No start condition. Cranking like fury though!

[andyt11]

Hi, I'm hoping for some quick pointers to help me diagnose a no start condition. Two little kids leaves not enough time to spend on these things.

The car is a stock 88 3.2 Carrera.

In 2016 I had an intermittent engine management problem that was eventually traced to a fault in the ECU and Mr Wong kindly rebuilt it and solved it for me. During that time I replaced the dizzy cap and rotor, the reference sensors and the plug leads. I did not replace coil or plugs at that time.

In 2020 my alternator died and I put a new one on and a new battery at the same time as that was testing bad too. the car has been reliable apart from that.

Today I drove the kids to the park, but when we came to get back in it just cranks over and wont start. Things I've observed while waiting for the tow:

• ICV thing on top of the engine vibrates with key on.
• Replacing the DME relay changes nothing.
• DME relay clicks once when key is turned on, and once more when cranking.
• Tach doesn't move when cranking.
• I don't smell fuel.
• I unplugged the dme and the ref sensors and cht and plugged them all back in in the hopes that I would disturb something and avoid the tow. Didn't work.
• Its got a strong battery and I could probably have driven home on the starter lol

The car is now back in the garage. The good news is it still doesn't start. Which means whatever is wrong has remained wrong and I'll eventually find it. Last time this happened in 2016, the fault was intermittent and it took forever to diagnose because half of the time everything tested good.

I'm going to check for a spark tonight, and I'll check the ref sensors and the coil.

I have a Bentley manual, but I'm really hoping someone says, "check x too", and I get to the issue sooner. Any thoughts are appreciated.

[andyt11]

May already be onto it.

The ref sensor tests at 1000ohms between pin 1 and 2. Spec is 960 plus/minus according to Bentley.
The speed sensor tests at 1640ohms and slowly climbing. Certainly its out of spec. What I don't know is whether it would stop the computer seeing the engine rotation.

[brighton911]

That's strange that the resistance climbs, I think you may be on to something. Even at 1640 ohms, it is out of spec and could cause issues. Certainly, if they are original replace the one or better yet, replace them both with the cheaper BMW alternatives. BTW, have you checked the resistance of the CHT? Also, if it the one wire version, replace it as it can cause a no start situation.

[andyt11]

The CHT is a two wire and I didn't think it would come into play until the engine was warm so I didn't check it.

Both the ref sensor and the speed sensor were replaced in 2016. I'm surprised if one has failed already but i guess I shouldn't count it out.

I have no spark.

I checked out the coil.
The coil has battery voltage at the + terminal.
Primary resistance is 0.9 ohms. Spec is 0.4-0.7ohms. So I'm just outside of that.
Secondary resistance is .57megaohm. Spec is 5.0-8.7. So just in.

I found this last night which combined with a bentley has helped.
911 3.2 No-Start Troubleshooting

[john70t]

Originally Posted by andyt11

The ref sensor tests at 1000ohms between pin 1 and 2. Spec is 960 plus/minus according to Bentley.
The speed sensor tests at 1640ohms and slowly climbing. Certainly its out of spec. What I don't know is whether it would stop the computer seeing the engine rotation.

I may be off on the wrong road, but aren't the Cam/Crank/wheel sensors inductive?
(meaning they generate a voltage signal via a blade moving over a magnet)
On the OHM test circuit, the DVOM sends out a tiny bit of voltage/amp to measure return, so the additional voltage created while cranking would affect this reading.
I thought inductive speed sensors are best tested using voltage output instead of ohms. Could be wrong.

There are DVOM inductive clamps sold which should quickly tell you if a pulse is going through the wire. There are also 'fake' spark plugs that clamp on an outside engine ground so the spark can be seen. No need to remove plugs. If half of them are working the engine should still run poorly.

You are also not smelling fuel (pressure/injectors/control circuit), and I'm assuming there is a separate fuel pump relay after the DME. The pump itself could be suspect. I'm not sure if there is a schrader valve at the rail to test that.

[mysocal911]

Originally Posted by andyt11

In 2016 I had an intermittent engine management problem that was eventually traced to a fault in the ECU and Mr Wong kindly rebuilt it and solved it for me.

Try calling him for help. It may have failed again.

[andyt11]

Earlier I stated that the speed sensor is out of spec. I'm actually hoping that when I dig further it turns out to be the ref sensor. All I know at the moment is that one of the two is out, but I haven't followed the wire through the firewall to figure out where it leads.

I haven't started looking at fuel yet, I don't have a spark.

What's a DVOM inductive clamp? Sounds like a way to test if the ref and speed sensor are sending a signal despite a high resistance.

[andyt11]

Originally Posted by mysocal911

Try calling him for help. It may have failed again.

I guess so. Shouldn't rule something out just because it was good a couple years ago. So were the sensors after all.

[andyt11]

I just got it started. I put my meter on the reference / speed sensor that was testing at 1600ohms last night. Today it tests at 959ohms, right on the money. I now have a spark too.

I put it all back together and the car fired up. I think for some reason, one of the sensors I installed in 2016 is on its way out. Perhaps engine temp was affecting it somehow?

Anyway, is everyone 100% sure the BMW Bosche part 0281002 is an equivalent?

[cabmandone]

I have the BMW Bosch sensors in my car. I'd have to go back and check the exact part number but I know I put the Bosch sensors in my car.

[john70t]

Originally Posted by andyt11

[LIST][*]ICV thing on top of the engine vibrates with key on.

The idle air bypass control valve vibrates?

I'm not sure they are supposed to do that. I thought they were analog. Could have a worm out solenoid or just the plunger seat or whatever gummed up. Those and the starter solenoids can be temporarily unstuck by tapping lightly with a hammer. Better minds could chime in on this.

