As many of you know, I'm doing a carb'd magnum 360 swap into my 90 vert and getting rid of the computer. I've decided to re-use the rear wiring harness and 50 pin bulk head connector because it appears that all that functionality is mostly self contained. When I say re-use, I'm going to re-pin/freshen up the connectors and wiring, but use the old wiring as a template.

So far, I've identified that the rear harness takes care of:

Rear ABS unit
Fuel pump and sending unit
Rear lights, brake, signals, etc.

The 50 pin bulkhead will keep all the interior wiring in tact, so all I have to do is map the pins to what I need in the engine bay. It's mostly straightforward, as it is the sensors for gauges, the lights, windshield wiper, washer, horn, and the interface to the rear ABS unit. A few pins go to the SBEC, like speed control, Emission reminder lamp and shift lamp (for manual, I have auto).

From what I can gather, the main pins for ignition in/out are:

1 - Main 12VDC battery feed from a fusible link
48 - Main 12VDC battery feed from a fusible link
49 - Main 12VDC battery feed to ignition circuits
50 - Main 12VDC battery feed to ignition circuits

47 - Ignition run/start back to engine bay
31 - Ignition start back to engine bay

I guess they used multiple fuseable links into the engine bay for 12VDC for different functions?

So, it appears, I can take the engine harness and redo it to my needs for the sensors, transmission, ignition circuit and headlights and call it a day? I'll have a separate circuit for the transmission overdrive/lockup from pressure sensors (PATC style), and a fan controller for the radiator.

Anyone see any major issues with this plan?

 

So, using my service manual, I've gone through all the AN pages and color coded, what stays, what's in the rear harness, what's in the engine harness, what's in the cab, and what goes. I think I've got a pretty good handle on most things, but ran into a snag.

It appears the odometer (distance sensor) from the rear of the tranny goes through the SMEC computer only. I haven't found where it comes out of the SMEC yet through the main firewall bulkhead to the gauge, but I have to believe it's somewhere.

So, how to convert? Is the sensor on the transmission simply an inductive type prox and submits a pulse? Tracing the wires on the diagrams, it appears that one wire from the SMEC goes to a splice that provides some voltage (12VDC?) to the MAP, O2, TPS and the distance sensor. Eliminating the MAP, O2 and TPS of course, so if I tie that wire to 12VDC (ignition running), and get the signal back and tie that to the bulkhead pin that goes to the odometer, is that all that is required. Or is the SMEC doing some other signal conditioning?

 

Umm ... the 90 used a hard cable between the speedo and the transmission.

I don't know where you get the idea that the odometer is run off the SMEC; there's no electrical connection to a 1990 speedometer/odometer.

(Yes, I bet the sensor on the rear of the transmission goes to the SMEC; the SMEC or SBEC uses it to determine how fast you're going. It's not wired to the speedometer/odometer, though.)

RwP

 

Yes, I know the Speedo is hard cable....wasn't sure if the odometer was part of that as I haven't removed the dash, but makes sense it should be driven off the same mechanical link. It just threw me a bit when I saw the "distance" sensor wired to the SMEC. Guess I can dispense with it altogether. Thanks Ralph.

 

Let's put it this way - inside the speedometer, there's another unit driven with these gears:

http://laspeedometergear.com/Odomete...rd%20Dodge.htm

that then drives the odometer.

Why, yes, I've replaced several of those in Ford speedos already, why do you ask? *grins*

RwP

 

Robert, is it possible the sensor on the rear transmission is used for the cruise control? Just taking a guess.

 

93, very possible....All of the cruise control stuff goes through the controller as well. One of the functions that I'll lose dropping the PCM, but no big loss.

Here's what I've identified that I'll have to rewire/manage. Info might help someone down the line:

AN25 - Transmission overdrive and lockup. The overdrive button from the cab goes to the PCM and the ground signal to the overdrive servo comes from the PCM. I'll rewire the ground to a PATC pressure switch for overdrive. Not sure how I'll integrate the button yet as it's a switched transistor signal from an overdrive module in the cab. The lockup will come from a PATC pressure switch also.

AN19 - the temperature sending unit differs from the 2.5L and 3.9L. In 2.5 it was sensor directly to dash. In 3.9L it seems to route it's way through the PCM. I'll just convert it back to the 2.5 style sensor as the dash was the same for both.

AN17 - will lose the idiot light check engine function...connections 3 and 20. Will also lose the maintenance required light, connection 56. The rest is the throttle kicker motor which I'll no longer have, and a signal for the starter relay which will be wired to the ignition switch.

AN18 - lose all the engine management...throttle kicker motor, coolant, MAP, Distributor, and distance sensor. If my truck was Canadian I'd lose the daytime running lamps

AN15 - Lose the O2 sensor, TPS sensor, Injectors, Diagnostic connector, and some wires to the A/C.

AN16 - Need to rewire the Auto Shutdown Relay ground. It currently comes from the PCM. Lose the Air Switch Solenoid, Purge solenoid and EGR Solenoid. I'm keeping the charcoal canister for fumes, will just need to wire a way to purge it. Also lose the negative side of the ignition coil (Using 1 wire HEI Distributor with integrated coil).

AN21, 22 - lose cruise control

AN27 - lose the 12VDC feed to cruise control...don't need it.

AN52 - lose A/C

 

Ahem.

Check the wiring again.

For a 1990, there's two sensors for the engine temperature; one goes straight to the dash, one goes straight to the ECU. No huhu (hint: 1990, still V-belt, it's under the A/C compressor for the gauge).

You can always dig up a 1988 or earlier cruise control; it's self-contained and bodges into the speedo cable.

A/C can be controlled; the ECU dumps the clutch signal if a) it's overheating or b) you're close enough to WOT; you can use a vacuum switch for b), and wire it with a temp switch for a).

You don't need the "CHK ENG" or "MAINT REQ" lights if you're dropping the ECU.

The ASD relay needs to be wired to "ON IN RUN OR START", so that it will turn the pump and ignition off when you turn the motor off.

The Overdrive is just a module to supply "OD ON" or "OD OFF" to the ECU; the PATC design doesn't need that internal switch, but if you must, use a ON/OFF pushbutton and wire that in instead.

If you want to keep cruise, but don't want to hunt up a 1988 or earlier unit and the speedo cables for it, there are units that will run off the VSS signal and control the current servo ... but dang if I know where or how much!

RwP

 

Thanks Ralph....When I was saying "lose" it meant I was gladly getting rid of those things. I wasn't saying I was losing them and missing them, so most of your comments reiterate what I'd planned anyway. As for the coolant sensor, yes there are two (actually 3, but who's counting). AN-18 and AN-19 show them and the difference in wiring between the 2.5L and 3.9L.

 

Hey Robert, this is a little off topic, but could you tell me if the harness connectors are hard to find? Thanks