1994 S10 4.3 4x4 Tahoe LT, Everything was working and replaced a jammed ignition key lock cylinder with a AC Delco D1457C. Cranks but no start, no activation of the fuel pump. Located the fuel pump relay, activated the relay coil and continuity was established through the power terminals. The original purchase invoice says the Blazer has "anti-chip protection", what ever that is. When to the dealership to check if I had the correct ignition key lock cylinder part and by the VIN, I had the correct part. To verify the fuel pump would work and the engine run, I jumped the power terminal of the fuel pump relay and the fuel pump came on and then started the engine, which ran fine. Checked power to the fuel pump relay none was noted which comes from the PCM. A locksmith told me, "the key lock cylinder is merely mechanical and there so no relearning for the PCM when a new key lock cylinder is installed, something else is amiss." When the ignition key goes to start then on position, there is no security light present. Was going to put the old key lock cylinder back in and check to see if the fuel pump would be activated and start but it jammed up again and can't turn it. Any Thoughts.

 

Any aftermarket security added?

The 94 should not have much in the order of key security. As the locksmith said, merely mechanical.

You could try and replace just the locked cylinder internals and put the OEM back. Or minutely compare them.
Checked all connections to the lock cylinder? Not that simply a cable came loose while changing it.

 

1994 S10 4.3 4x4 Tahoe LT key lock cylinder replacement - no fuel pump activation

Thread Update: I’ve had some interesting developments in the basic problem of the coil of the fuel pump relay not being power by the PMC. For the mechanic guru’s, please correct any mistakes you may recognize or additional opinions you may have.

I have seen several post threads concerning this problem of which the actual remedy is not given. So if someone out there has a good answer let us know, if not, I intend to share the information I have accumulated.

I am now in possession of a set of GM factory shop manuals: S/T Truck 1994 Driveability & Emissions and Electrical Diagnosis Manual and S/T Truck 1994 Service Manual for $138.00. Also have purchased a Actron Elite AutoScanner CP9690 for $204.00 which is in the shipping process. i was looking at 3 scanners, the Actron CP9690, Innova 3120 and OTC 3211 all of which are OBD 1 & II. All said, they covered some of GM’s 1994 vehicles, but didn’t list them, so I called their customer service and was told the Innova 3120 and OTC 3211 did not function with my 1994 Blazer, however when I talked with the Actor CP9690 representative which ask questions like the others, but also ask if it was a L series. Not knowing what that meant, he ask if it was 4 wheel drive, I said yes, and he said the CP9690 did read the vehicle. I needed a good OBD 1 anyway and it reads OBD II thru years 2013. Tidbit, In the GM Service Manual p. OA-3 Vehicle Identification Number (VIN) the 4th digit denotes Code, Line and Chassis Type. (Code) S - Sm Conventional Cab (line) and 4x2 is (type), and (Code) T - Sm Conventional Cab (line) and 4x4 is (type), only 2 codes are given, the S or T, mine is a T.

I’ll give you a brief summary of where I’m at and intend to follow with a more detailed evaluation of the information I have accumulated so there will be additional information I have discovered to shorten your processes of this repair. One thing I want to share about the VIN. the 8th digit which denotes Code - Producer - Type - RPO (Regular Production Options). Three listings W - CPC - 4.3L V6 CPI - L35), (Z CPC - 4.3L V6 EFI - LB4) & (4 - CPC - 2.2L L4 MFI - LN2). I have seen references to the L35 and didn’t know what to answer, but my 8th digit code is W which has the RPO - L35 designation. Just a tidbit. One thing I would like to say about the GM manuals is they are very readable and easy to understand the information, one does have to learn where to access different information and their locations in the manuals.

Currently my blazer is working. Put the new key in the associated new key lock cylinder, turn the ignition to ON and the fuel pump activates for 2 seconds and shuts off, then turn the key to start and the engine starts and runs great. Tidbit - The thing about the anti-chip protection in my initial post which swayed me to think about “key security” to shut down the PMC (powertrain control module) - computer and having to go through a computer relearn process is not the case, the anti-chip protection probably had to do with protection for the vehicle’s paint as in a clear coat. What I am going to explain next is even thou the blazer is working is I have a unsolved same problem lurking. Taking a break!

