplug any of the connectors? Sometimes they corrode and just unplugging and plugging back cleans the contacts off enough for them to work.

I've solved many electrical problems during trips with this method.

Those tiny relays don't use much current so they are easy to block by corrosion.

Anything electronic, especially when a computer is involved, is especially vulnerable to this minor corrosion because they use very low voltages.

[jhaggerty]

Bus is running fine now. I still want to replace the fuel pressure switch. I have the parts manual, but cannot find the pressure switch. I have looked in the electrical, fuel and engine section. Can anyone tell me what section it is located in, or better still the parts #. It is an 83 MC9 with an 8v71. Thanks

Jim

[Jriddle]

In my book Jan 79 Part number 8F-28-3

John

[JackConrad]

Or, call Luke at US Coach  856-794-3104   That is where I got my replacement fuel pressure switch.  Jack

[jhaggerty]

Thanks for the info.

Jim

[Chopper Scott]

Ask for a two wire switch if possible. If you get a one wire switch don't use thread tape!!!