tomrc

OK, I've been working on this for 3 days and it's time to call in the big guns. Here's what happened. I was trying to get a tie wrap around the cable going to my heater valve in the engine bay next to the firewall, when the connectors to speed and ref sensor connectors just crumbled and otherwise made a mess. So fine I get out my soldering iron and put a new (but different type)connector on both and went to start it up. It chugged along and then caught, but it was running on only 2 cyclinders. Great so I take the connector off and solder everything together without a connector, same result. I know it's running on only 2 cylinders because I can pull the middle two fuel injector connectors and the car runs the same. Although the car seems to idle still at about 900 rpm, the tach reads waaay low like at 200 RPM and when revving the engine it still reads low.

Anyone have a suggestion? I checked the resistance of the sensors and they are well within spec, I don't have an O'scope to check the square wave. I did check the sensors and they are tight, I doubt that they moved.

Am I in limp home mode or is it something else?

Thanks in advance (I hate saying tia),

michaelathome

I am not familiar with that situation, is it possible that while working on them you tugged a bit too much and now the sensors aren't gapped correctly?

Also you got all three wires in there right? Not just 2 of the 3? The unshielded ground is pin 3. Just checking.

Michael

tomrc

I guess it's possible, but the sensors are pretty tight I tried to move them by hand but couldn't. Would I need an o'scope to know for sure? I read the procedure for aligning them from Clarks, but I'm not too sure I understand it, so I'm hesitant to loosen them up to re-align them.

All three wires are connected correctly.

Thanks,

michaelathome

Tom,

Did you add/remove any of the length of the wires? Any change to the wire length will change the amount of resistance on the strand. When I did mine I was sure to cut them longer then removed what I needed to get them to the exact length.

Good luck!

Michael

tomrc

Right now they should have less resistance because the lenth is slightly smaller and the wires are soldered together. I may be able to borrow an o'scope tonight to see if I'm getting a good waveform from both sensors. All the computer is looking for is a good >2V sawtooth right?

Thanks,

tomrc

OK, I was able to check the sensors with an o'scope and I do have very good signal at both the reference and the speed sensors. A friend and I troubleshot it down to the cap and rotor, I think. I'm still a little skeptical but I'll give it a try. There is no spark at the middle two spark plugs. There seems to be ample pulse from the coil to support 4 plugs though.

Thanks to all that replied, I'll let you all know what happens.

tomrc

Well, I'm still down. I put a new distributor cap on and it didn't help. From what I can tell I'm only getting half the reference pulses that I should be getting. The way I'm figuring that is the firing order is 1-3-4-2, I am getting spark on cylinders 1 and 4 so, I am skipping every other cylinder.

What would make my reference sensor skip every other pulse, or would it be the DME? Peak to peak voltages are good on an o'scope.

Anyone know what the frequency of the reference sensor during idle? Should the frequency be about the same as the speed sensor? The speed sensor has a much higher frequency.

Thanks again in advance,

Mike C.

Tom, The reference sensor only gives one pulse per revolution which tells the DME crank position (a single target on the flywheel goes by). The speed sensor looks at the flywheel ring gear teeth and so would have a much higher frequency. I don't think I've ever heard of a problem as you describe with the ref sensors. Typically they either work or they don't and the car will not run at all if they are not working. I can't imagine a scenario where the ref sensor would miss every other pulse. I know it's unlikely that 2 plug wires would go bad at the same time but I would check them out too.

Jason-85944

My guess....you have injector wires that have broken inside on the harness on the back of the motor. I had a problem similar to this and had to splice in a piece of wire as a jumper for the injector to fire properly. I could be wrong though.

-J

tomrc

I'm not getting any spark out of the distributor from 2 and 3, nothing nadda. They're just dead. The other two will arc just fine to ground. This was tested right at the cap without the wires. Also the new cap had arcing tracks on the contacts of 1 and 4, but 2 and 3 were clean. It seems the DME or something is shutting them down.

Will the computer shut down spark if the injectors don't fire? I haven't checked the voltage to the injectors yet.

I really appreciate the help guys, Thanks.

Zero10

Nope, what happens is as follows, the reference sensor tells the engine when TDC for #1 is either approaching, or when it's at TDC, I can't recall for certain.
From there, it uses the speed sensor for 2 purposes, to determine engine speed, and thereby to time the spark for cylinders 2/3, since they occur when 1/4 are at BDC, and there is no second reference mark for firing them.

This goes hand-in-hand with your tach problem, since it is driven indirectly by the speed sensor. What is happening, is that the DME is seeing the reference sensor pulse, before it is going to fire the spark for cylinders 2/3, so effectively, you're only getting spark on 1 and 4. Now, why this is happening is anybody's guess, but I do believe that is a breakdown of your problem.

From there, check the signal at the DME on the speed sensor.
My cheap-o trick, is to get my multimeter out, and set it on RMS AC, and check the voltage at the sensor. On a properly gapped speed sensor, I see roughly 9VAC. You need >2.5V peak voltage for the DME to read it as a logic high, and actually see the signal.
So, if you have access to an oscilloscope, that is the best tool for troubleshooting this, and simply attach it to the wires at the DME, if memory serves me, Clark's Garage mentions the pin numbers at the DME.

It's entirely possible that the shielding or wiring beyond the conectors (on the DME side) is damaged as well.

Zero10

While I'm at it, you're zip-tying your heater valve? I used an elastic band on mine when I couldn't get the heat to shut off, the problem then is a broken clip in the driver's footwell. Beyond that, if you still have warm air even when the heat is off, your heater valve is bad (the diagpham in mine broke, and this is my symptom).

tomrc

Thanks Zero10, it's starting to make sense. I've been concentrating on the reference sensor but paid little attention to the speed sensor (I saw a decent signal and called it a day). So the speed sensor (or the cable) may be telling the DME that it's going slower(?) than it is, and thus the DME is slowing down the output pulses to compensate. I have the Clarks garage sensor procedure printed out, it does say which pins to check, I'll do that tonight.

I have an early 944 and it doesn't have the notorious drivers footwell clip. All summer I used a long stick to turn the valve off in the engine compartment, I just had the crazy idea that I would fix it so I can open and close it from the cockpit before winter, (I know, crazy kid). The problem is the clip for the cable that goes to the heater valve is missing so there's nothing holding the cable sheath while you operate the control lever inside. So I was trying to tie wrap the sheath, it works somewhat but the heater control valve lever is getting hung on the firewall. Another problem another time though.

Zero10

Right, early 944. So it's a cable-actuated heater control valve.
See, it's not exactly that the speed sensor is telling the DME that the engine is turning slower, because it is most likely seeing all of the ring gear teeth going by, however the output signal is probably borderline, causing only some of the voltage spikes to be visible to the DME, which reads it in a strictly logic sense, requiring as I mentioned, at least 2.5V to read as a logic high.

If everything checks out at the sensor, check it at the DME just for good measure. I've seen the wiring inbetween go bad/be broken before.

So, for your heater problem, I'm not 100% sure I understand, but it sounds like a clip/bracket is missing from the heater valve itself? Necessitating replacement of the valve I suppose.

tomrc

Alright I've had about had it if I don't get this thing running right this weekend it's getting parted. I can't afford to start buying parts to swap in and out of it. I have a thread on Pelican http://forums.pelicanparts.com/showt...92#post2125792. Also.