So, my car has started the usual 3800 hesitation a while back, but has continued to get worse. When the car is fully warmed up, it will not even rev above 3800 under load. The last time I had my intake manifold off, I noticed that the engine ground was a bit dried up, but I sanded it down, and made sure it was bolted down nice and tight. I figured this was the the root of my grounding problems so I set out to reground the ECU to the body under the dash.

After looking at the FSM, I found that pins 2C, 2R, 3A, and 3G. The FSM also says that the pin out provided is from the Control Unit Side. I assume this is the point of view when looking at the plug side of the connection, not the wire side. With this vantage point in mind, I spliced all 4 wires into a ring terminal that I grounded with the bolt holding the kickplate down.

In short, these were not the correct wires. The car would not run and I ended up blowing the ECU 30A fuse. After removing the splices, resoldeing the harness, and replacing the fuse, the car runs as it used to.

Now to my real question
I cut back the black tape on the harness and found 4 wires that merge into two approximately 10ga wires. These are all black wires with brown spots. I would assume these are the ECU ground. I am assuming I have confused the picture in the FSM. By "control unit side" Does it mean literally looking at the ECU, or looking at the ECU side of the plug on the wiring harness? Should I just go ahead and ground these 4 wires?

Thanks in advance,

Alex

 

if it dies when you hit 3800rpm then I would say that the secondary injectors are not coming on at all which might be more then a simple "extra ground" fix

 

Not trying to be a smart-a** or anything..but have you seen this...

http://www.aaroncake.net/rx-7/grounding.htm

Might help...?

 

I know I need to add an extra ground, which is what I tried doing but blew the EGI fuse. I just want to know if the 4 wires that converge into two 10ga black wires are the ECU grounds. If so, Ill just ground those.

 

There are no 10 gauge wires in the emissions harness.

What harness are you cutting? The emissions (ECU) harness? Engine harness?

There are two ground wires in the emissions harness that everything else splices into. They end up going to the ECU plugs, the sensors, and end up at the ring terminal at the top of the engine.

 

You look at the wires as they go into the ECU plugs. Example is when looking into the wire side of the small plug, the far right, top wire is 3A. The wire below it is 3B. You work your way from right to left in a up/down fashion.

The gnd wires should have been black in color except for 2C which is brown with a black stripe on 86-87(early) cars.

 

I had kind of guessed these 2 wires were the ones that bolt under the intake. I eyeballed the 10ga estimate, so no doubts I was wrong there. My point was that the 4 grounds (from plugs 2 and 3 of the emissions harness) converge into the 2 main ground lines that lead to under the intake manifold. Since I am not sure where this wire is faulty under the engine bay, would it be best to just splice into the 2 larger wires and ground to the body in the cabin? Thanks for clearing up my confusion on the FSM perspective of the picture, Hailers.

 

That seems to have helped, on series four cars, for some of us.

You pull the small plug off. You follow the black wires up the harness (you have to unwrap some insulation for a couple of inches) until you find the area in the harness where the gnd wires come together. You'll also find the SHIELD for the 02 sensor in that area. Make sure you don't accidently splice its center condutor to the shield or your new wire. Anyway, at that point some of us have soldered a new wire to those existing wires and teminated the other end of the new wire to one of the studs on the ECU attach bracket.

The wire at 2C connects to all the sensors in the engine bay. It IS the gnd for those sensors in the engine bay. Just saying don't accidently break the connection of 2C to those other wires that are spliced to it from the sensors in the engine bay. Just solder on to it without breaking the circuit to the other sensors.

 

You can see 3A and 2R off to the right of the jpg. The 2C is in the middle and follow it out to the sensors. It's a gnd for them through the internals of the ECU. The bottom line is that the ECU's gnd is thru the 3A and 2R, so 2C does get gnd thur those wires.

You know, there was a factory service bulletin. It had you add a gnd to the boost/pressure sensors gnd wire and terminate it on the engine/thermostat housing. Actually it wasn't a boost sensor problem but a AFM problem if you read that bulletin. It is just easier to add that gnd to the boost sensors gnd wire rather than the afm's gnd wire. When you do that, you also add a extra gnd to the other sensors in the engine bay by default.

Anyway, I just spliced a new gnd wire to 2C and combined it with 3A and 2R and bolted the end to the ECU chassis stud. It works for some, not for others. Why? Got me. It was a early series four fix.

Note: IF I remove the new gnd wire from the ECU stud...........I get hesitation again. Put it back, hesitation gone again. Worked for me and some, not others.

