88 f150 map sensor

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Jan 20, 2004 | 11:02 AM

irishman200232

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88 f150 map sensor

ok here we go, i went to check the map sensor, haynes manual says the connector should between 4-6 volts with key on no vacuum and not connected, it read 4.9(so far so good) then i plugged it in and the book said it should read 5.0 volts(voltage varying type) it only read 2.4 volts, is this thing bad? Also my symptoms were high idle, it starts about 1000 rpm then slowly goes to about 1500 rpm but it tries to kick down but goes right back up, i replaced tps sensor check IAC for volts, all tested good.
Help me with this headache!@@@ thanks for all your help@!

Jan 20, 2004 | 11:39 AM

EPNCSU2006

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From: Concord, NC

No. The book is wrong. 2.5 Volts is the correct reading for when it is plugged in. I thought the same thing, and so bought a new sensor thinking that was throwing off my mass air conversion, but I got the exact same reading (2.5 V) with the new sensor. I then found in another book that the reading should be 2.5V.

Jan 20, 2004 | 11:49 AM

irishman200232

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where else do i look?

Jan 20, 2004 | 05:45 PM

EPNCSU2006

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From: Concord, NC

Look for what?

Jan 21, 2004 | 12:32 AM

bigric

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From: Round Rock, TX

When you replaced the TPS, did you make sure that it's below 0.99V at idle? If it's anything over 0.99, you'll get that same behavior. After adjusting it, snap the throttle repeatedly and make sure that it's always going to the correct voltage or less and adjust as needed.

Have you cleaned out your IAC? Just because the voltage is correct doesn't mean that it's not sticking; it just means that it's being cycled correctly. Take it off and clean it out really well with carb cleaner.

And, BTW, the Ford MAP sensors work on frequency rather than voltage, and although the voltage will probably give you your answer, testing the frequency will give you the best answer. Except I don't remember what the frequency is supposed to be near...

Last edited by bigric; Jan 21, 2004 at 12:34 AM.

Jan 21, 2004 | 07:41 AM

fishin1976

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85-90ish Ford Fuel injection had voltage varying MAP sensors, after that they were Frequency varying type. So you you are both correct. The voltage varying type are much easier to test. The frequency type must have a oscilliscope to test it. ( or a least a Digital meter that has a RPM function.

I agree with IAC theory, clean it out and make sure you do not have any vacuum leaks.

Jan 21, 2004 | 09:28 AM

EPNCSU2006

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From: Concord, NC

I think with either type, voltage varying or frequency varying, with the vehicle off (no vacuum), the signal wire should show 2.5 volts, and will change with vacuum if it is a voltage varying type. You can test the frequency varying type with a tachometer set on the 6 cylinder scale. I don't know exactly what frequencies, but a repair manual should have a table. I definitely would clean the IAC first off, as MAP sensors rarely go bad.