North American Turbocoupe Organization

PCM Ground
Not B Anymore Offline
I read somewhere that there are two ground wires from the PCM to the inner fender next to the battery. The ring terminal at that location on mine used to have two ground wires coming from it, but someone clipped one of them off before I bought the car. So are both wires for the pcm or is one for something else? Anyone know??
Brian Leavitt
'86 TC 5-Speed -- MS2x w/COP | 83 lb. injectors | T3/T4 50 Trim Stage 3 .63AR | Full 3" Exhaust - No Cat | Motorsport FMIC | Ranger Roller | Ported E6 | Walbro 255HP | Kirban | 20psi | 120-amp 3G | 8.8" 3.55 rear | '03 Cobra Wheels

Jeff K Offline
Brian, I dont know if the 86 is the same, but on the 87-88, PCM pins 40, 60 go as 2 wires to a connector in the harness near the firewall, and then head over to the batt, where there is a 2 terminal connector near the batt that goes to the batt neg terminal via a pigtail on the factory neg cable. The 2 wires also briefly merge into 1 wire briefly somewhere in the harness. IMO, a bad idea with 2 corrosion prone connectors in the very important ground circuit, but then 25+ years ago, in the early days of EFI, I dont think the engineers really understood the importance of good PCM grounds.

I left the factory PCM ground system intact and ran 2 new 12 ga wires directly from pins 40, 60 directly to the neg batt terminal (no connectors to corrode, etc). Pins 40, 60 have to sink lots of transient currents like the 25+ amp peak currents every time the injectors fire, so the less resistance in the PCM ground circuit the better.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized

84TBirdTurbo42 Offline
Senior Member
i dont think they understood either, remember the very early pcm controlled vehicles eec-1 i believe, ran on a 10v vref. think about that happens if you have a battery thats slightly weak. haha.
Chris Perry
1984 Ford Thunderbird Turbo Coupe. Dead, NY rot killed her
1986 Thunderbird shell, swapping parts from the 84.

Not B Anymore Offline
After reading your post, Jeff, I looked over the EVTM again and saw a few things that I, for some reason, missed the first time. Pins 40 and 60 are grounds as you said. They first travel to Splice 540 in the EVTM. Pin 30 is also a ground on 5-Speed cars and goes to the same splice, as do the grounds for the boost pressure switch and the EEC power relay. From there it looks like it's a single wire to Connector 434, and then on to Ground 211 (The one next to the battery), then to the negative batt. cable. So 5 wires in to this splice and one (pretty small - maybe 14 gauge?) wire out.

So in my case, the wire that was clipped off the ring terminal (by a PO) was most likely the one that ran to the negative battery terminal. So now there are two thing I'm going to do. First is run a wire from Ground 211 to the negative battery terminal. Second is run a dedicated ground from the pcm to the battery as you mentioned. Thanks for the info.
Brian Leavitt
'86 TC 5-Speed -- MS2x w/COP | 83 lb. injectors | T3/T4 50 Trim Stage 3 .63AR | Full 3" Exhaust - No Cat | Motorsport FMIC | Ranger Roller | Ported E6 | Walbro 255HP | Kirban | 20psi | 120-amp 3G | 8.8" 3.55 rear | '03 Cobra Wheels

Joe F Offline
Posting Freak
My solution several years ago was to replace the entire ground lead. I started out with some 8-gauge super flexible copper power wire. On the battery end, I crimped and soldered a ring-style connector that begins it's route under the negative battery connector's tightening bolt.

That wire then runs to the wheel-well ground position where another ring connector was installed in-line. To accomplish this about 1/4" of insulation was carefully removed from around the wire while leaving it uncut. Then a "U" shape was formed at the bottom of the ring where the wire would normally be inserted and crimped. The ring's "U" was "wrapped" around the wire, crimped, soldered, and insulated with silicone rubber to keep moisture out. Finally the wheel-well connection point was cleaned up with a wire brush and the ground ring secured with a screw, star lock washer, and some more silicone tho keep the moisture out.

The last step was to continue the improved ground run to the harness where the funky two-prong ground connector was located. This was removed and the 8-gauge ground was instead soldered directly to the harness wires and the connection insulated with some heat-shrink tubing.

The result? The engine runs smoother than it has in years due to the improved grounding for the EEC. Also, before this mod the engine would occasionally "hiccup" - it would dip down from normal 900RPM idle, almost seeming to die, then quickly rev back up to about 1500RPM, at which point it would once again drop down to normal idle. This annoying behaviour ceased completely once the stock ground was replaced with the new-and-improved version.

Hope this help someone sometime. Sorry for the lengthy post but a picture's worth a thousand words and I have no picture!
JR's Place - My '87 Turbocoupe

TurboE Offline
Posting Freak
Great info! I had a ground issue on a previous turbo coupe I had, but it was solved mostly from the engine bay engine and chassis ground, but I never thought of the EEC side of the ground. Nice to know if I see some odd things happen.
-88 TC Black
5spd, Precision SC50 T3/T4, QH/SD Tune, Gillis, AFPR, 255FP, WB O2, K&N, Ported E6, 3" DP, ATR 2.5" Duals, 3:73 Rear, Konis, Eibachs, 18" Voxx Wheels, X Drilled Rotors.
-06 G35 Coupe Diamond Graphite
-97 Pathfinder

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