1991 GMC Safari Intertia Switch

[cubsfan1977]

Does anyone know if a 91 GMC Safari has an intertia switch??
My van has been running fine, but the other day one of my back tires went off a curb, and went it hit the ground my van shut off, the battery and everything works fine, it just won't turn over, we have already replaced the relay for the fuel pump, but heard some vehicles are equipped with this emergency switch cut off in case of an accident.
If it does have this, where is it and how do I reset it?

Thanks
Mike

[old_master]

Astro / Safari vans are not equipped with an inertia switch. The oil pressure sending unit will shut down the fuel pump to prevent damage to the engine in the case of an oil pressure loss. A malfunction in the fuel pump circuit, a loss of engine oil pressure, or turning the key off, are the only ways the fuel pump will be shut down.

[Blue Bowtie]

First, the Old Master is correct in stating that almost NO GM vehicles had an "inertia switch" (actually, a kinetic switch) for the fuel pump. GM vehicles with HID headlamps DO have such a switch (one-time fusible) for shutting down headlamp ballast output on impact, but not for the fuel pump. These switches for fuel pumps are commonly found on Ford and other vehicles.

The oil pressure sending unit will shut down the fuel pump to prevent damage to the engine in the case of an oil pressure loss. A malfunction in the fuel pump circuit, a loss of engine oil pressure, or turning the key off, are the only ways the fuel pump will be shut down.

Not to start anything, but that is a common misconception, and was once errantly taught even in factory service technical training. If you study the schematic for your GM vehicle, you'll discover that the auxilliary oil pressure switch is not in series with the fuel pump relay, but in parallel. This has been and continues to be the case for all domestic GM control schemes from 1982 EFI systems to the present. This means that it is not a "safety shut-off" as many people believe, but is merely a redundant path for pump power in the even of a relay or other pump power control circuit problem.

Aside from understanding the schematic, there are two tests that will verify this. The most obvious is to simply disconnect the wire at the oil pressure sensor and note that the engine still starts, runs, and operates normally. This is not quite so siple on a Safari, however, because of the location of the switch:



The second test is to pay attention to your vehicle. Listen to the pump when you first turn on the vehicle. It should run for a two-second prime cycle (via the ECM/PCM and pump relay) even though there is NO oil pressure. Later PCMs will also operate the pump for two seconds on power-down if the engine has not been run. Further, the PCM will energize the pump relay at any time it is receiving reference pulses from the CMP, CKP, or any ignition resolution sensor. MAGIC! All without oil pressure.

You can also listen to the pump on shut-down. Notice that it is receiving power while the engine is running, and should continue to run even after the engine is shut off, since the residual oil pressure (above 7 PSIG) is holding the auxilliary pressure switch closed. You will often notice that the pump slows down a bit after the engine shuts off due to the lower voltage (battery voltage versus charging system voltage), and will continue to run up to ten seconds or more if the engine and oil are cold, and the residual pressure takes longer to bleed off. Ever wonder what that humming was after you walked away from your van on those cold days?

But don't take my word for it - I could be mistaken. Check for yourself. At last check, I've been right about this for the last 25-some model years. The operator is responsible for shutting off the engine in case of oil pressure loss, so monitor you indicators and instruments.

[old_master]

[quote=Blue Bowtie]

Not to start anything, but that is a common misconception, and was once errantly taught even in factory service technical training. If you study the schematic for your GM vehicle, you'll discover that the auxilliary oil pressure switch is not in series with the fuel pump relay, but in parallel. This has been and continues to be the case for all domestic GM control schemes from 1982 EFI systems to the present. This means that it is not a "safety shut-off" as many people believe, but is merely a redundant path for pump power in the even of a relay or other pump power control circuit problem.

I studied the schematics closely, and noticed that on the fused side of the ECM fuse, which is hot at all times, there is a splice. One wire feeds the fuel pump relay, the second wire feeds the auxiliary side of the oil pressure switch then directly to the fuel pump, a parallel circuit. I service a fleet of Astro/Safari vans, and of the three I have tested so far, a '96, '98 and '02, every one remained running after disconnecting the fuel pump relay. This proves your theory of a redundant, or back up circuit. However, all three also stalled after disconnecting the oil pressure switch, (with the fuel pump relay connected). What all this means is that the ECM is programmed to shut off the fuel pump relay trigger circuit after the engine is running. By doing this it allows the oil pressure auxiliary switch to control the fuel pump. It also explains why the fuel pump continues to run after the key is turned off for a few seconds until the residual oil pressure bleeds off. I can't say what the results will be with other year vehicles until I test them, but I will edit this post as I find out. Unless I have 3 ECM's with the same problem, the oil pressure switch is in fact a safety shut off. It is possible that some ECM’s are/were programmed to leave the fuel pump relay energized after the engine is running. If that’s the case, it would bypass a perfectly good safety feature, and waste money on unnecessary parts doing it. Hmm, maybe that’s one of the reasons why they’re in financial trouble.