No manifold pressure indicated

Need some help,

On a brief practice flight I observed the manifold pressure drop from 20 to 0 while adjusting the prop. It dropped fast enough the needle seemed to bounce off the peg. Almost as quickly, I realized I was still making power, so the flight home was uneventful, but the needle didn?t budge.

From reading through several manifold pressure gauge threads, this doesn?t seem to be the typical way a Van?s gauge fails. They either peg high or gradually begin to read low. Perhaps this is so obvious no one would ask.

The ground and power connections at the gauge appear to be intact. Despite the usual contortions, I haven?t even been able to eyeball the sending unit that must be there somewhere.

Where would you start? Here's hoping no one says sending unit??

Yes I had a blonde moment. I agree with Mel and Vic.


Good luck
Bill

William said:

I would start by confirming the map line has not come disconnected from either end. Follow the line from the engine to the gauge. If still connected check the map line for leaks.
Good luck
Bill

Click to expand...

If the line had come loose and the instrument was working properly, it would read ambient, not "0".

I agree with Mel. This is an indication problem, not a line problem.

Vic

Thanks gentlemen, I appreciate your responses.

Having located the sender, I see the splices to the gauge wiring are suspect. in addition, on closer inspection, the shield ground at the gauge was partially frayed and may have been in contact with an adjacent terminal....so I have a few things to try before replacing any hardware.

manifold pressure gauge update

Repairing the frayed shield ground and tightening all the connections did not solve the problem. The sender splices looked dicey, but proved to be solid.

The leads from the sender to the + and - gauge terminals measured just over one volt, but weren't identical. I presume the differential is translated to produce the appropriate needle deflection, so exact values must depend on the circumstances. A very prompt response from Van's tech support suggested these readings were in the neighborhood.

Given these findings and the inaccessibility of the sender, we chose to just replace the gauge.....and we're back in business with a reading very close to atmospheric pressure.

Takehome thoughts: (...new at this so don't expect anything too profound)

- A Van's manifold pressure gauge can fail abruptly to the low side. This is contrary to several other observations on manifold pressure indications in which they peg high or slowly decrease.

- The + and - terminals should read something just over 1 volt, but won't be identical with an engine at rest.

- My van's manifold pressure gauge failed after 500 hours, so in this instance I think it was a value purchase. YMMV.