MAF/ECU wiring question

[Second Shadow]

blacktop S13 sr20det in 93 s13 chassis.

a couple months ago I started running really rich, and just this week I finally found the problem, bad ground on the MAF. it had continuity to ground using the ohmmeter so I thought it was good, but when I finally rechecked it with the voltmeter it had 160mV, and that raised as the engine revved.

well, I cut the wire on the pigtail and grounded it to a bolt on the chassis and capped off the wire on the harness, problem was fixed. However, I've looked up the wiring diagram and that ground(I believe) goes to pin 17 on the ECU and I can't help but think that that pin should be a ground. if the wire was dropping like 10-20mV and just wasn't good enough for the MAF thats one thing, but 160mV that raises as the engine revs (it broke 200mV pretty quickly) concerns me.

is it possible the ECU isn't grounded properly or perhaps has gone bad on that circuit? really curious if anyone knows what would give me voltage readings like that on an ECU "ground" pin. thanks.

[PyR0NiAk]

Hopefully OM3GA can step in on this one...

[OM3GA]

That definitely sounds like a strange issue. For starters i would definitely check ALL grounds to verify that they are not corroded and also that they are attached. I would definitely start off by trying another ECU that may definitely be the problem.

What would be more likely is that the ground going through the harness would actually be bad somewhere in there. Possibly try checking the resistance between the wire that was cut and the pin on the ecu plug. That is great that you have pin-pointed your problem, i would definitely suggest trying another ECU and if that is not the case the only thing it could be is wire in the harness.

Good Luck!



Pin Wire Description
101 W/B Fuel Injector # 1 Drive Input
102 O/L EAI AIV Control Solenoid Output
103 G/B Fuel Injector # 3 Drive Input
104 B/P Fuel Pump Relay
105 R/W Fuel Pressure Control Module
106 G/Y A/C Relay
107 B Fuel Injector Ground
108 B Fuel Injector Ground
109 R ECU Current Return
110 Y/B Fuel Injector # 2 Drive Input
111 O/B Turbo Boost Pressure Control Solenoid
112 L/B Fuel Injector # 4 Drive Input
113 SB Auxiliary Air Control, Idle Speed Control
114 Br/Y (unknown function)
115
116 B Fuel Injector Ground
1 R/W Ignition Signal # 1
2 R/Y Ignition Signal # 2
3 Y/R Tachometer Signal Output
4 R/B ECU Relay
5
6 B Ignition Signal Ground
7 G/B Receive (Control Unit Data Reception) RX
8 R/G Ignition Signal # 3
9 R/L Ignition Signal # 4
10 G/Y A/C Relay
11
12 P/B Automatic Transmission Gear Select
13 B Ignition Signal Ground
14 G/W Clock (Synchronization Signal) CLK
15 G Transmit (Data Sent From Control Unit) TX
16 W Mass Air Flow Sensor Signal Input
17 B Mass Air Flow Sensor Ground
18 L/O Engine Coolant Temperature Sensor Input
19 W Oxygen Sensor Input
20 W Throttle Position Sensor
21 B Sensor Ground
22 W Cam Angle Sensor 180 Degree Signal
23 G/R CLK Data Link Connector For Nissan Consult
24 / Malfunction Indicator Lamp (Add Wire For Check Engine Light)
25 O/B Exhaust Over Temp Warning Lamp Output
26 L/Y Exhaust Temperature Sensor
27 W Detonation/ Knock Sensor Input
28 W/L Automatic Transmission TPS (Kick Down Switch)
29 B Sensor Ground
30 W Cam Angle Sensor 180 Degree Signal
31 B Cam Angle Sensor 1 Degree Signal
32 Y/G Vehicle Speed Sensor
33
34 O Start Signal
35 G/O Park/ Neutral Position Switch
36 B/R Ignition Switch
37 Lg/R Throttle Position Sensor Power Output
38 B/W ECU Power
39 B ECU Ground
40 B Cam Angle Sensor 1 Degree Signal
41 L/G A/C ON Switch
42 Br/Y Automatic Transmission Gear Select
43 Pu/W Power Steering Oil Pressure Switch (Helps Idle When Turning Wheels)
44 Lg/R Automatic Transmission Gear Select
45 L/W Intake Air Temperature Sensor
46 R ECU Backup Power
47 B/W ECU Power
48 B ECU Ground

[PyR0NiAk]

What would be more likely is that the ground going through the harness would actually be bad somewhere in there. Possibly try checking the resistance between the wire that was cut and the pin on the ecu plug. That is great that you have pin-pointed your problem, i would definitely suggest trying another ECU and if that is not the case the only thing it could be is wire in the harness.

Pretty much what I was thinking as far as the bad ground in the harness but figured I'd wait and see if you could suggest something else. As soon as I read this thread, I thought of you.

[zachattack_5491]

Be sure to let us know what you find out. I'm having a very similar problem, but I never even thought to check for voltage on the ground. Let me know if a new ECU helps, because I think I'm having almost the same problem as you.

[Second Shadow]

okay so I did some more checks and here's what I got:

key in on position, engine off:
ECU pin 39(ECU ground): 0V drop to chassis ground, approx. 60 ohms
ECU pin 17(MAF ground@ ECU): 0V drop to chassis ground, approx 70 ohms

engine running @ idle:
ECU pin 39: approx 10mV, 0.0 ohms to chassis
ECU pin 17: approx 10mV, 0.0 ohms to chassis
note: the resistance change doesn't necessarily mean anything, once the circuit using this ground is live it can throw off readings, in fact resistance lead to lead was like .4 ohms, and through the chassis it was dead on balls 0.00.

now comes the fun one, @ the MAF with engine running @ idle:
white/black wire: approx 14V battery voltage
white: 1.42V signal(I believe this is in spec).
black: 0.0V obviously as it's grounded to the chassis with a 20" wire.
**old "MAF ground" wire that was capped off and is supposedly going to ECU pin 17:
.4 to .7 volts, bouncing erratically.

I have no idea wtf is up with that wire. I thought for a second maybe it's broken, but then how does it have half a volt on it? how does it also only have like 1 ohm to ground at the same time? I'm starting to think maybe it's broken, but the broken tips are just barely touching, enough for the meter to see continuity, but not provide a proper path for current to ground, and then maybe it's actually picking up bleed over voltage from the electromagnetic fields the other wires might create. yeah, it sounds like crazy borderline BS to me too, but wtf else could cause something like this.

hopefully in about a year or two, I'm gonna do a wire...well it's not a tuck cause I don't wanna tuck anything, but I'm going to go through and remove all the excess wires not doing anything. so maybe when that happens and I have the whole harness out I can trace where that wire is going and what's causing this, but for now I think I'm just gonna be happy the engine's running right.