Tex06 Posted January 24, 2017 Share Posted January 24, 2017 Worked on Rambo last weekend, installed the headlight relay harness, replaced the valve cover gasket, vacuumed out the inside. Started up afterwards and got a CEL. Checked it today and got 12-14-13-55. 12 & 55 were expected. 13 &14 deal with the MAP sensor. Tried to start and it turns over and coughs but doesn't quite catch, even when holding the gas pedal (sorry, I'm still a carb guy with carb habits). Disconnected the MAP, hard to start but will, just doesn't like to idle (duh no MAP signal). Grabbed a spare MAP and swapped it in (high quality GM junkyard unit). Exact same issues. No start with, hard start disconnected. Already reset the codes by disconnecting battery and touching + to -. Tried it all again. No difference. Any ideas? I'm currently looking over my connections again... Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted January 24, 2017 Author Share Posted January 24, 2017 Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 24, 2017 Share Posted January 24, 2017 From an old post of mine: Don't worry about the GM markings on the MAP body. I've seen OEM sensors with and without. Your MAP connector has three pins, A, B, and C. Pin A is sensor ground (BLK/BLU wire), pin B is the MAP output to the ECU (RED/GRN wire), and pin C (VIOLET wire) is the 5VDC reference input voltage from the ECU.To test the connector inputs, unplug it from the MAP sensor, turn IGN ON, and measure across pins A and C. You should see very close to the 5VDC reference voltage. To test the MAP output, plug the connector back on, start the engine, and measure the voltage across pin B by back-probing the pin B connector and a good ground. With the vacuum line hooked up to the sensor, this voltage will vary between ~ 0.02V to 4.94VDC depending on engine RPM and intake manifold air pressure if the sensor is working. An analog meter is best to use if you have one for this test. A steady or no voltage indicates a bad MAP sensor. Link to comment Share on other sites More sharing options...
Tex06 Posted January 24, 2017 Author Share Posted January 24, 2017 Yep next step is to back probe. Gonna have to wait until tomorrow, left the meter at work. For now I'm gonna run it with the MAP unplugged. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted January 25, 2017 Author Share Posted January 25, 2017 Got a quick chance to test voltage....only seeing 4.81V at the connector. Not good....I'm gonna back probe this afternoon, if this is like my TPS issue then there's corrosion somewhere I need to find. Question: Is this 5 VDC the same as the 5VDC at the TPS? I'd like to check it in reference to the TPS ground (a known good signal ground) and see if I can eliminate the ground or the +5V side from my troubleshooting. Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 25, 2017 Share Posted January 25, 2017 The 5VDC reference voltage for every sensor that uses it comes off a single ECU pin. To check if it's ground side corrosion dragging the voltage down, check the ref. voltage first using pin A of the MAP for meter ground, then the battery negative post. If the readings are the same, the MAP has a good unrestricted ground. 4.81 VDC actually isn't that bad. I don't remember the exact tolerance, but it it should run fine if the MAP is responding to changes in vacuum based on engine load and RPM as it should. Link to comment Share on other sites More sharing options...
Tex06 Posted January 25, 2017 Author Share Posted January 25, 2017 Will check when I get back. I know my TPS didn't like 4.8, when I cleaned up my ECU pins it came up to 4.94 and ran much better Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted January 25, 2017 Author Share Posted January 25, 2017 4.87VDC between MAP and negative terminal on battery. Getting a reading of .3 VDC on the signal side when the key is on. I disconnected the MAP and started the truck with my back probe in place. 0.3V. Connected the MAP and saw no change in value before it died. Repeated this but held the throttle mostly open and the motor kept running, still read .3V, died when I let off the throttle. I'm guessing it's reading too much vacuum? Either way I'm buying another sensor. This one is a Napa sensor less than a year old. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted January 30, 2017 Author Share Posted January 30, 2017 Ruh row. Same issue. Reading 0.3V with power on. Unplug MAP, reads 0V (no feedback in line). Gonna swap this for another MAP I guess. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted February 2, 2017 Author Share Posted February 2, 2017 New wrinkle. The second new MAP acted exactly the same as the previous one. So I back probed all 3 wires, just in case I had the wrong signal wire and found this. As soon as I plug-in the MAP sensor, the supply voltage drops from 5V to 0.5V. My first thought is bad connector possibly? I'm looking to see if I have a spare to test. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted February 3, 2017 Author Share Posted February 3, 2017 Found (and halfway repaired! )the issue. By some dumb stroke of luck I found a '92 jeep factory service manual and bought it. I used the included wiring diagram to TS my issue. Broken solder on the MAP 5V supply wire. So basically my MAP sensor was pulling down my ECU through the output line,and that's why the truck would die. Ohmed everything out using the diagram, tore open the wiring harness, found the break (at the solder joint), repaired break and started rewrapping everything. Oh and tested...the truck starts so much easier now! It's the purple and white wire, where it goes into the black heat shrink was the issue. Power comes from pin 6 (also supplies 5V to the TPS, that's why the solder joint exists). Considering I had issues with my TPS getting low voltage before, I'm going to guess this was part of, if not most of the issue all along. Another shot of the purple and white wire. The MAP sensor connector is green and on the left Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
HOrnbrod Posted February 3, 2017 Share Posted February 3, 2017 Absolutely mandatory to have the correct FSMs for your rig for electrical troubleshooting issues. W/o it you're flying blind. Good job tracking down the harness issue. :cheers: Link to comment Share on other sites More sharing options...
Tex06 Posted February 3, 2017 Author Share Posted February 3, 2017 Absolutely mandatory to have the correct FSMs for your rig for electrical troubleshooting issues. W/o it you're flying blind. Good job tracking down the harness issue. :cheers:Would posting the fuel injection wiring diagram break any rules? Since I have them, I figure I can share. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
HOrnbrod Posted February 3, 2017 Share Posted February 3, 2017 Why not? I do it all the time. :yes: Link to comment Share on other sites More sharing options...
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