My 1990 chev ignition switch is not ingaging to start truck it has a manual transmission

Information for the combined neutral safety switch and reverse switch, but you are guessing that it is the problem.

https://www.google.com/search?q=1990+caprice+classic+replace+neutral+safety+switch

Diagnostics is the way to solve. I am not sure if it has an immobilizer, but you disconnected the battery. If there is an engine immobilizer, it needs to be reset.

https://portal-diagnostov.com/en/2020/07/04/chevrolet-caprice-classic-1991-1990-92-engine-performance-4-3l-self-diagnostics-tbi-tests-w-codes/

This is the best way to fix it. An auto electrician with a pro grade scan tool computer. The "let's try replacing this" method of auto repair doesn't work and is expensive.

https://www.google.com/search?q=mobile+auto+electrician+near+me

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sounds like the fuel pump is not getting fuel to the injectors - check the fuel filter and if that is not plugged up, then check the fuel manifold pressure - if it's about 10psi or more (when the ignition switch is on) then it's probably not the fuel system. it the ignition system okay (sparks to the plugs?) you could have a burned out coil or spark plug wires - or spark plugs - but since it was running, I'd check the fuel system first

Not enough of a description of the problem. Does the car crank at all? How long was it sitting?

could be ignition switch, crank sensor or cam sensor, neutral safety switch. If you can get a scan tool, with OBD I plug for gm you will be able to pull code/codes. Better than removing /replacing parts until you find problem. Two of my customers have had the same problem, it was the ignition switch on one and the crank sensor on the other.

time to replace the starter solenoid

4.2L 6-Cylinder

1. Set parking brake. Shift automatic transmission to Park or manual transmission to Neutral. With A/C off, start engine and attain normal operating temperature.

2. With ignition OFF, connect an ignition timing light and a calibrated, expanded-scale tachometer.

3. Disconnect 4 " and 10 " Hg (CEC System) vacuum switch assembly wire connector (located at top of valve cover). Disconnect and plug distributor vacuum advance hose.

4. Start engine, increase engine speed to 1600 RPM and check ignition timing. Adjust timing to specification listed on Emission Control Information label (if necessary). Tighten distributor hold-down clamp and recheck ignition timing.

5. Turn engine off and remove timing light and tachometer. Connect No. 1 spark plug lead, if disconnected. Unplug and connect distributor vacuum advance hose. Connect vacuum switch assembly wire connector.

NOTE: If timing light has an adjustable advance control feature, turn control to OFF position.

Timing is set per the emissions tag under hood.

1. Disconnect battery ground cable.
2. Place lock cylinder in the Run position.
3. Remove steering wheel.
4. Remove turn signal switch. It is not necessary to completely remove the turn signal switch from the column. Pull switch rearward enough to slip it over the end of the shaft. Do not pull the harness out of the column.
5. Remove attaching screw and lock cylinder set. Do not drop the screw into the column.
6. To install, align cylinder key with keyway in housing, as shown in Fig. 1. Push the lock completely inward, install attaching screw and torque to 40 inch lbs., on standard columns, or 22 inch lbs., on tilt columns.

Change modulator valve on transmission.

have your bmw dealers check switch it might need to be replace.

how to replace ignition switch in 1990 chev 1500 pickup long bed