The hood is probably grounded to minimize any outside electrical interference from confusing the computer and its sensors, like from a passing 18-wheeler chatting it up with a high-power CB radio. The hood ground seems pretty unlikely culprit for OP’s symptoms. It is very difficult to measure low resistance accurately using ohm-meter. Usually that is done using a voltage drop measurement. Other than the one signal ground pin in question, I doubt this is a grounding problem. Suggest to be very cautious about that pin, b/c if you ground an ECU pin that shouldn’t be grounded that could very easily cause irreparable damage the ECU. If that pin is supposed to be grounded but isn’t, suggest to hire an auto-electric expert to figure out why.
I’ve heard a couple theories on hood grounds
When they started putting A.M. radios in cars the strap was to ground the hood to suppress electrical noises from the charging system and ignition system reaching the antenna, The pivot points in hinges didn’t make very good ground connections while driving down the road.
I’ve also heard that the wind blowing across the hood under the right conditions while driving down the road could create static electricity and sparks could be seen jumping from the hood to the body.
I don’t know if they are right, just what I’ve heard.
Yes, it’s possible. Paint doesn’t conduct electricity well and the wind rubs the paint continuously. An analog is walking across the carpet with rubber soled shoes. The rubbing creates a charge in both cases. Since the paint doesn’t conduct electricity it builds up charge until it dissipates to the closest conductor, the metal hood usually. It is basically a capacitor. The sparks would be electricity dissipating into the air since it discharges in all directions.
Whoever mentioned the signal wire got my attention. A bad one can cause a very rough running engine. The thing has only got four cylinders so losing one can have a big impact.
I’ll look into that signal ground wire I mentioned a little more and I’ll also try that wiggle test @TwinTurbo mentioned and also just check for the correct voltages at different signal wires. I’ll report back once I get some time to check
@Mustangman @TwinTurbo Ok, so apparently pin 46 (signal ground) is a special filtered ground that isn’t directly connected to ground so that’s why it seemed open. I did a few resistance tests listed in this other forum post for a full size truck: The #46 pin ground is hot, should it be? - Ford Truck Enthusiasts Forums, but it passed all the tests. I also saw in this article here: EEC-IV and Grounds - GrandMarq.NET - Panther Headquarters, that grounding pins 40 and 60 to chassis could cause issues, but I checked and both pins were grounded straight to the battery terminal. I checked for continuity and resistance at every sensor that the wiring diagram says that pin is connected to and they all checked out fine. I also did a voltage drop test and saw no more than 0.04v drop anywhere. I couldn’t check for voltage drop on any of the wires relating to pin 46 because I can’t get to the wire on the sensor without taking the connector off. I also moved around the wires at the ECU harness a bit and saw absolutely no change when moving any of the wires. Correct me if I’m wrong, but to do the voltage drop test, I put the negative side of my multi-meter on the negative battery terminal and then positive side on whatever I was testing. I also tried to put a load on the system by turning a lot of stuff on like lights, radio, blower fan at full speed. I did see something odd though, when I moved the wires for the canister purge solenoid and also the intake air temp sensor, I swear I heard a small buzzing noise for a second. This probably isn’t related, but my gauge light fuse blew again after replacing the fuse about a week ago and maybe using the gauge lights for one or two 18 minute drives.
Re: ECM pin 46. On the diagram I’m looking at, pin 46 seems to be directly connected via B/W wire to splice S127. Other gadgets connected to same splice: Knock sensor -, MAP sensor -, IAT sensor -, ECT sensor -, throttle pos sensor -, and connector C199 (used supposedly only during vehicle ass’y). S127 is supposedly located near the TPS. The diagram doesn’t show any connection from S127 to chassis ground. I’m guessing it is some sort of driven-ground, meaning the ECM provides the ground reference itself, electronically, via pin 46. It would measure close to 0 volts with a DVM, but has no connection to chassis ground. It’s likely connected to the output of a feedback amplifier located on the ECM. Difficult concept to explain briefly, but this is common technique in electronic instrumentation design to reduce the effects of electrical interference. If my speculation is the case, that pin should definitely not be connected to chassis ground, as that could zap the amplifier circuit providing the active ground.
