94 CMP sensor install (specific question)

[tbunch]

I'm new here so bear with me. I'm not looking for a how to on this, just a specific answer so I can move on today. I'm completing a rebuild on my 94, 4.0 and am re-installing the CMP sensor and trying to get it aligned with the engine on a stand to save the trouble of doing it in the vehicle. I have a good understanding of the workings of the sensor and believe I have installed it correctly, engine is at 26* ATDC, align the vein on the window, test with multimeter and find the exact point where battery voltage drops to zero on counterclockwise rotation then tighten hold down.

All seems well, but my question is this; in another members writeup on this same install they referenced that the vein on the sensor is to align perfectly with the far right mark on the window (no reference to checking voltage). However, on mine, if I tighten it down at the point where voltage drops to zero as described in the haynes manual and other install guides, the vein is not perfectly aligned. I presume that going by voltage would be more accurate, but I'm curious if any one knows, or knows how fincky these things are? the vein is only misaligned by a couple milimeters, but id rather not go through the whole install and have the motor run like junk because I was to lazy to ensure correct installation. Hopefully someone has some knowledge on this.

This is the 94-95 window style sensor so there are no special install tools, just eyes, multimeter and patience.

THANKS!

 

[RonD]

Well come to TRS

Cam sensor runs off the cam gear so rotates only once for two full rotations of Crank

So first thing is to make sure you are using 26deg ATDC for #1 Power stroke, could be intake stroke.
With #1 spark plug removed you rotate engine manually feeling for the compression stroke at #1 spark plug hole, when felt then next 0deg TDC on crank pulley will be start of Power stroke.

And then no, it isn't that precise, as far as CMP sensor.
The computer can see the power added to crank spin each time a cylinder fires via the CKP(crank position) sensor.
CMP is mainly used to time sequential fuel injection, computer will learn when best timing is by varying the off-set from CMP pulses.
Same as it does for spark timing.

Computer also has a sanity check for CKP and CMP pulses, CKP pulse is the main one, if CMP #1 doesn't sync with CKP #1 then computer will disregard CMP pulses and run fuel injection as Batch Fire, then set a code and turn on the CEL(check engine light) to notify driver of the problem.

If CKP sensor has a problem then engine won't start, lol, that's the Big Kahuna of sensors, and the only sensor that can cause a no start.

 

[tbunch]

Thanks for the detailed response!

So if I'm reading into your answer correctly it sounds like the sensor shouldn't be sensitive enough to cause major drive ability or idle issues as long as its close?. The motor is out on a stand right now otherwise I could trial and error easy enough as you would a traditional distributor, so I'm restricted to setting this in either by relying on the voltage readings when supplying power via my battery and some alligator clips along with a multi-meter, or rely on the visual marks in the window of the sensor.

My understanding is that the sensor should relay full battery voltage when the circuit closes and 0 voltage when open, which is how it relays proper spark timing (as you said). I'm not really concerned with no start conditions, more with poor/erratic idle. My thinking is that if I tighten the hold down at the exact point where the voltage swaps over (from 12 to 0v), assuming I've correctly placed the crank at exactly 26* ATDC position,it will be exactly in sync with the crank and all will be well with the world, even though the vein does not line up exactly with the marks in the sensor window?

Im probably just overthinking it, but its much easier to adjust it on a stand than it will be when its crammed behind the intake haha.

 

[RonD]

Yes, that would be fine if 26deg ATDC Power stroke for #1 is correct, not intake stroke.

 

[tbunch]

great, it is on the compression/power stroke at 26* ATDC so it sounds like, since its close in the window and correct with the multimeter it should be good to go hopefully, thanks!

 

[adsm08]

Yeah, that sensor has some fudge factor in it. I stabbed mine in the the engine in the vehicle, never checked the voltage, just eyeballed the vein and mark, with one eye closed to boot, and drove off.

I don't have any major issues. That one feels like it has less get up and go than my batch-fire 4.0, but that is a common complaint, and the sequential fire engine is pushing through higher tranny gears and bigger tires, so those come into play as well.

 

[tbunch]

Good deal, I kinda figured as much, I guess at the end of the day if it runs wrong then the hold down clamp is already broken loose so adjusting it would be quick, ill leave it as is and not worry about it anymore. thanks for the input.