I have a 91 (zf6 conversion) that has had two drives with the new larger motor. Based on several checklists (and the fact that I got it to run by adding a quart of oil), I feel that it is a oil pressure switch or bad ECM.

So how do you simulate the oil pressure switch? Ground it? 12v?
Is the oil pressure switch the single wire connector near the distributor?
Or is the 2/3 wire connector near the distributor? (I'm thinking no)
Or is the 2/3 wire connect just above the oil filter the pressure switch?

Other data for your questions
measurements via diacom thingy
The car cranks over at 9.36v with a huffing sound
The car has reasonable fuel pressure 40 something
The neutral/clutch switch works beacuse it cranks over
injectors show 12+ ohms
injector harness show +12V available
Diacom show 30 for MAP
Diacom shows 0.0/0.0 for injector pulse width/Duty cycle
Oil pressure gauge in car reads 15-20 psi during cranking
Diacom shows 100-200 cranking RPM
spark confirmed
grounding single wire oil pressure feed has no effect
Diacom recorded 0v for TPS?

 

For a stock car in working order:

The oil pressure switch has only one terminal. I do not know where it is located on a stock '91 L98.

If you have fuel pressure then the oil pressure switch is not your problem.

At 5 PSI it closes. So if the indicated 15-20 PSI durring cranking is true then the switch should be closed and you should be able to check that the fuel pump is running.

The switch all by itself cannot cause a car not to start. Bad wiring can, however.

All bets are off once the modifications begin.

 

Diacom shows 0.0/0.0 for injector pulse width/Duty cycle


Houston we have a problem....Something is commanding the ecm to not fire the injectors.. VATS? or the wiring to it?

 

If VATS related you wouldn't have cranking
ECM has to see pulses from dist to fire injectors during cranking. Is TPS set correctly?

"The distributor will send a string of pulses to the ECM in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.If the ECM sees the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods.
The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch.

The ECM also monitors the TPS and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver is trying to clear the flooded condition .
Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Oil pressure switch has 2 terminals ; 12V feed and wire to pump.
(Org and red wires ?) Jumpering these should make the pump run full time.
1 wire unit is for gauges.

Gauge sensor and OP switch are behind dist; one above filter is oil temp sensor

 

For my 91...
My experimentation was the one wire did not effect the gauges.
My experimentation was the three wire is the oil press gauge source.

 

Diacom show 30 for MAP


How can the map be 30 kpa cranking?????? That is 3/10's of an atmoshpere or 20 inches vacuum... you can not do that at 200 rpm and probably not at idle either.. I would check map sensor and wiring to it and ecm and what diacom says key on engine off for map... I would think around 100 kpa there and would expect something like 75 or 80 cranking...

The key is the ecm not commanding a pulse so until it does it is not going to start. it is an SD system so no MAF to go bad there are not many things that would cause a command of zero for PW...

The VATS signal to the ECM could be missing as well. If the vats module sees the resistor it will along cranking but if it does not send a signal to the ECM or the wiring is bad it will not allow the injectors to fire and a possible reason for the zero PW. Just because the car cranks is not proof everything in VATS is working only that it reads the key and the signal to the start enable relay is good...

I can only dream up 3 things that would cause zero PW commanded.

One is clear from flood ie. WOT at cranking. If tps volts are right and you don't have the pedal to the floor ..BTW.. and the ECM is seeing around .54 volts it can not be that.

If VATS signal is missing from VATS module to ECM another reason for zero PW.

Diacom shows or an off the wall sensor reading that never got programed like to 30 kpa at cranking... I am real curious now what Diacom says the map sensor reads with key on engine off to see if they have this number inverted as in 30 kpa vacuum.. It would surprise me if they do as it is a Manifold Absolute Pressure....
Dave

 

The switch has two wires. At the rear of the intake and to left of dist. Providing it does not have the later combined switch/transducer.

Huh?

The OP's 91 may have a single device with three wires performing the function of the two separate ones.