Help with a no start

sherald · 12-22-2005, 05:19 PM

I was, I thought, finished with a restoration my '88 924S. New belts, new clutch, rebuilt torque tube and CVs, water pump, PS pump and misc. other R&R.

After all was done she started right up. Went for a test drive and and she was running so sweet. Then parked it and later tried to restart and she won't fire.

I started diag. the DME system and I have a no spark condition. Checked the ohms on the speed and reference, ok, check the temp sensor, ok, checked the grounds, Check. So, I'm thinking bad DME computer. I borrowed a known good one but still no joy.

The only thing, I can think of, that I haven't done is put and O-scope on the speed and reference sensors and verify voltage to be sure they are adjusted correctly after that I'm stumped.

Any suggestions??

Wolf1 · 12-22-2005, 05:36 PM

You didn't mention the DME relay. Have you checked that?

earlr85944 · 12-22-2005, 05:42 PM

also check for a signal to and the coil itself

sherald · 12-22-2005, 05:46 PM

Yes, left that out.

New DME relay and new coil and new Magnacor wires.

I have 12V to the coil but now voltage out of the coil so the primary is not switching. So the dme is still looking for a start signal and is not seeing one. I have check most of the connections to the dme connector with a meter and so far all is as it should be.

Any other ideas? You guys are doing great so far.

earlr85944 · 12-22-2005, 06:26 PM

this may help, from clarks garage?:
new clutch...... maybe the sensor clearance was disturbed?
you probably dont have a scope, try the mulitmeter test,

http://clarks-garage.com/shop-manual/ign-02.htm

earlr85944 · 06:34 PM

never really tried this but it sounds good:

"Another way to check the operation of the speed and reference sensors it to crank the vehicle and watch the response of the tachometer. If the tachometer jumps during cranking, the speed and reference sensors are probably good. If it does not jump, it indicates a problem with the speed and/or reference sensor or the DME computer. If it's the speed and/or reference sensors, it could be a bad sensor or the clearance gap is too large (see "Adjusting Sensor Clearance")."

also you can have 12v to the coil and still have a bad coil. I believe the negative side of the coil collapses and charges the coil to fire, thats why the tach reads off of the neg side o something like that

sherald · 12-22-2005, 06:36 PM

Thanks but I think I have Clarks memorized by now.

Not only were the sensors disturbed they were replaced. The old ones were stuck in the bracket and the bracket had to be removed to get the old sensors out. So, I used Clarks method of putting a shim on the old sensor for a gauge.

I ohmed out the sensors and they checked out. I don't have and O-Scope but have a friend that is bringing one over in the morning so I can check the signal. I'm hopeing that it will show that sensors are set too high.

What I don't understand is that it ran great and started a couple of times after the clutch job then stopped.

I'm stumped but not giving up. I have too much invested now to turn back.

My co-worker say I have a Porsche monkey on my back and they are planning an intervention.

Keep em comming guys, these are great ideas, I just need one I haven't tried yet.

sherald · 12-22-2005, 06:40 PM

Earl,

The point about the Tach is a good one. The tach is NOT moving when I crank the engine. So, I really hope it is just the sensor height. I will be able to adjust that when I get the scope in the morning.

Zero10 · 12-22-2005, 06:54 PM

Haha, you say you have Clark's memorized, then in an afterthought, mention one of the most important points yet. No tach movement.

Speed/reference sensor gaps are my bet.
You can check the output with a multimeter. Set it to VAC, I use the 200V range, but it doesn't matter too much. I expect >7VAC on the meter if they are gapped properly. With a fully charged battery, I get 11.5VAC on my the speed sensor when cranking.

SoCal Driver · SoCal Driver's Garage

Do you have power at the injectors with the key on?

sherald · 12-23-2005, 03:11 AM

Good point Zero, I did forget to mention the tach. When I tried to read the sensors with my meter it was eratic and I couldn't get a reading ( Ihave a cheap meter I need to replace).

But like I said I have a geek friend that has an O-scope and I will be checking the sensors with a scope to see if I am getting the right voltage and sine pattern

SoCal,

I havn't checked the injectors with a noid or a light but do have pressure on the rail. Will the injectors fire if the DME hasn't signaled the coil to fire?

sherald · 12-23-2005, 11:17 AM

Update:

I found the problem.

With the O-scope I got a good signal on the speed sensor but the reference sensor was dead. So, we pulled them both out and checked them by clamping them in a laithe and setting the sensors close to the chuck head. Then we spun the chuck and measured the output with the O-scope and all was well, so what was up?

