boost vs prime pump circuit?

[gabez]

Hello, 

I have a quick question that I cannot find an answer in the POH. On Sunday my prime pump wasn't working. I checked the voltage and it showed 11.2 v. I said crap my battery is dead, I  then tried to crank the plane and it did. I then tried the high boost pump and it worked, so I used that to prime the engine. Eventually started the plane, it showed a load. I flew for about 30-45 minutes. Stop at the fuel station and restarted the plane with the prime pump (which now seemed to work). 

 

My question, is the prime pump on a different circuit than the high boost pump? ( this is a 1984 MK20 231 - no low)

 

Thank you

Gabe

[DonMuncy]

It is my understanding that the pump is the same, but it squirts the fuel in a different place. I'm sure someone will correct me if I'm wrong.

[N201MKTurbo]

I believe it is the same pump and squirts into the same place. Low boost just has a dropping resistor so the pump runs slower. Prime is the same as high only with a momentary switch.

[kortopates]

You should have 3 fuel pump related switches. High Boost, Low Boost and Prime - so not sure why you say "no low" since you'll have all 3. But its all done by the same pump as Rich and Don say above. But the Prime button used to put the fuel into the top of the induction manifold through a diverter valve till a critical mandatory SB (which didn't become an AD) called for removing the diverter. With the diverter removed, the prime button works the same as low boost switch, except of course that the Prime button is momentary, but will deliver fuel directly to the cylinders now. The guarded high boost just does it at twice the rate.

Maybe you should make sure your engine has complied with the SB to remove to the diverter if this is not familiar to you. 

[gabez]

25 minutes ago, kortopates said:

You should have 3 fuel pump related switches. High Boost, Low Boost and Prime - so not sure why you say "no low" since you'll have all 3. But its all done by the same pump as Rich and Don say above. But the Prime button used to put the fuel into the top of the induction manifold through a diverter valve till a critical mandatory SB (which didn't become an AD) called for removing the diverter. With the diverter removed, the prime button works the same as low boost switch, except of course that the Prime button is momentary, but will deliver fuel directly to the cylinders now. The guarded high boost just does it at twice the rate.

Maybe you should make sure your engine has complied with the SB to remove to the diverter if this is not familiar to you. 

you are absolutely right I do have a low boost I had to go back and look at my panel. So with all that said any thoughts on why the prime didn't turn the pump on but the high boost did? perhaps the resistor on the prime dropped the voltage (already low) below what the pump can work with? 

 

What is the SB # if you don't mind asking you, I will look at my logs

Edited May 8, 2023 by gabez

[81X]

20 minutes ago, kortopates said:

You should have 3 fuel pump related switches. High Boost, Low Boost and Prime - so not sure why you say "no low" since you'll have all 3. But its all done by the same pump as Rich and Don say above. But the Prime button used to put the fuel into the top of the induction manifold through a diverter valve till a critical mandatory SB (which didn't become an AD) called for removing the diverter. With the diverter removed, the prime button works the same as low boost switch, except of course that the Prime button is momentary, but will deliver fuel directly to the cylinders now. The guarded high boost just does it at twice the rate.

Maybe you should make sure your engine has complied with the SB to remove to the diverter if this is not familiar to you. 

My 231 prime button will pump at the same rate as the high boost, not the low boost.  
 

Just in case anyone wondered, the low boost goes through some kind of resistance or voltage reduction box to slow the pumping rate- it’s behind the panel on the pilot side in mine.  

[PT20J]

OK, we figured it out. But remember that there are a lot of different Mooneys from carbureted to fuel injected (Lycoming is different than Continental) to turbo. So, please, next time tell us which airplane and engine you have.  

[1980Mooney]

On 5/8/2023 at 8:45 PM, PT20J said:

OK, we figured it out..... So, please, next time tell us which airplane and engine you have.  

In defense of the OP, he did state it, but just not where we expected. It was at the end of the last sentence in his original post. "( this is a 1984 MK20 231 - no low)" - and not in his profile.

I was also originally confused. I drifted into this thread midstream and didn't read his post carefully. I was looking for it in his profile - but it was not there.

[PT20J]

12 minutes ago, 1980Mooney said:

In defense of the OP, he did state it, but just not where we expected. It was at the end of the last sentence in his original post. "( this is a 1984 MK20 231 - no low)" - and not in his profile.

I was also originally confused. I drifted into this thread midstream and didn't read his post carefully. I was looking for it in his profile - but it was not there.

My bad -- I somehow missed that. Apologies to @gabez

[gabez]

33 minutes ago, PT20J said:

My bad -- I somehow missed that. Apologies to @gabez

all good @PT20J We did a deep cycle on the battery and tested good so we shall see what happens over the weekend. 

[Aerodon]

On 5/8/2023 at 12:54 PM, gabez said:

Hello, 

I have a quick question that I cannot find an answer in the POH. On Sunday my prime pump wasn't working. I checked the voltage and it showed 11.2 v. I said crap my battery is dead, I  then tried to crank the plane and it did. I then tried the high boost pump and it worked, so I used that to prime the engine. Eventually started the plane, it showed a load. I flew for about 30-45 minutes. Stop at the fuel station and restarted the plane with the prime pump (which now seemed to work). 

 

My question, is the prime pump on a different circuit than the high boost pump? ( this is a 1984 MK20 231 - no low)

 

Thank you

Gabe

The M20K has a 'box' that either delivers full voltage to the pump for hi, or medium voltage to for low.  Then the prime button delivers power to the pump and the diverter valve to direct the fuel into the primer nozzles.

There is CSB19-01A to remove the diverter valve - needs some minor plumbing changes as well as the correct injector nozzles.  If this has been done, then either the primer button or Hi fuel will do exactly the same.  (one switch is momentary on, the other is on/off).

Its feasible that your installer removed the power to the primer diverter and the fuel pump, that's why it is no longer working?

The OEM wiring is a bit 'goofy' for all of this, IIRC the primer is powered by an inline fuse off the Hi fuel switch.I don't like inline fuses hidden behind the panel, so I had access and space for a separate primer circuit breaker.

Send me your serial number, I have the electrical drawings for all the M20K's.

 

Aerodon

 

 

 

 

CSB19-01A.pdf

[gabez]

49 minutes ago, Aerodon said:

The M20K has a 'box' that either delivers full voltage to the pump for hi, or medium voltage to for low.  Then the prime button delivers power to the pump and the diverter valve to direct the fuel into the primer nozzles.

There is CSB19-01A to remove the diverter valve - needs some minor plumbing changes as well as the correct injector nozzles.  If this has been done, then either the primer button or Hi fuel will do exactly the same.  (one switch is momentary on, the other is on/off).

Its feasible that your installer removed the power to the primer diverter and the fuel pump, that's why it is no longer working?

The OEM wiring is a bit 'goofy' for all of this, IIRC the primer is powered by an inline fuse off the Hi fuel switch.I don't like inline fuses hidden behind the panel, so I had access and space for a separate primer circuit breaker.

Send me your serial number, I have the electrical drawings for all the M20K's.

 

Aerodon

 

 

 

 

CSB19-01A.pdf 703.33 kB · 0 downloads

my primer button has been working just fine since I took ownership of the aircraft. just last Sunday tho the button didn't work. The high boost did work fine and then after refueling the prime button worked again. The battery was showing 11.7v so perhaps it was low, I had it done a deep cycle so I will find out tomorrow .... fingers crossed.