according to the 86 service manual, code 34 means MAP signal voltage is too low. If you have the 1986 PDF it's on page 488 (aka "6E2-A-44" under "driveability and emissions"). Theory of operation is on pg510. (The 1985 manual isn't on the web as far as I know).
Find the manuals here:
http://www.fieronews.net/fusion/downloads.phpThey don't give any great ways of testing this sensor. On pg 515 they just say "if the wires and hoses look good, swap it out". But your signal voltage is low either because of the sensor or because of wiring. A vacuum leak wouldn't cause this.
There are 3 pins, one is +5v, one is ground, one is signal. The signal voltage is a variable voltage between ground and 5v.
The TPS and MAP both share the same +5v line from the ECM, diagrams show it branches off to those 2 sensors.
Try unplugging the sensor, turn the key on, and find pin C on the harness connector. Check voltage between that and ground (which is on the center pin). With the key on you should get a steady 5v at pin C. This voltage is regulated by the ECM and should be very close to 5.00 and shouldn't move. You can also check it with the engine running if you want, again, it should be steady.
The 5v is the reference voltage that the sensor uses to produce a lower signal voltage on pin B.
You could also check resistance between the signal pin and ground. On my car, I found something like 40K ohms there, but that's from memory. I believe this is an intentional pulldown, but if your resistance there is very low or shorted it would cause a low signal error.
With the connector unplugged, check that same resistance on the MAP connector between the signal and ground pins. If it's shorted through the sensor, that would cause the low signal problem. It might be better to check that with vacuum connected and engine running, but I'm not sure this resistance will really behave correctly without any power to the sensor.
If you have a WinALDL setup, you can read the MAP signal values with the engine running that way.
If you want, you could poke through the insulation on the wires and probe them while connected to the sensor, with the engine running. If you do that, you can check voltage on the signal wire (pin B according to diagram). GM says it should be between 1.0-1.5v while idling. On my car it was in the high end of that range at warm idle, but I'm having issues too so that doesn't mean much.
If you can't find any shorts in the wiring to the sensor, or a low reference voltage, and your TPS isn't giving any errors, it's likely the MAP is bad.
I don't believe a leak in the vacuum line will cause this error. Code 34 is MAP signal too low, which means very low pressure, not something that would happen with a leak. A vacuum leak should cause code 33.
[This message has been edited by armos (edited 05-07-2012).]
