| Author | Message | ||
| Craig (Ceieio)
Registered Member Username: Ceieio Post Number: 313 Registered: 12-2004 Posted From: 206.212.230.165 Rating: N/A |
Greeting fellow bus nuts. I woke the bus up yesterday from her long winters nap. All is well except that the not gen light is on. Everything was working just fine when I parked her last October. I have been able to trouble shoot it down to the air pressure switch that enables the field relay that powers up the regulator. The air pressure switch closes a connection from the field relay to ground. That is not happening and I assume a connection needs to be cleaned or the switch is gummed up. If I ground this terminal the "not gen" light goes out and the voltage on the batteries jumps up to charging level. Here is my problem... I don't know where the air pressure switch is, nor do I know what it looks like. Can someone give me a hint on where to find the air pressure switch (and what it looks like)? Bus is a 1973 MC 7 8v71/auto. Thanks! Craig - MC7 Oregon | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 723 Registered: 1-2003 Posted From: 67.142.130.46  Rating: N/A |
Craig, If you no longer have the A/C blowers or other major electrical loads, you can probably just bypass the switch and forget about it. I'm open to being corrected by someone who is more familiar with MCI's, but I believe the intent of this switch is to keep the electrical load off the alternator until air pressure has built to brake-release levels, thus assuring full engine power is available to the air compressor. The way most conversions are designed, it's hard to imagine that air pressure is going to build significantly more slowly with the alternator connected. FWIW, my Neoplan has no such air pressure switch. The alternator starts charging even before the starter is done cranking. If my tanks are empty, it will take two or three minutes for the master Low Air warning alarm to shut up, but we'll be charging that whole time. HTH, -Sean http://OurOdyssey.BlogSpot.com | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 384 Registered: 8-2006 Posted From: 207.231.75.253 Rating: N/A |
Good evening Sean, You hit it on the head, as usual. If he doesn't have OTR air, or something else with an air-tensioned belt, the switch could go. The purpose of the pressure switch on an MCI is to keep the load off the alternator until the air pressure builds sufficiently to tension the drive belt. It has the added effect of keeping the air conditioning disabled until the alternator starts charging, or maybe that is the intended purpose, by the back door. Mine has another relay which keeps the not gen light out while the gen field is held out by the low pressure switch, I guess to keep drivers from reporting a defect as soon as they start a bus with no air? Regards, George | ||
| Craig (Ceieio)
Registered Member Username: Ceieio Post Number: 314 Registered: 12-2004 Posted From: 206.212.230.165 Rating: N/A |
Thanks Sean and George. I have thought about bypassing it... I will probably poke around some more before I give up and go that way. My MC7 (factory) alternator is driven by four v-belts that are air-tensioned; I was concerned that I would cause premature wear on them if I bypassed the switch. (Say after running the inverter on the house batteries while parked - could put a load on right away.) It doesn't look to me like the belts slack off when the air drops, but I am not sure as my look at the belts was static with no load. Thanks, Craig - MC7 Oregon | ||
| John MC9 (John_mc9)
Registered Member Username: John_mc9 Post Number: 587 Registered: 7-2006 Posted From: 66.217.107.49  Rating: N/A |
"Thanks Sean and George. I have thought about bypassing it... I will probably poke around some more before I give up and go that way. " I personally wouldn't arbitrarily "bypass" it, Craig. Without sufficient air pressure, the belts will slip. If there's enough of a draw on the alternator, the belts will wear quickly, or slip off the pulley causing more damage than it's worth. If the air pressure reads properly and is sufficient to release the brakes, then the belts should tighten also. If that's not happening, then locate the reason, and fix it. Bypassing any indication of a problem, doesn't make it "go away"! | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 24 Registered: 11-2007 Posted From: 121.218.32.208 Rating: N/A |
"It doesn't look to me like the belts slack off when the air drops, but I am not sure as my look at the belts was static with no load. " Inside the tensioning cylinder is a strong spring that provides about half of the required belt tension needed to deliver the full 300Amps. I would be inclined to get the pressure switch working if you intend charging house batteries. I can set up my system to do that and initial charging current is close to 200Amps which might be enough to slip belts if there is no air pressure. | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 1221 Registered: 12-2000 Posted From: 76.68.120.61  Rating: N/A |
Hello Craig. I suspect that the Penn-Air switch that triggers the low air warning as well as engages the alternator is just sticking. On an MC8 it is mounted in the rear electrical junction box. Stick your head in the driver side back side door and have a look to see if there is an air line to one of the boxes mounted to the junction box to the front of the coach. Take off the cover and spray it, maybe need to make a little adjustment to its settings, long time since I looked in there, memory a little foggy as to what you'll see. For those with air tensioned drive belts, you do not want to load the alternator before you get proper belt tension...they'll throw off, and then you get to play that game.... Craig, let us know how you make out! happy coaching! buswarrior | ||
| Craig (Ceieio)
Registered Member Username: Ceieio Post Number: 315 Registered: 12-2004 Posted From: 207.101.213.58 Rating: N/A |
Hello all - Here is what I found for future users of the search button. I Found the Penn Air switch as described (attached to the side of the rear electrical junction box). The switch was indeed sticking. I tapped it with a wooden dowel after opening up the cover and it freed up. What is inside the box is a couple of springs with threaded rods running through their centers. One spring/rod combo is labeled to set the turn on air pressure for an electrical switch while the other spring/rod combo sets a turn off (over pressure) point for the same electrical switch. Both rods are threaded and adjust the spring tension to change their respective settings. Each setting has an indicating arrow that points to a scale calibrated in psi. The spring adjustment for mine is set at 65 psi indicated cut in on as read by the pointer/scake. The overpressure side seems to be set above 120 psi (and is therefore unused). Not finding a chapter for setting this thingie up in my book, I will lube it up and then set it up with regulated shop air (I believe it is supposed to kick in at 60 psi per the schematic) and not trust the scale in the box too much. Once it kicks in reliably at 60 psi of regulated air, I will button the whole thing up and call it a day. Thanks to all of you for the help! Craig - MC7 Oregon. | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 1226 Registered: 12-2000 Posted From: 76.68.132.196 Rating: N/A |
Hello Craig. You need to be sure that the belt tensioners are getting air before the switch engages the alternator. As I recall, the switch picks up its air supply from the same circuit as the belt tensioner? If not, besides the unreliable adjustment points in the switch, you have to check at what real pressure point is the belt tensioner being pressurized. Check to see if there is a pressure protection valve supposed to be plumbed in there somewhere ahead of the belt tensioners. Either way, you want to confirm it's doing its job, and that some previous owner hasn't made it ineffective. happy coaching! buswarrior | ||
| Thomas Kurzawski (Yellow427cobra)
Registered Member Username: Yellow427cobra Post Number: 1 Registered: 4-2008 Posted From: 207.250.46.118 Rating: N/A |
Hello there, I have been following this with interest but also need some help. I have a 89 RTS. The "GEN Stop" light is constantly on. The gauges show good charge. What can be the problem and how can I correct it short of taking the bulb out. Thanks for all your help TK |