Help! L81's Manifold Differential Sensor (a.k.a. MAP sensor) troubleshooting

[GerryLP]

Hello everyone,

a couple of Fridays ago, at the end of a very long and day, the vette stalled a couple of times while driving, and to re-start, I had to press on the gas pedal hard, and once it started, it would make a huge cloud od smok. I was confused about Gail's behaviour.

Something told me to start with the manifold differential pressure sensor, which in hopes of simplifying things, I am just going to call a MAP sensor. :ugh

Anyway, when I follow the firt step in the trouble shooting charts for bad MAP sensor, I got a voltage equivalent to nearly 5,000 feet of altitude, and I thought, "oh well, it looks like it is reading right, buti'll go ahead and apply some vacuum and see if it changes like it should, just like it says on the next step.....if I can only get it to move with vacuum applied. That's right folks, it was stuck at the same reading even with 20 inches of vacuum applied. ad


So then, I tried to follow the troubleshooting chart, and it kept pointing towards a bad sensor, but when I hooked-up the new sensor, it also did read the same voltage. When I followed the decision branch to the right, it points to a bad ECM; however, it says to, 'check for terminal 1 at ECM terminal to see if it is grounded, and if it is not grounded, then replace ECM'. I am paraphrasing here, since I do not have the manual with me right now.

In the hopes that someone could save me another weekend of troubleshooting, I was hoping that someone could recommend a short cut, and help me avoid purchasing a new ECM before checking a few other checks. I believe that ECM are not returnable, so if I buy one, I need to be able to be prett certain that it is an ECM problem

Inputs requested.

 

[bill81vette]

Gerry,how about finding the ground for the ECM and make sure it is in good working order, eg not corroded,broken,check for continuity in the wire itself....hnoeshnoes

good luck!!!

 

[GerryLP]

Bill,

I will do that. Thanks for the suggestion. The weird thing is that I searched earlier for set codes, and the ECM keeps flashing code 12 ( as you know, one flash of the "check engine" light, and then two quick flashes).

I did find that pin "A" on the connector to the sensor has an open somewhere between the connector and Pin 7 on the ECM connector, so IAW the electrical drawings, Pin "A" (aka terminal 452) splices with terminal "452" for the throttle position sensor somewhere near at the 6-pin connector on the right side of the engine (connects with the harness that runs along the RH valve cover). So I spliced pin "A" on the sensor connector to pin "C" at the 6-pin connector, and so now it has a signal, but when I repeated the sensor test, it still registering 5,000 feet, and it does not change when I apply a vacuum source.

p.s. I just realized that I did not repeat the checking of sensor terminal "B" to "C" for grounding after I restored the signal from "A" to ECM terminal 7 by the jumper. So I will try that as well.