TDC Sense

[lilredtruck]

OK, I'm an old distributor guy and don't know how the newer DIS engines discern where TDC for #1 cyl is, with the 7 pulse crank sensor or the 56 pulse cam sensor ala LX9. My LX9 has/had the crank sensor, cam sensor, and harmonic dampener (blue) sensor. Dave says to get a sensor from WOT which is a crank/HB sensor/7 pulse. Can you tell me how any of them know TDC?
Brian

[Google Adsense]

[pocket-rocket]

They get it from the crank sensor. The cam sensors are just used for sequential injection below something like 2000 RPM for better low speed emissions and smoother idle. Once the RPMs go above the threshold the PCM ignores the cam signal and reverts to batch firing the injectors like the old multiport engines.

[bszopi]

Pocket-rocket is correct, but to further expand on it, it has to do with the 7th notch in the trigger. The 7th notch is relatively close to another notch, so when the sensor passes those 2, the signal sent to the PCM is different, thus telling it the location of the crank.

Brad via Tapatalk

[WrathOfSocrus]

It is also keyed just like the crank so you can't put it on the wrong way.

[lilredtruck]

OK, so far so good. The blue sensor at the crank pulley is counting seven pulses, of which two are close together, indicating TDC of #1 cyl., correct? But ignition only occurs on every other TDC (rather, before) of #1 cylinder. Is the blue sensor sufficient with the OEM LX9 crank pulley to the '94 Cavalier harness/PCM? I tried to detect any places on the pulley that had magnetic influence and found none. What 'trips' the blue sensor?
Brian

[WrathOfSocrus]

It's a bit more complicated than that. The three different sensors each perform different jobs. Your 94 cavalier I believe would have a Gen 2 3.1 engine. If you are trying to put a 3500 into this car using the stock wiring then you don't need to wire the cam sensor because your car doesn't have one. You can just leave the sensor in place on the 3500 to fill the hole and not need to plug it in. The sensor on the crank pulley of a 3X00 is a 24x trigger. It doesn't exist on either the 3500 or the 3.1 so there is nothing to wire there either. That's the good news.

The 7x sensor in the 3.1 is in the middle of the crankshaft and has a sensor in the middle of the block on the firewall side. The 3500 has a 56x sensor that wont work with your computer. That is where the TCE trigger comes in. It bolts onto the front of the crank pulley. There is also an adapter for the sensor to be relocated there. You use the old crank sensor out of the side of the old engine and put it up front in the bracket. That's the only thing it needs for ignition timing.

With the coil packs, there are 3 that simply fire 1-2-3 and repeat. the wires go to corresponding cylinders so they both fire at TDC, just that one is in the compression stroke and the other exhaust stroke. The crank sensor feeds directly to the Ignition Control Module (ICM) and from there to the computer.

[lilredtruck]

A little clarification. The LX9, ala '04 Malibu, is going into a '92 Isuzu P'up, RWD, replacing a 3.4L GMPP engine with TBI. The '94 Cav harness was recommended by Dave along with its PCM because it is ODB1. I am trying to get all the wiring issues resolved this winter, hoping to complete the mechanical part come spring. Dave has completed (?) this swap in his Camaro. I have not heard from him lately so I opened this issue to all for response. I did note that the injectors are wired in pairs since 1 and 2 are only a few degrees apart in compression/ignition stroke so pairing ignition between one cyl. igniting while the other is exhausting is clever. In the LX9 harness, injectors and coils are not paired but separate.
Could you clarify about the crank sensor? Is it the blue crecent that is on the LX9 at the crank pulley or the black "plug" that was on the left side of the engine sensing a crank throw? Apparently the ODB2 on the LX9 uses all 3.
Brian

[WrathOfSocrus]

I was under the impression the 3500 didn't have what you refer to as the 'blue' sensor. The engines that did use it have a 24 notch ring on the back side of the pulley. The 3500 pulley does have a step on the back that appears like it could possibly fit the ring on there. Since your engine is an '04 perhaps it was used on that year. In any event what I said above still applies. The black plug on the back side of the motor (as it came in a fwd car) is the one that senses the crank throws. However the 56x trigger will not work with your computer. You would have to leave that sensor in place and get one that matches the car your harness came from. Then you need a trigger. The only bolt on solution to get the 7x trigger for a 3500 to work with your computer is the TCE external crank trigger. Installation is simple.

Finding one is another matter. If you need one in the immediate future you need to start blowing horns and whistles now. You can try getting a hold of TCE or WOT to see if any are available or if there is a batch being made in the near future. Occasionally people buy them and have a build fall through so you could also try making a Looking For ad in the For Sale section on here or some of the forums for cars that came with 60* motors.

There are other options. I believe there was one where they removed the 56x trigger from the middle of the crank and had a ring made to replace it. I also recall seeing a setup using an ABS sensor off of some truck and a custom trigger. These are not bolt on, not easy, and while they have been done before I doubt you are going to find too much help going those routes since they are custom and the fact that there are TCE triggers out there to make it a bolt on deal.

Are you going to be using the stock 3500 pulley setup? If you are then there should be no issue. If you use the pulley setup from a RWD application I believe the pulley is different and you need a newer version of the trigger. Just saying that FYI if you buy one used and use the RWD pulley so you don't end up stuck in a jam.

[Superdave]

OK, lets back up a few steps and clarify a few things, then get into the gory details with your setup.


the 94 Cav harness is OBD1, which also happens to be MPFI. All that means in context is that the Harness and ECM are set up to use the 87-04 Distributorless ignition system used on the gen 2 and gen 3 engines. This MPFI system uses 1 sensor to determine the crank position (7x). There are 6 notches in the reluctor wheel that line up with TDC on each cylinder. Being an even fire engine 1&4, 2&5, 3&6 are paired together, every time either pair of pistons is at TDC the ICM fires the plugs (minus the spark advance). This is the waste spark system.

The extra notch is the reference notch, this is filtered out by the ICM and sent to to the ECM, then used in calculations to determine engine RPM which is also used to fire the injector drivers.



There are 2 injector drivers in the OBD1 ecm, It runs in a bank fired configuration which means that it alternates between sides of the engine. You need to get the wiring correct in this aspect or weird fueling problems might show up. The green wire is the ground for injectors 1,3,5 and the blue is ground for 2,4,6. The hots (pink/black stripe) all go to an ignition switched 10A fused lead.



So, this is what you should have..

1. Disregard all the stock 3500's sensors (knock, crank and cam) You do not need any of them other than to prevent oil leaks.
2. 7x external crankshaft trigger installed with a standard OBD1 (3.1) sensor, un-modified harness runs to the Ignition control module, OBD1 harness plugs into the big plug and the 2 pin is chassis ground & Ignition switched power.
3. Injectors wired as described above
4. 3.1 knock sensor
5. Correct OPSU for your application (Gauge or switch)
6. ECM correctly wired in
7. A good tune to run the engine since the 3.1 tune will not.

I think that covers the important parts.

[lilredtruck]

OK, guys, I think I'm mostly settled. I'll look for the WOT trigger setup - is TCE another vendor? My 3500 crank pulley has no notched ring but it did have the blue sensor. My 3.4 has a knock sensor; same deal? Where to I find the number of my PCM? There is a raft of numbers available but I see no numbers on the can in the truck or on the Cav PCM. OPSU is oil pressure sending unit? Dave, you mentioned earlier that some of the harness went to the PCM and some into the bulkhead connector going to...? Is this (old cam sensor wires) what I will use for the 7x crank pulley sensor?
Brian