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Old 05-20-2020, 09:45 PM   #1
Mini-Skoolie
 
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Hello, new member here from Ohio

I just bought a bus today and look forward to starting the conversion very soon. Itís a 2001 Thomas MVP 72 passenger with a Cat 3126 and a Allison MD3060 5 speed. Not my preference of engine but at 140,000 miles and only $2700, I can learn to like it until something goes wrong. I drove it 220 miles to get it home and it runs out good, with a top speed of about 72 mph.

The oil pressure gauge is shakey, going from 0to 30 and all over the place. If the pressure was truly low or zero, it wouldnít run. Another problem is no turn signals or hazard lights. No noise, no lights, no nothing, so I have to get that fixed before I can get it back InThe road. Nice riding older bus and appears to be an ok to do my first build with.

Been searching google to find out body info. I have some switches I donít know what they are. Two are labeled pump and have a icon of a water spicket? Any good sources to find out what Thomas put on this bus? Thanks for any help you can provide. I try to search for my topics before I post any problems so I donít annoy you all with the same questions.

Bill
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Old 05-20-2020, 10:08 PM   #2
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The oil pressure gauge is shakey, going from 0to 30 and all over the place. If the pressure was truly low or zero, it wouldn’t run. Another problem is no turn signals or hazard lights. No noise, no lights, no nothing, so I have to get that fixed before I can get it back InThe road. Nice riding older bus and appears to be an ok to do my first build with.
Engines can run for some time with low or fluctuating oil pressure. Zero, sustained, is another matter. Possibly a bad pressure sending unit, but I would verify true pressure with a mechanical guage. You could have a serious and looming problem that you need to deal with before doing a build-out.

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In searching google to find out body info. I have some switches I don’t know what they are. Two are labeled pump and have a icon of a water spicket? Any good sources to find out what Thomas put on this bus? Thanks for any help you can provide. I try to search for my topics before I post any problems so I don’t annoy you all with the same questions.
Likely windshield washer pumps. Buses will have one for each side of the windshield.
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Old 05-20-2020, 10:16 PM   #3
Mini-Skoolie
 
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Thanks for the info. Since that motor runs a HEUI pump, the electronics shut the motor off if the injectors don’t have enough oil pressure supplied to the HEUI unit... or so I’m told. I’m new here and don’t know how to add pics of the switch’s.?
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Old 05-20-2020, 11:02 PM   #4
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Low or fluctuating pressure may or may not be enough to affect HEUI.
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Old 05-21-2020, 12:33 AM   #5
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HEUI engines won't even run if the oil pressure is below a certain number. HEUI engines by their very nature require high oil pressure to run the fuel injection system.



My money is on a bad gauge/sending unit. Test with a mechanical gauge like others have said.
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Old 05-21-2020, 06:27 AM   #6
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the gauges are notorious on many of the bluebird / thomas busses.. is this an MVP RE? if so there is a second gauge in the rear engine compartment which you can check..



if the oil pressure is truly at fault, you'll usually notice a big difference in gauge motion from cold to hot. when cold the gauge may stay higher or "jiggle" a lot slower.. simply because of the properties of engine oil..



its normal for some gauge motion on a HEUI engine.. as the injection system requires more oil you may see the Main Oil P drop a few. I notice it on my T444E that the oil P flcutautes between 35 and 45 (hot engine low RPM under load) .. less so at higher RPMs..



as for the lights.. check fuses and circuit breakers first.. could be something simple..



here in ohio you might need to order a set of floats for that thing and just take the river everywhere since all the roads are flooded!!


the water spigot switches are likely your heater loop boost pump.. on a big bus esp at idle in really cold weather the coolant flow was slow and the heaters near the end of the loop would blow cool air so they have booster pump to increase coolant flow.


-Christopher
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Old 05-21-2020, 06:30 AM   #7
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If your gauges are like mine, try pushing in on the gauge panel to see if it makes the gauge work. mine has a bad connection and sometimes acts like you're describing.
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Old 05-21-2020, 07:20 AM   #8
Mini-Skoolie
 
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Thank you for the info. I’m going to apply a manual gauge to check the actual oil pressure. It makes sense about the pump switch Being for what you mentioned.
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Old 05-21-2020, 08:50 AM   #9
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oil pressure reading

First, imagine how tough a sending unit must be. Bite a running engine, put your teeth on it.... that is the life of the electrics inside a oil pressure sending unit. Or anything else bolted to the engine.

Second, yes install a mechanical gauge to cross check. That is what I usually do. If the oil pressure checks good, then you have to figure out what is at fault, gauge or sender. Could be both.

Third, your gauge is reading the engine oil pressure that is related to engine oiling, not the huei engine oil pressure, I think that is like 1,000 times greater than engine oil pressure.

On many of my race cars, I have a short flexible line to engine sending units so I can isolate engine vibration. If the budget was bigger I would direct mount because the gets rid of a possible failure mode, but more parts replacement due to sending unit failures. High frequency vibration seems to do a good job of killing parts.....

Glad for you that you have your bus. Have fun.

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Old 05-21-2020, 09:14 AM   #10
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Quote:
Originally Posted by Weesew View Post
Thanks for the info. Since that motor runs a HEUI pump, the electronics shut the motor off if the injectors donít have enough oil pressure supplied to the HEUI unit... or so Iím told. Iím new here and donít know how to add pics of the switchís.?
Electronics won't shut the engine off with no oil pressure. It shuts off by design because the hpop boosts the oil up to a few thousand psi to fire the injectors. No oil in the engine, no oil to boost to create injection pressure, and therefore no fuel injection event.

Pressure is just the resistance to flow. You can have little to no pressure and still have oil flowing and lubricating. The hpop only needs oil provided to it, so you can have 0 oil pressure, and the engine will still run, because it's still being provided with oil. Granted it might run poorly, especially under load, but will still run nonetheless.

It won't run without oil in the system though. So if you change it and forget to add new oil, it will start and shut off after a few seconds.
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