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Trying to determine if my MAP sensor test procedure was correct. I first tested reference voltage at 4.8v. Good. Then, backprobing the center wire (blue on 1986 F150 5.0) I get 2.5v at 0" of vacuum. The voltage does not change when up to 20" of vacuum is applied with hand held vacuum pump. I determined that this was a "no response" per Haynes Manual and move on to frequency test. For frequency I used my dwell/tach. Backprobing the center wire got me 1400 rpm on the six cylinder scale at 0" vacuum. And at 20" vacuum, the rpm drops to 1000rpm. Haynes calls for +/- 310 rpm at atmospheric pressure and 200 rpm at 20". I understand that my readings are very high compared to the called for readings but then my readings are linear in comparison to the lower readings. Just 4.5 times higher.
With no guidance, I bought a new Delphi MAP sensor. It tests at 700 rpm on the same 6 cylinder scale and drops to about 550 rpm with 20" vacuum applied. Wondering if I should have bought a Motorcraft.
His procedure states to set the tachometer to the 4 cylinder position.
Well, this is the same procedure I used. Of course using the six cylinder scale would just give you a lower reading by 33%. 310 as opposed to 464.
I ordered in and received this afternoon a new Delphi brand MAP sensor from Rock Auto. It measures 700 rpm on the six cylinder scale or 1150 on the four cylinder scale. Guess I'll tiptoe through the Rock Auto return mine field. I'll check on a Standard brand tomorrow. I have not been able to locate a genuine Motorcraft.
For frequency I used my dwell/tach. Backprobing the center wire got me 1400 rpm on the six cylinder scale at 0" vacuum. And at 20" vacuum, the rpm drops to 1000rpm. Haynes calls for +/- 310 rpm at atmospheric pressure and 200 rpm at 20".
Have not yet found an answer to my original question. However, a former neighbor who has been in the Ford service business for forty years lent me a "known to be good" MAP sensor from his tool box. It tests exactly the same same as my original.
I have always used a meter that can measure frequency directly. I cannot answer why the tach/dwell method is erroneously showing the output is out of specification.