Yeah that’s basically what I read when I was researching it. I’ll probably take apart the dash soon and check out the wiring in there to see if anything is open or touching metal in places it shouldn’t be because I need to figure out the gauge light fuse problem. Seems like the ECU wiring is ok. I also wanna check the voltages at each sensor wire on the ECU just to be sure nothing is weird there. Its an easy thing to do so I might as well
If I had that symptom on a Ford of that era the first test I’d do is to measure the amount of EGR applied vacuum, if such a thing is easy to do. There should be almost no vacuum on the EGR control port at all at idle, then the vacuum should increase gradually with increasing rpm.
I’m assuming you mean the vacuum solenoid when you say the control port. There’s two vacuum lines coming out of the solenoid, one goes to the EGR valve and the other goes to the intake manifold plenum, there’s also a line going from the EGR to the EGR pressure feedback sensor. Which would I measure at?
Did I miss that part where you replaced the EGR feedback sensor? You mentioned all of the other EGR components. If not, replacing the feedback sensor should be your first step. A faulty feedback sensor will set an EGR code and cause the engine to run rough at idle, miss, and run rich.
Yeah, I replaced the pressure feedback sensor and saw no change in how it ran.
I still wonder if the problem may be coolant leaking into one of the cylinders from a defective head gasket or a cracked cylinder head as a I suggested in an earlier post.
I would suggest letting the truck sit overnight or longer and before starting the engine, remove the spark plugs one at a time and look for coolant on the firing tip. This isn’t a perfect test for your problem, but it’s free and you aren’t throwing parts at the problem.
I’m not familiar with the EGR system configuration your particular make/model, but the way it works on my vehicles, the egr valve itself is vacuum actuated, has an internal vacuum motor, and has a vacuum line going to it. The EGR valve opens in proportion to the amount of vacuum applied. My Ford truck’s EGR has a single vacuum input, and it connects to a carb vacuum source, ported from above the throttle valve, so the vacuum to the EGR’s vacuum actuator is greatest when the throttle valve is open (higher rpm, higher engine loads, etc), which is what you want to control the amount the EGR valve opens. My Corolla on the other hand, its EGR valve has two vacuum line inputs. The main vacuum input is from the EGR vacuum modulator (which is where I’d measure the vacuum), which approximately duplicates the truck’s carb port function; the second input connects to an electric solenoid actuated vacuum (air) valve, and allows the power train computer to disable the EGR valve under certain operating conditions, for example under wide open throttle, when the driver presumably wants access to all the engine power available , for passing etc.
fyi, a discussion on the early 90’s Corolla’s EGR configuration for comparison. It seems like it is pretty similar to Ford’s method used on your Ranger
I’m pretty sure I would measure it at the line going from the solenoid to the EGR valve. Also, I noticed whenever I turn on the headlights that my RPM drops about 50 or so, its a difference I can hear, is that normal? I’ve seen conflicting things like that’s normal because the alternator has to work harder, which makes it harder to turn, but other people say the car should compensate for that and the idle should stay the same. I finally got a MPG reading too to see how rich its running, and I’m getting about 11.8 MPG city. I got a video of the idle walking all over the place at cold start too in case that maybe helps solve this.I didn’t touch the throttle at any point during this video. https://www.youtube.com/watch?v=lAviZ9Mgzmo
My Corolla’s idle rpm increases 50-100 rpm (by design) when I turn the headlights on. Whether than applies to an 86 Ranger, don’t know. It doesn’t happen on my early 70’s carbureted Ford truck. I’ve never noticed the idle rpm decrease on the truck when I turn the headlights on though. On the Corolla an “idle up” solenoid is turned on whenever the headlights are turned on, which allows a little more air into the intake manifold to increase the engine rpm. Same idea applies to the rear window defroster circuit. Turning the steering wheel also increases the idle rpm, done in a similar way, but the air valve is part of the power steering pump.
I expect both the vacuum hose from the EGR modulator or the one from the air-valve solenoid would measure the same, b/c the two are connected together inside the EGR valve. Probably the connection is a small hole, so the response time might not be immediate. Speaking of the Corolla system. Just guessing it is configured more or less the same on your Ford. Easy enough to know for sure, just measure both of them.
Perhaps a smoke test for a hidden vacuum leak.
I’ll try that. Probably will rent a vacuum pump from autozone to read how much vacuum the system is under too
What’s your car-repair and diagnosis diy’er skill level? I ask b/c most diy’ers would already own that tool Since car repair is an inherently unsafe activity, if you lack the necessary experience, you’d probably be better off hiring a pro shop to resolve this issue for you. In the meantime try to find someone experienced, who will give you some hands-on experience.