 

That helps a ton! One question though, what does 3G ground? If I just ground the 2 larger wires, as in solder in a ground to the cabin and leave them connected to the harness, will this take care of all 4? I believe it would, but just dont want to make any more errors than I already have.

 

3A, 3G and 2R all gnd *some* circuits inside the ECU. I've no idea which/what or why one wire couldn't have done the gnds for all the internal circuits.

You want to leave all the wires intact and just solder a new wire on to them. Like cut the insulation off the wires and solder a wire on them.

It's just my memory, but I thought three of them all came together inside the harness a few inches from the plugs. Like maybe six inches up the harness.

I'd solder at the wires and at the other end of the new wire(s) crimp on a ring terminal and bolt it to the ECU bracket. Done.

 

I have personally had a lot of success with this same type of fix (see link below). First, a couple suggestions - you must use a heavy gauge wire because if you combine 4 or 5 wires in need of a ground, you will be carrying a significant current through your new ground. I ended up melting through one before I got it right. Next, I also suggest finding the O2 sensor wire which is shielded (has a braided ground surrounding it) and pull that shield/ground into your bunch of ground wires. This will only help the performance of this component. I found that all ground wires, including the O2 ground ultimately had a black with brown/maroon dots cover which made them a little easier to identify. Good luck - hopefully you are yielded the same results because it has made my car completely reliable versus how it ran in the past.


https://www.rx7club.com/2nd-generation-specific-1986-1992-17/hesitation-dead-battery-stalling-dead-tach-fixed-757035/

 

Hrm, so my problem is better, but not solved. I grounded two of the larger 12 or 14ga ground wires which have the blue tape on them. My idle, hot startup, low speed surging, and hesitation are all better, EXCEPT...

When in gear, the car will redline fine at part throttle. Under full throttle, the car will not budge over 3800. Not exactly sure what it is. I tried grounding the MAP at the sensor (not sure if it was needed with the grounds I added to the emissions harness), but I must have patched into the wrong wire since I immediately blew the 30A Comp (?) fuse or the one closest to the engine. I grounded the black wire with a yellow stripe. It all other wires were brown with stripes, iirc. Guess I remembered wrong.

Anyways, any insight on the problem?

 

Forgot to clarify, at full throttle underload, it will NOT rev any higher than 3800. It will occationally buck up 1000 rpm or so then stop again, but its very intermittent. ALSO, this problem seems to exist much less or not at all for the first 5 minutes or so of driving the car cold. No idea why

 

Someone is color blind or on the wrong sensor. The 86 pressure/boost sensor has four wires. Black/White (12vdc), Brown/Black (gnd), Brown/Red (output) and Brown/White (5vdc ref voltage).

Brown/Black is the one that should have gotten the extra gnd wire. On SOME cars this is pure black.

Try something out. PUll the vacuum hose off the pressure/boost sensor. Plug that hose so it won't leak. Then go drive at full throttle over 3800 and see what happens.

 

Im not color blind, just dumb. the Black/white wire on mine is so dirty and faded that it looked black/brown, which I got confused with the real brown/black ground wire. Ill try the MAP trick tomorrow when I dont have 1/32 of a tank of gas. Also, I just found your thread yesterday regarding the orifice in the MAP line. I replaced my line a few months ago, but I think that is an unrelated problem.

 

So, I grounded the brown/black wire on the MAP harness and it had no effect. The car still hesitates TERRIBLY at anything over 1/2 throttle over 3800 rpm. I tried your method, Hailers, of plugging the MAP line. This made the problem worse. The car wouldnt accelerate at all over 3800. Does this mean the MAP is bad? On a side note, this problem is almost non-existent when the car isnt fully warmed up. It still stutters, but will redline under full throttle.

 

Anyone?

 

Anybody have anymore idea's?
I am having the exact same problem with my '88 T2.

When cold it'll somewhat rev over 3800 rpms but when warmed up it falls on its face if I go over 1/4 throttle in gear. I have checked and done everything mentioned above and a few of my own idea's with no luck.

 

A few people have mentioned the injector resistor pack under the AFM as being a culprit. There is one en route to my house via USPS, so Ill report back in a couple days to tell ya if thats the answer. BUT, I think the 88 cars came with high impedance injectors, which would negate needing the resistor pack Im talking about. Not sure on that though.

 

My '88 has an '87 wire harness which saved me the hassle of having the '88 high impedence injectors.

Im going to check out my resistor pack this weekend.