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raizo

Rotax 912 oil pressure fluctuations

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The weakened wire would have effectively caused the equivalent of ground faults. Ground faults are the most common cause of problems in any electrical system, but most particularly in cars where everything is grounded through the body. That's part of the reason that in aircraft wiring the standard practice is to run a seperate ground wire from every electrical device directly to a common bus bar and also have a good sized connection between the bus bar and the battery.

 

I'm intrigued that you say the connections are soldered to the battery in your aircraft! Can you post a pic of the type of battery you are using pls?

 

Also, you might or might not be aware that soldered connections are usually avoided where possible in aircraft wiring, in favor of crimped joints. Soldered connections make the end of the wire very stiff and vibration causes metal fatigue at the joint.

 

 

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Just for anyones info in case it helps someone else in the future.

We were getting a few fuel and oil pressure alarms recently. The pressure would dive and then come back up. We tested fuel flows and pumps extensively, checked all lines, checked oil systems. Checked ground terminals. Nothing found.

 

Battery which is supposed to last 15 years (lol) was getting tired on cold mornings, so we decided to replace it. Wires are soldered to battery. While pulling back heat shrink on terminal and wire the - wire came off. It definitely wasn't loose or unattached, but it was significantly weaker than the + attachment.

 

We proceeded to install new battery. Aircraft starts perfect and all alarms have ceased.

 

Me thinks perhaps the solder had begun to crack and the slightly poor termination was causing the occasional alarms. Strangely they only occurred during late warmups or climb out. or possibly the battery was actually dying and causing the alarms.

Further to Scott - I second all of your comments and add:

 

  • The best that I have ever got out of a 12 volt battery in a high vibration environment like a vehicle/mobile plant/tractor/boat/aircraft is about 8 years, with 3-5 being the norm. I replace my batteries at the slightest hint of slow cranking performance. To do otherwise is to unnecessarily stress the starter motor & associated wiring.
     
  • From my automotive experience, I have installed an additional (to existing system) 8 amp earth/ground wire to my composite aircraft's electrical circuit, that links all earth/negative terminal points (including 3 on engine) together. I have yet to experience a dud ground on my aircraft.
     
  • Make sure all connections are "good" (have nil resistance/continuity and are tight) and are protected from corrosion by using electrical grease.
     
  • Support wiring against movement/vibration.
     

 

 

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The weakened wire would have effectively caused the equivalent of ground faults. Ground faults are the most common cause of problems in any electrical system, but most particularly in cars where everything is grounded through the body. That's part of the reason that in aircraft wiring the standard practice is to run a seperate ground wire from every electrical device directly to a common bus bar and also have a good sized connection between the bus bar and the battery.I'm intrigued that you say the connections are soldered to the battery in your aircraft! Can you post a pic of the type of battery you are using pls?

 

Also, you might or might not be aware that soldered connections are usually avoided where possible in aircraft wiring, in favor of crimped joints. Soldered connections make the end of the wire very stiff and vibration causes metal fatigue at the joint.

Thanks, Im aware of that and posting for the benefit of others, so I think your post is very valuable. We did not test for earthing probs or battery fault so I cant categorically say it was either, but definitely more likely earthing in my opinion.

 

The reason we proceed with replacement is our battery is a manufactured battery made up on demand from cells, so no refund would have been forthcoming and we already had the replacement in our hands.

 

As far as soldered, my aircraft is 24 rego, though I am going to ask the manufacturer to approve a change to spade with locking screw/bolt, for two reasons.

 

1. To comply with maintenance regs about removing power while working

 

2. For reasons you just stated

 

BCR-0859-1008.jpg

 

Battery type

 

BCR-0859-1008

 

 

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Would also like to add that cable size from batt-solenoid-starter and main return/earth to batt is very important.

 

[email protected] pointed out some time ago Rotax minimum spec is 16mm2 cable. Many aircraft seem to be well under this, to save weight I imagine.

 

Also use industrial cable lugs rather than the lighter, thinner auto lugs.

 

I have used "conductive carbon grease" between my connections for better conductivity and to reduce corrosion.

 

There are various graphs on the net showing batt distance from starter (cable length) and the dia of cable required to avoid voltage drop.

 

Those with Rotax 4 stroke engines can do serious damage costing thousands if power at the starter is not good and strong as this allows the sprag clutch to slip.

 

I now have high quality 16mm2 multi strand cable and a strong odessey battery and voltage drop is down to 10 volts during starting rather than the 6 or 7 volts previously.

 

 

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16mm2 is pretty small - about 5mm in diameter. I can't imagine anyone using cable smaller than that for the starter?