[cabmandone]

Yeah you can feel a vibration in the IACV. It's one of the first things to look for if you're having an idle issue.

[andyt11]

An update.

I've confirmed which sensor is which, and I've checked the wires from the engine bay back to the DME. I gave them a good wiggle at each end too looking for some sort of continuity break. Didn't find any.

So...

On Saturday 6 hours after the breakdown:
The speed sensor was testing at 1600ohms across pin 1 and 2.
The reference sensor was testing around 1000ohms across pin 1 and 2.
I had zero spark.

On Sunday with the engine stone cold:
The speed sensor was testing at 959ohms across pin 1 and 2.
The reference sensor was testing around 960ohms across pin 1 and 2.
I had a spark, engine ran.

Monday with the engine cold to begin with:
I tried to replicate the idea that as the car warms up, the speed sensor goes wonky. I idled it for 15 mins. I appreciate it wont get super warm but i didn't want to go for a drive and end up on a tow truck again.
Cold, the speed sensor was testing at 959ohms across pin 1 and 2 cold and went to 1150ohms when warmed up.
Cold the reference sensor was testing at 960ohms across pin 1 and 2 cold and went to 1160ohms when warmed up.
I had a spark, engine ran.

It still runs. If I disconnect the speed sensor or the ref sensor, the car wont start.

Two questions:

I was under the impression that loosing the ref sensor would result in a no start. But if I remove either from the system, it results in no start.

I'm torn whether to take a punt on replacing the speed sensor. I'm no longer convinced its defective, although the results on Saturday would suggest it is.

Any thoughts appreciated.

[mysocal911]

Originally Posted by andyt11

An update.

I've confirmed which sensor is which, and I've checked the wires from the engine bay back to the DME. I gave them a good wiggle at each end too looking for some sort of continuity break. Didn't find any.

So...

On Saturday 6 hours after the breakdown:
The speed sensor was testing at 1600ohms across pin 1 and 2.
The reference sensor was testing around 1000ohms across pin 1 and 2.
I had zero spark.

On Sunday with the engine stone cold:
The speed sensor was testing at 959ohms across pin 1 and 2.
The reference sensor was testing around 960ohms across pin 1 and 2.
I had a spark, engine ran.

Monday with the engine cold to begin with:
I tried to replicate the idea that as the car warms up, the speed sensor goes wonky. I idled it for 15 mins. I appreciate it wont get super warm but i didn't want to go for a drive and end up on a tow truck again.
Cold, the speed sensor was testing at 959ohms across pin 1 and 2 cold and went to 1150ohms when warmed up.
Cold the reference sensor was testing at 960ohms across pin 1 and 2 cold and went to 1160ohms when warmed up.
I had a spark, engine ran.

It still runs. If I disconnect the speed sensor or the ref sensor, the car wont start.

Two questions:

I was under the impression that loosing the ref sensor would result in a no start. But if I remove either from the system, it results in no start.

I'm torn whether to take a punt on replacing the speed sensor. I'm no longer convinced its defective, although the results on Saturday would suggest it is.

Any thoughts appreciated.

Have you asked your DME ECM "rebuilder" these questions? It's probably still under warranty, right? Most rebuilders provide technical support even after the warranty period.

[cabmandone]

My understanding is that if either is lost you lose spark. As Dave said, I'd reach out to Steve. Keep in mind that the sensors aren't the only thing getting warmer as you drive. Intermittent issues can be a real P.I.T.A.

Just to be sure, You are disconnecting the sensors and testing resistance rather than back probing right?

[andyt11]

Originally Posted by cabmando

My understanding is that if either is lost you lose spark. As Dave said, I'd reach out to Steve. Keep in mind that the sensors aren't the only thing getting warmer as you drive. Intermittent issues can be a real P.I.T.A.

Just to be sure, You are disconnecting the sensors and testing resistance rather than back probing right?

I'll call Steve today. I despise intermittent issues. And now the car is running so of course everything is working.

And no question is a daft question. I'm checking resistance on the sensors yes. I did check for continuity back to the DME too. I wondered if I had a loose connection perhaps that would be causing it not to see one of the sensors.

Tempted to go for a drive and see if I can get the speed sensor to hit 1600 ohms again while the other does not.

One other snippet of info, I checked the CHT, its at 3.3megaohms. Bently says 2.2 - 2.8 at room temp is acceptable. Its only about 60f at the moment so I've put the heat on and I'll check it again when the garage has warmed through.

Edit: At 64f I'm at 2.75megaohms and decreasing as the garage gets warmer. Which makes total sense.

Further edit: Steve advises to swap the sensors as there must be something up. And if there was, I wouldnt get spark or fuel, which is what I'm seeing.

[cabmandone]

I wouldn't get too hung up on the CHT. It's not going to cause a no fuel/spark situation. I'd go with the man who knows. Steve has you on the right track. The DME needs to see signal from the S&R sensors for the DME relay to energize the fuel pump IIRC. I didn't see that you were also having a fuel issue. I only remember you mentioning no spark.

[andyt11]

new sensors arrived this week. I got lucky and got the old ones out of the bracket without having to remove he bracket. The bit I'm stuck on is getting that gromet out of the sheet metal! I've read "just stick a screw driver through one of the holes and give it a twist", but man is it in there good. Taking a frustration break.

[andyt11]

just for the benefit of future readers, the new ref sensors worked. Car fired right up. thanks for all the help.

[GH85Carrera]

Good news.

When my second set of sensors went bad, heat was indeed the culprit. Cold they were ok, hot was just like the car was haunted. Replace them, cured!