The pick-up (allen screw) was sheared off for the reference sensor all the way down to the flywheel surface. Apparently it was sheared during the clutch installation and was hanging on just enough to start the first time. Then during the test drive the centrificical force finished the job. So, no pick-up, no start signal, no go.

Thanks for everyones help.

SoCal Driver · SoCal Driver's Garage

Good that you found the problem.

When I suggested checking the voltage at the fuel rail it's the second voltage test one should do with a non-start. First is to check for voltage at the coil then the injectors with the key on. This tells you two things. First is the ignition switch is working, power at the coil, and second that the first set of contacts in the DME/fuel pump relay is working, power at the injectors. This contact also powers the DME.

Second test is to use a spare spark plug in the number one spark plug cable. Lay the side of the plug on the intake and crank the car. This will tell you several things; no spark can be the reference sensor (most likely) or the coil to dist wire or the rotor is not turning (timing belt broke) or is out of alignment (screw fell out).

Leaning out of the drivers seat with your head down you should here the fuel pump run for a half second after you stop cranking. This tells you the DME is getting power and closing the second set of contacts in the DME/fuel pump relay. Another means of checking those first set of contacts. Still one should check for voltage at the injectors because the wires have been known to break inside the loom.

sherald · 12-23-2005, 07:26 PM

SoCal,

Thanks, that is great information. I hope I don't need to continue on with the diag.

I said I found the problem, but fixing it is another story. I'm sure you can appreciate how hard it will be to get that broken pick-up out and replace it.

One way of corse is to drop the gear box and exhust again and pull the bell housing. However, what do you think of drilling a hole in the bell housing on the exhaust side of the engine. there is a nice flat area in the right spot there to make a hole large enough to get to the broken pick-up and replace it and will only have to drop the test pipe and cat (they are welded).

I really don't want to drop the gearbox and pull that bell housing again.

sherald · 12-23-2005, 07:36 PM

SoCal,

You seem to know your 2.5L engines pretty well so let me ask you a question. The other reference sensor (TDC) that is mounted in the same bracket with the speed and reference sensors, what is it for? Mine is disconnected and by the looks of the connector has been for several years. Is it used by the DME or by another system?

Just wondering because I can't even find where it connects.

Thanks,

SoCal Driver · SoCal Driver's Garage

Jack up the front of the car. Put a couple of jack stands under the cross member. Crawl under the car and take the metal pan off of the bottom of the bell housing to get to the pin -- or what is left of it. You will need the height of the pin. Think there have been past posts that give the stand off height. Use good locktite to hold it in.

After you replace the pin put the cover back on.

The third sensor that is not connected to anything is for the dealer computer set up. Kind of a cross check for the other reference sensor. Don't think it's a workable substitute.

sherald · 12-24-2005, 05:04 AM

Well, I don't have a belly pan that covers the bell housing. There is a cover on the front of the bell housing, a dust cover of sorts, that allows you access to the two pick-ups for that thrid sensor we were talking about. The only place were I can even see that broken pick-up is through the hole for the sensor, it is in the center of the flywheel.

Do you think drilling a hole in the bell housing would be a problem? It sure would make getting to that pin a lot less labor.

Its Christmas eve and I'm not going to worry about it anymore until Monday.

Thanks for your help and you have a great Christmas.

Steve

SoCal Driver · SoCal Driver's Garage

Item 21 is the cover. Item 20 is the plug for the inspeciton hole. Between the two of them you can access the pin.

What part of NASA do you work in?

sherald · 12-24-2005, 08:33 AM

Yes, part 21 was what I was calling a dust cover. I have tried both the inspection plug (too far to the rear) and the cover (too far forward). The cover gives you access to the two pick-ups for the inspection sensor (unused) and is right at engine side of the flywheel.

The pick up for the reference sensors is between those two places. I have had the car on a lift and pulled the starter and looked at it from about every angle. Short of pulling the bell housing or drilling a hole I can't get to it except through the hole for the sensor and I can't get in from the top with the tools I need to get the old one out and put a new one in. Very frustrating it felt sooo sweet after all the work I put into it. Over $5K of R&R new tires and wheels and rebuilt the entire drivetrain from the flywheel back with mostly new parts.

Anyway I work at Marshall Space Flight Center. I am the Lead Engineer at the Materials Combustion Research Facility. We do high pressure Oxygen compatibility testing for the manned space programs. Basicly I get to burn and blow-up hardware and get paid for it. Is this a great country or what?

earlr85944 · 12-24-2005, 11:33 AM

Being that you work at NASA use duct tape for repair.

If you have a clear shot at the part, drill the bell housing, it would be such a small breach as not to hurt its structural integrity.