 

 

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If you are measuring voltage drop at the starter (where you should) it's a good indicator of the state of the battery and the ability of the leads to carry current. If you are trouble shooting starter circuits , crank the motor with no ignition for say 20 secs and feel all leads and components, (solenoids lugs etc). for temp rise. I would earth the engine in two places for safety or you risk cremating cables and pipes. Nev

 

 

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16mm2 is pretty small - about 5mm in diameter. I can't imagine anyone using cable smaller than that for the starter?

Add the insulation to that and you're looking at 9-10mm total dia...... The cable I installed is double insulated so looks huge!

 

It has (from memory) 3000 odd strands of 0.08mm, which makes a difference over normal cable too.

 

One German manufactured aircraft I looked at had a very small battery (on the firewall) and small cable so as soon as the battery started to fail, it couldn't get the revs to fire.

 

Very minimal setup you might say.....

 

 

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Hi Guys,

 

I have recently changed the engine in my plane and it has the honeywell oil pressure sender, on the old engine it had the old VDO sender, question: is my old oil pressure gauge compatible with the honeywell sender or will I need to buy a new gauge for it to work properly?

 

David

 

 

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Hi Guys,I have recently changed the engine in my plane and it has the honeywell oil pressure sender, on the old engine it had the old VDO sender, question: is my old oil pressure gauge compatible with the honeywell sender or will I need to buy a new gauge for it to work properly?

 

David

Short answer - Don't know! but giving it a go is unlikely to do any harm. Or you could swap out the new sender for the old (assuming it was working okay when you took the engine out).

 

 

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Sorry Dave..need a new gauge..its a totally different output from the sender. Instead of being a resistive sensor like your old one the honeywell is a 0 to 20 mA one..totally different

 

 

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Or buy a vdo std oil sender and replace the honeywell with it. I have gone from the honeywell back to a std type as my honeywell stuffed up. But mounting the older stle direct onto the block will shorten its life. I have remote mounted mine and it works a treat. My MGL can take either input so I didnt have to get another steam gauge

 

 

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Dave post # 1048 Another NEW Savannah XL on its way this is what I did. My Honeywell died at 150 hrs but my MGL RDAC can take both types of sensors so I just needed to get the std oil pressure sender about 50 bucks I think. Then I got the fittings to make it remote as the engine vibrations wear the little wire wound resistor inside the sender..this way it should last much longer

 

 

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Dave post # 1048 Another NEW Savannah XL on its way this is what I did. My Honeywell died at 150 hrs but my MGL RDAC can take both types of sensors so I just needed to get the std oil pressure sender about 50 bucks I think. Then I got the fittings to make it remote as the engine vibrations wear the little wire wound resistor inside the sender..this way it should last much longer

Yes I would like to make it remote but because it it is an LSA 24 rego I would need factory approval, I think that would take too much time unfortunately, I have ordered a new 52mm Aviasport gauge to suit the Honeywell.. I should then be able to take the old Mitchell gauge out and drop the new one straight in the hole, anyone need a oil pressure gauge going cheap??? thanks anyway mate for your advice.

 

David

 

 

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an update to the original post,after a long investigation ,and by confirming fluctuations with a mechanical gauge,we are are no closer to solving the problem,we have fitted the new mushroom valve and spring,no change ,tried a new sender,no change,completely rewired the aircraft,no change,But today we found a collapsed oil line ,right where it enters the oil cooler,and after researching the web have found this may be our problem,so will order new oil hoses and fit this weekend,if this is not our problem will look at removing the oil pump,but as previously stated this has been a problem since we installed the 912 since new

 

Will keep you posted

 

 

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Short answer,NO replacing the collapsed oil line did not solve our problem,now at a total loss as to where to go next

 

 

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Short answer,NO replacing the collapsed oil line did not solve our problem,now at a total loss as to where to go next

I had 2 oil pressure sender units from Rotax sent that were not the right ohm range which would make the oil pressure fluctuate

 

Can't remember now what the vdo sender number was

 

I think there are about 10 different ones out there from VDO with all different ohm ranges

 

My advice is to borrow one off a mate who has a 912 to confirm it is not just the sender

 

If it fixes it then look on his for the part number

 

 

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Thanks Alf but we tried a new sender and then verified the problem by fitting a mechanical gauge which proved the readings we are getting from the sender, we have changed oil and filter 3 times so cant be the filter????

 

John

 

 

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Thanks Alf but we tried a new sender and then verified the problem by fitting a mechanical gauge which proved the readings we are getting from the sender, we have changed oil and filter 3 times so cant be the filter????John

I sympathise with your frustration John - but as the great detective Sherlock Holmes was supposed to have said (& I paraphrase) "if you have checked all the likely causes then it must be the unlikely one(s)".

 

The oil pressure sender/gauge system - highly likely but it would appear not to be the offender in this case (verified using the master gauge)

 

The oil filter scenario was, at best, a highly unlike cause.