 

1994 S10 4.3 4x4 Tahoe LT - no fuel pump activation

So the main problem is/was the fuel pump relay coil was not activated by the PCM which all occurred after I replaced a non-working key lock cylinder with a new one. The old key lock cylinder was stuck in the ON position and could be moved to Start, but could not be moved to the Off position. The GM manual says don’t reconnect a battery with the key in the ON position, only in the OFF position because this could result in damage to the PCM. Also warns about handling sensor devices without grounding yourself to relieve any static electric that could transfer through handling the sensitive sensors and cause damage, especially the PMC. Not knowing this, I pulled the coil wire off to kill the engine and then disconnected the battery negative cable because the ignition key was stuck in the ON position. When through this process probably 10 or more times. So if you have this type of problem, shut it down until the key lock cylinder is replaced. Repair time would normally take about 1 1/2 hours. I ordered a AC Delco part so it wasn’t ready available, also find out what is required for tools and what to do before starting.

My blazer has the fuel pump relay located to the right of the power brake booster at the top of the firewall, looking from the front to the back. Two relays are mounted together, the fuel pump (FP) relay on the left side and the intake manifold tuning valve (IMTV) relay on the right. The two relays are interchangeable (Important). When I took the FP relay out and put the IMTV relay in it’s place, the fuel pump worked. Started the engine which ran rough because the IMTV relay was not connected. Bought a new relay for the FP and put the IMTV relay back and the vehicle is working fine. From some of the checks I had made previously gave me pause that something else may be wrong, but maybe not. The fuel pump voltage wire 440 ORN is hot at all time. It is fused with the 15 amp ECM-B fuse in the fuse block. The 120 GRY is hot when the relay switch is completed and powers the fuel pump. The 465 DR GRN/WHT is 12V from F6 fuel pump relay control in the PCM. In testing for voltage at the DR GRN/WHT wire connector with the key in the ON position, it is only a 2 second activation of 12 volts by the PCM. When I tested this terminal with my digital Fluke voltmeter (VM) positive lead on ORG/WHT wire relay terminal and negative grounded. It showed a momentary, less than a second, 12 volts then faded down.

So the first thing is do in the case of no fuel pump 2 second activation when the key is in the ON position which results in a engine crank and no start condition is uncover the fuse panel located under the dash, driver side and check the ECM-B 15 amp fuse and the ECM 10 amp fuse for 12V on both sides of fuse. If good, located the FP relay. Disconnect the relay, check voltage on the ORG terminal, should have 12 volts. If good re-connect FP relay to address a faulty terminal connection. Put the key to ON is there a 2 second fuel pump activation. If no activation, remove the FP relay and substitute the IMTV relay. Put the key to ON is there a 2 second fuel pump activation. If yes, replace the FP relay.

At this point I will address the oil pressure switch. My blazer is supposed to have one but haven’t seen it yet. The oil pressure switch is a normal open switch run in parallel circuitry with the fuel pump relay. It closes when enough oil pressure produced by the engine is reached. If the fuel pump relay fails while driving the oil pressure switch would keep the fuel pump activated until the engine was turned off, causing the oil pressure to drop, causing the oil pressure switch to open. The 440 orange 12V hot wire powers both the oil pressure switch and the fuel pump relay. A ECM B (ECM Battery) fuse 15 amp, hot at all time, supplies the power. When the individual internal relay switches closes, power flows to the fuel pump. A faulty oil pressure relay should not affect the workings of the FP relay. Checking the oil pressure relay if the engine were running, simply disconnect the FP relay and the engine should keep running normally. This is what I did, so I know there is one and it is working. Otherwise one could crank the engine (bad for the battery and starter) until the oil pressure switch closed and power would be supplied to the fuel pump. So with the problem of no fuel pump 2 second activation when the key is in the ON position, the oil pressure switch is not involve. Taking a break!