Hello there,

Many people on this forum have been super helpful, and I’ve posted about this issue quite a few times at this point, so some may be familiar with my problem. In short, my car has had issues stalling intermittently, and after much troubleshooting we’ve narrowed it down to a wiring issue. The signal wire from the map sensor to the ECM is sending constant 4.98 volts to the ECM, when the reference voltage is already 5, so it’s basically not adjusting the voltage it sends back at all. We’ve taken as much apart as we feel comfortable doing, and the issue isn’t visible anywhere.

My mechanic is saying the only thing he can think to do is to shotgun a whole new wiring harness. For various reasons, mostly financial, I really don’t want to do that. Is there any workaround to this? For the record, this is my daily (it’s got 36k miles and virtually no rust). If it was a passion project I’d be willing to do whatever I needed to get it going. I’m looking into getting a different car and selling the cutlass, but getting rid of a super well-running car because of an obscure wiring issue seems… not right. Any advice on the situation would be a massive help. Thank you all again.

 

Don't replace your wiring harness hoping it will cure a driveability problem. Here is one of your other threads:

https://classicoldsmobile.com/forums...upreme-173722/

Did you ever speak to your technician about replacing the ECM in your car?

 

He tried replacing the ECM, checking/adding a few grounds, cleaning the electrical connectors, and unplugging other sensors that run off the five volt reference. After all of that the sensor still sent a constant 4.98 volt signal back to the ECM, so he thinks there’s wires rubbing somewhere that he can’t see.

 

You need an Oldsmobile service manual to see how to test that MAP sensor. 4.98 VDC to a sensor using 5 VDC logic is probably the power wire. You need to figure out how the reading from that sensor changes and what effect it has on the engine.

What's the driveability problem your having again? I can merge the threads again if needed.

 

No need to merge the threads again, I don’t want it to become a big monster thread now that I have more direction.

It stalls while driving, specifically after driving, sitting, and then being driven again. The technician I took it to hooked it up to a scan tool that shows all the sensors’ data in real time. As far as he could tell, the only thing that was off was that the MAP Sensor signal wire was putting out a constant 5 volts. As a result the ecm has no idea what to do with the fuel. Also, I should have mentioned earlier that he already tried replacing the MAP sensor and had no luck there.

 

I’d start with disconnecting the battery, unplugging the ECM and the map sensor. Backprobe the map sensor signal wire at the ECM and map sensor using the proper type of terminals (it’s very easy to damage electrical terminals using the wrong kind of probe) and make sure you have continuity from the map plug to the ecm.

Next, if that checks out, carefully and meticulously un-tape/unwrap the engine harness from the map sensor to the ecm. Find the map sensor signal wire and inspect it all the way from the map to the ecm. Look for pinched or damaged insulation.

There is no way I’d replace the entire harness for this kind of problem.

 

Gotcha. I definitely won’t replace the entire harness. He said that he looked about everywhere he could without taking apart major things. I think he mostly looked under the plenum and down around the power steering pump. Wouldn’t hurt to do my own looking though.

A buddy of mine is wondering if we can just wire the map sensor directly to the ECM. Do you think there’s any merit to that idea? Or is it pretty integral that it follows the factory routing? I’m very new to these kinds of electrical issues.

Thank you much for your input

 

To wire the MAP sensor directly to the ECM you need to know what pins it goes to. Do you have a full wiring schematic with an ECM pinout? This would be a preferable troubleshooting step before replacing an entire wiring harness, but first you should do some testing with your multimeter like Matt mentioned.

 

I do have a wiring diagram, but I’m not sure whether or not it includes the ECM pin outs. I’ll have to take a look at it later, as of right now it’s in my car which is at the shop. You say it’s a preferable troubleshooting step; does that mean it wouldn’t be a good long term solution? I’ll definitely look for continuity in the signal wire, as well as the other wires just for good measure. Getting to them may be a bit tricky though.

Thank you as always!

 

DOH!

You could replace the wires between the ECM and the MAP sensor, but it wouldn't be very practical. Have you seen the coupler that attaches the ECM to the wiring harness?

 

I’ve not, everything regarding the wiring has been the technicians doing. I haven’t gone around and looked at the wiring myself, only thing that he showed me was the scan-tool’s reading of all the voltages.

 

Sounds like you need a different shop. If the MAP is showing full on with 2 different sensors, there's probably a different problem, like a vacuum leak.
Are those engines prone to intake gasket failures? A lot of GM engines have that problem.

 

I haven’t seen anything specifically talking about the intake gaskets being bad. We didn’t think it was a vacuum leak because of how inconsistent the issue, but that would make sense. He’s specifically saying the barometric pressure sensor in the MAP sensor is off; would a vacuum leak cause that?

 

^^^THIS^^^

Didn't you mention in the thread I linked that you've already paid the shop to replace multiple sensors?

 

The sensor is measuring Manifold Absolute Pressure. If the pressure is the same as the atmosphere with no vacuum......

 

At this point I’ve taken it to two different shops. The one I mentioned in the previous post replaced the MAP sensor and crank sensor (no charge since it didn’t help). The shop I took it to recently had fancier diagnostic equipment and replaced the MAP sensor as part of troubleshooting but didn’t charge me for it. He’s good, but I think he got sick of meticulously looking at it and pitched the idea of shotgunning a new harness. Either way I’m getting ready to buy a new daily, so going forward I’ll probably do most of my own work on it.

 

Fair enough. I’ll do the carburetor cleaner test later, or I think a buddy of mine said he has some sort of specialized equipment for it? I’m not sure. Either way this is a preferable direction to replacing the whole harness. Thank you for your help!

 

Yup, a smoke machine would yield the quickest, most accurate results.
What if the first shop knocked a line off?
Keep in mind, it could literally be almost anything.
Anecdotal story time! I swapped a 3800 Series 2 and 4L60E from a 2002 Firebrd into my 85 Bonneville G body. It would throw lean codes, TPS codes, MAF codes, etc. I found what was possibly the last DBW TB assembly in the country, bought it and swapped it out. They are not very serviceable; I tried. The problem persisted. The ACTUAL problem was a clogged fuel filter. Some dvmba$$ (me) put shrink tubing on the fuel pump and sender connections in the tank. The fuel filter was clogging with black dust from the heat shrink breaking down.

 

You can ID the vacuum leak with a cigar/cigarette/vape pen.

 

Yeah, I wouldn’t put it past the first shop to have knocked something loose, the second shop was actually very unimpressed by the first.

Thank you for the story, was quite entertaining, and provides a good example of an issue actually being caused by something else. I’ll definitely proceed with testing for vacuum leaks.

 

A buddy of mine (the same guy I usually talk about, he’s been teaching me how to work on stuff), wasn't jazzed about the idea of using carburetor cleaner because the liquid is explosive and bad for rubber parts. He actually built his own contraption out of a propane tank that yields the same results as a carburetor cleaner without the danger. I’ll probably do that to start, but if it’s tricky to find exactly where it is I’ll look into a smoke related procedure

 

There are videos on YouTube about using propane to ID a vacuum leak. I just posted the Scotty Kilmer video because I've linked to it before. Using a cigar emulates a smoke generator tool professional technicians use to ID vacuum leaks.