 

I think you have to put aside the notion that this is a "good engine" just because its new or reconditioned - errors in construction/assembly

 

do happen.

 

At the risk of offending (I apologise in advance) for stating the obvious - it would seem to me that you must go back to basics.

 

I think I am correct in saying - Oil pressure in an internal combustion engine is generated by an oil pump and combustion blow bye (positive crank case pressure) past the piston rings.

 

The oil pump should give relatively constant pressure readings, especially when limited by a pressure relief system. SO it is unlikely the pump is responsible.

 

The pressure relief system could be acting up and instead of providing progressive relief is suddenly releasing, then on large pressure drop,slamming shut again - could cause a rapid pulse in the oil pressure reading that could get worse as the engine warms up - bit of a long shot but I think a

 

possibility

 

Crank case pressure is a more likely culprit - incorrectly fitted/wrong piston rings (or piston(s) and a very common problem, a partial or totally blocked crankcase pressure relief system - this would definitely produce rapid pressure fluctuations.

 

Unlike most modern engines, I don't think the 912 has a conventional pressure relief valve to get blocked/damaged/fail. Crank pressure is handled through the oil return pipe to the remote reservoir/can, that in tern is open to atmosphere.

 

SO

 

I would investigate excessive crank case pressure either due to an excessive leaking piston/cylinder or a restricted oil return system or a blocked reservoir breather.

 

Start with the simple -

 

Is the reservoir breather fully open to atmospheric pressure. If OK move on -

 

(as a long shot - are you running an oil vapour catch can on your breather ? if so remove it. It should not be required on a 912 & could be restricting the breather)

 

Is the oil return line unobstructed and hot oil able to flow freely. OK move on -

 

Is the crank case being over pressurised by blow by - not quit sure how to check for this but I imagine oil will be sputtering with lots of hot gas into the reservoir, rather than flowing.

 

 

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Thanks for the reply Skippydiesel,

 

I sent a rather long and descriptive email to Gary at Bert Floods last night(again) hoping they will respond with some positive action, stripping the motor is a drastic and costly measure if this has to be done I hope Rotax will cover the cost as the motor is out of warranty

 

 

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Hi John - If I were in your shoes, I would be going pale at the thought of a costly BF strip & rebuild.

 

Sorry to harp on, but have you checked that the oil tank breather is completely open/free to atmosphere? and that there is no restriction in the oil, crankcase to tank, return line (including the fittings at both ends)?

 

 

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I read through the whole story just now. I sympathise with you. What a nightmare. Most of your diagnosis efforts (and those of fellow readers) seem to be centered around the gauge(s) . I was thinking about it and came to the conclusion that the pressure does indeed flucuate! No amount of oil, filter, hose or gauge manipulation will change that. OK. The oil pump can hardly change pressure at fairly high speed.What retains the pressure on a Rotax?

 

Is the PRV at the "end" of the system or the begining? I mean is the PRV next to the pump or somewhere else?

 

I believe that you must look (again) at your pressure relief valve. I can't imagine anything else in an engine that will cause pressure to suddenly change. You wrote " Changed mushroom and spring" I assume that is the relief valve?. Did you have to lap it onto its seat? Is there some trick part that directs oil to the gearbox? Or jets that cause the pressure to be retained. (on a plain bearing engine the big ends and main bearings do this) On a roller bearing jobbie there must be a restriction that causes the pressure to be retained. Otherwise there would be none. I'll see if there is an "exploded" engine view somewhere. Good Luck.

 

 

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Many exploded views but not much help. But lots and lots of warnings about priming/purging of the oil system. I would pull out the oil pump section and have a close look. Not a lot of tear down and easy access. Purge oil lines when you're finished. (whatever that means)...Cheers

 

.

 

 

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. Purge oil lines when you're finished. (whatever that means)...Cheers

 

Hi Geoff - hope your not a 912 owner/operator because the above advise is very very important to the start up lubrication of your engine after a complete oil change.

 

I say couplet because most 912 oil changes consist of draining/refilling the remote oil reservoir/tank and not the crankcase.

 

If you drain the crankcase, there is a sequence of events and the use of positive air pressure, to ensure that the oil is circulating correctly in the engine before you actually start it.

 

Failure to do the purge correctly, puts your engine at risk, due to oil starvation.

 

One of the reasons for 912 owner/operators to do the "gurgle" check of the lubrication system as part of the pre-flight checks, is to ascertain if the oil is circulating.

 

Rotax go to some lengths to explain how to do "the purge".

 

 

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If the oil pressure is fairly low within the operating range, could you shim the spring a bit just to see if a slightly boosted oil pressure takes out the fluctuations?

 

And as skippy stated, the return oil system and breather hoses are fine?

 

 

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