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Author Topic: Which Relay is It ? MC 8  (Read 2271 times)
BC Bus
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« on: December 09, 2008, 12:50:58 PM »

On an MC 8 looking at the rear Junction box  behind the driver's side tag axle which relay is the 12v one that comes from the 12v post on the Alternator.  On the MC 8 that I just bought all of them have 24V relays in place and I know that is wrong.  PS.  My service manual is enroute but I need help with this today if possible.

Thanks in advance
« Last Edit: December 09, 2008, 01:55:51 PM by BC Bus » Logged

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Melbo
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« Reply #1 on: December 09, 2008, 01:39:36 PM »

I don't have a junction box on the passenger side in my 8

did you mean the drivers side??

Melbo
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« Reply #2 on: December 09, 2008, 01:55:09 PM »

Yes I meant driver's side.  Ceniality has it's advantages..everyday you meet all new people
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« Reply #3 on: December 09, 2008, 02:07:05 PM »

Should be the 3rd one up from the bottom - inside the J box - Discharge and Blower Cut-In relay.
It gets activated with 12 volts from the "R" terminal on the alternator.
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Sojourner
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« Reply #4 on: December 09, 2008, 04:37:27 PM »

Here an attachment of the rear relay junction box.

FWIW

Sojourn for Christ, Gerald
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Ps 28 Blessed be the LORD, because he hath heard the voice of my supplications. The LORD is my strength and my shield; my heart trusted in him, and I am helped: therefore my heart greatly rejoiceth; and with my song will I praise him
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« Reply #5 on: December 10, 2008, 07:44:20 AM »

Thanks everyone so far.  The blower cut-in relay is buzzing and vibrating and the No Charge Tell Tale light is flickering, but only part time, if you rev up the motor, and back off, rev and back off, eventually the no charge light goes off and the blowers start and it i is charging fine.  This all happens with full 120 PSI, not low air.

Here is my theory:  Is it true that the ground for the blower cut-n relay goes through a relay that is energized by the air pressure? If so that relay is probably bad or not grounding. 

I will try grounding the GND post of the blower cut in relay to see if that stops the flickering.
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« Reply #6 on: December 10, 2008, 09:56:54 AM »

I'm having a problem  with that circuit also, is your system actually charging , i would check it with a muliti meter at the batterys, check it shut off and then with it running . The ground switch you are referring to is  keep the alternator from charging until the air pressure is up ,if you have a belt drive alternator,  mine says on the j box cover that this switch is not hooked up this wayon mine it only controls the tel tale , iwould take the ground off the switch , should be the only switch in the j box with air going to it. run that ground to chassi ground in the j box or better yet on the alternator ground on frame. This will eliminate the air switch, i called mci support , this is the way they told to to diagnose it, on my system my voltage is actually flucatuating around however it is still  charging, they had me remove the postive wire going to the voltage regulator and run a wire from battery to this post on regulator, it stopped this flucating problem. SO my problem is somewhere in the regulator circuit , my coach is in florida , had to come home befroe i got it figured out , however i think it is the circuit braker that supplies the power  to the field relay circuit.
   Kevin
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« Reply #7 on: December 10, 2008, 11:00:27 AM »

If you keep the air tensioner for the belt drive alternator, you really want to get the Penn switch that controls the no gen/low air/blower inhibit circuit returned to proper functioning.

Allowing the alternator online and under load with slack belts, the belts will be harmed by the heat of slipping, and you risk throwing them off.

happy coaching!
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« Reply #8 on: December 10, 2008, 11:32:41 AM »

My coach has had the air tensioner taken off, i have a gear drive alternator so mine it not wired up for this.
  Kevin
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Sojourner
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« Reply #9 on: December 10, 2008, 11:00:08 PM »

Thanks everyone so far.  The blower cut-in relay is buzzing and vibrating and the No Charge Tell Tale light is flickering, but only part time, if you rev up the motor, and back off, rev and back off, eventually the no charge light goes off and the blowers start and it i is charging fine.  This all happens with full 120 PSI, not low air.

Here is my theory:  Is it true that the ground for the blower cut-n relay goes through a relay that is energized by the air pressure? If so that relay is probably bad or not grounding. 

I will try grounding the GND post of the blower cut in relay to see if that stops the flickering.

Is it true that the ground for the blower cut-n relay goes through a relay that is energized by the air pressure?

Yes.

The Penn air control is DPDT (double-pole/double-throw) switch.
No.1 pole is connected with the “Discharge Relay & Blower Cut-In” coil to ground after it reaches 100 psi and open at 70 psi.
No.2 pole is connected with the “Alarm & Low-Air Tele-Tale” lamp and flasher wiring to ground after it reaches below 70 psi

The Discharge Relay & Blower Cut-In is 12v (half of 24v) SPDT (single-pole/double-throw) switch.
No.1 pole is from the “R” terminal/alternator to relay’s coil to ground.
No.2 pole is from the “Master Switch/Rear-bus-bar to “Discharge-Rear” and Discharge-Front or “Not Gen” lights with ground.

If the charging system is within the specification then the following.

I am not an MCI full fledge and shop experience mechanic but here what I think.

Flickering or fluctuation at idle can be either one of two things:
1)   One stator (that is connected to "R" terminal) winding within alternator is slightly weak
2)   Discharge Relay is getting tired to hold armature down firmly.

I have 3 attachments of photos.

FWIW

Sojourn for Christ, Gerald
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Ps 28 Blessed be the LORD, because he hath heard the voice of my supplications. The LORD is my strength and my shield; my heart trusted in him, and I am helped: therefore my heart greatly rejoiceth; and with my song will I praise him
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« Reply #10 on: December 10, 2008, 11:02:16 PM »

#2 photo
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Ps 28 Blessed be the LORD, because he hath heard the voice of my supplications. The LORD is my strength and my shield; my heart trusted in him, and I am helped: therefore my heart greatly rejoiceth; and with my song will I praise him
Sojourner
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« Reply #11 on: December 10, 2008, 11:04:44 PM »

#3 photo of MCI-8 idiot lights panel.
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Ps 28 Blessed be the LORD, because he hath heard the voice of my supplications. The LORD is my strength and my shield; my heart trusted in him, and I am helped: therefore my heart greatly rejoiceth; and with my song will I praise him
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« Reply #12 on: December 11, 2008, 01:40:08 PM »

Gents,
 BC mentions revving the engine and finally light goes out and relay stops buzzing.I'd make sure that the charging system is functioning properly,including condition of both batterys.
The "R" terminal of the alternator is an output - equal to half the output voltage found at positive post of alternator. If you don't get at least 13.2v or better while charging, it doesn't have enough voltage to close contact of Blower relay.The ground side of that same Blower relay is controlled by the Penn air pressure switch. Make sure the ground connections of that switch are fine, clean and secure. If all these checks are fine and it's still not working, jump Black-12 wire on Penn switch to a known good ground and blower relay should energize - if relay has power from rear J-box and is in operating condition.
Gerald, thanks for posting those pages from your manual. Cool


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artglass
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« Reply #13 on: December 19, 2008, 08:23:32 AM »

was wandering what you  figured out your problem was?
   Kevin
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« Reply #14 on: December 20, 2008, 09:42:27 AM »

The system was charging fine but the idiot light and the blower cut in relay was not closing.  Replace that relay and all is working again fine.  I received my shop manual so it is a lot easier solving problems now. 

I put in a vanner equalizer and am now wiring 12v headlights without the cente tap, series stuff.
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