84 911 non starter.

[Liam O]

Hi,

Drove my car to work last week and parked it up. After my shift I couldn't get it to start, car normally runs great although I admit it only covers about a thousand miles a year.

First I swapped out the DME relay but that turned out to be ok

Then I called out an auto electrician and he replaced the coil, as it turned out this wasn't the problem either but I thought what the hell at least that's one less thing to go wrong in the future.

Ended up having the car trailer-ed home I had a quick look last night and found there was no spark at the plugs.

The last time I had this problem was about six years ago when I replaced either the speed sensor or the reference sensor can recall which one.

Also last night my son checked both sensors for resistance and he said there was no reading at all on the middle one on the mounting rail, I'm pretty sure that's the one that wasn't changed.

Car is a 3.2 with 86,000 miles on it.

Any thoughts on this would be well received

[ChrisVCR]

I am going through the same thing with my '84 Targa. Crank but no start. I replaced the DME relay but no go. I checked the speed/reference sensors and the speed was ok, the reference did not read well and it was actually cracked at the plug on the rail. As I was replacing that, I could see the speed one had already been replaced. After replacing the reference sensor, still no start and no spark. I finally took it to shop and they told me it was the DME itself.

I know there are many threads on here about checking the DME itself and even taking apart to check for cracked solder connections. Sending mine off to be refurbished seems like my next step.

[gtc]

Seems like there have been a lot of '84 DMEs going bad lately. Mine went kaput about a year and a half ago, but Ingo was able to fix it up quickly and affordably.

[Liam O]

I think I'll have a go at replacing the remaining sensor before I do anything else. HOpefully that will solve the problem.
I'll post up when I've done that.
thanks.

[hayday]

It is most likely the reference sensor...

[kidrock]

Originally Posted by hayday

it is most likely the reference sensor...

+1. Double check your sensor connectors on the left-hand side of the engine first before replacing.

[ischmitz]

Checking the sensors is unfortunately not always conclusive. Some read O.K. with an Ohm meter and could still be intermittent. If the meter doesn't show the 1000Ohm then it is definitely the first thing to replace after the DME relay. Keep in mind these sensors live a rough life with all the vibration and temperature cycling near the engine - I am amazed they last that long.....

Also always have a spare DME relay with you, cheap insurance and often the cause for issues with DME cars.

Best,
Ingo

[kidrock]

Originally Posted by ischmitz

Checking the sensors is unfortunately not always conclusive. Some read O.K. with an Ohm meter and could still be intermittent. If the meter doesn't show the 1000Ohm then it is definitely the first thing to replace after the DME relay. Keep in mind these sensors live a rough life with all the vibration and temperature cycling near the engine - I am amazed they last that long.....

Also always have a spare DME relay with you, cheap insurance and often the cause for issues with DME cars.

Best,
Ingo

Ingo, I don't want to be a thread thief but I believe that my next question is valid to the PO's thread.

In the past, I have also found that ohming the sensors was inconclusive, as the test provided decent numbers but actually replacing the sensors revealed that they indeed were the culprit.

Is the oscilloscope the best way of testing these sensors, or can the o-scope be intermittently inconclusive as well?

[ischmitz]

I fully agree with your observations. The Ohm meter test is a quick-check at best. If it Ohms out bad you have identified the issue, if it ohms out good it'll be inconclusive.

The scope is better since it shows a real wave form. If you know the peak-to-peak value of a good sensor and then see one that is considerably lower you have id'ed a bad one. This will also show if the spacing is off since sensitivity goes with the square of distance.

However, consider a bad shield (braided wire mesh). During cranking and with the scope connected there won't be much EMI from the ignition and the scope trace might look perfect. Once the engine runs the sensor signal becomes compromised by the EMI from the ignition and that's what causes drop-outs. So unless you have a break-out box where you can image the sensor signal while the engine runs it'll be hard. Condition of spark plug wires will make a difference, too.

Then there are temperature-related issues and issues at certain RPM ranges from mechanic vibration. Again, this might only show up intermittently at certain conditions.

I had a failing CPS on another car that only acted up on hot days between 1 and 10 minutes after engine shutdown. So getting gas went like this. You stop, fill up, try to pull away from the pump and nada. Cranking and no start, angry looks from people behind you. Wait 10 minutes and it starts up like nothing happened. The explanation was that the engine heat-spikes right after shutdown. Once sitting for a little the temperature goes down and all is well. During normal driving there is enough cooling to not get into the critical range. I replaced the sensor and never had the issue again.

Ingo

[kidrock]

Ingo, thanks for that great explanation.

Liam: if you find that the crankshaft position sensor is indeed your issue, make sure to go ahead and replace the speed (RPM) sensor as well. They are one and the same. If you do a search, you will easily find several threads on replacing these with the considerably less-expensive BMW sensors.

Good luck, and keep us posted.

[Liam O]

Sorry for my delay in posting guys, I done a check on the sensor i had already replaced a few years back and I did get a reading.
The other sensor, which has been on the car for god knows how long gave me no reading at all.
I'm inclined to assume this is my problem, although I know its not always as straight forward as this.
I've heard BMW do the same part, does anyone know of the part number for this?

I should be able to confirm if this is my problem when I get around to swapping it out over the holidays.

Thanks to all for your input.

[darrin]

searching the forum for "bmw 911 sensor" brought up this thread: CHT and Reference sensor protective cover cracked question: which provides the part numbers you're looking for

[Liam O]

Thanks Darin.

[Liam O]

Thanks Darrin, sorry mate.

[darrin]

Liam -- no worries!!!! hope this solves your no start, remember how frustrating my no start was . . . (turns out my dme computer had some bad soldier connections)

[Liam O]

Well I finally got round to replacing the bad sensor with the BMW one today,
Car fired up straight away.

At first I was a bit apprehensive when I noticed that the last digit on the part was different, then I thought what the hell, Pelican only charged me $75 dollars for the sensor, If I bought it from Porsche in Glasgow it would have cost me about £180.

I was fairly confident this would solve my problem, but you never know.

Well cant complain only two non starts in 7 years of ownership and both from reference and speed sensors.

Many thanks to all for their contributions,

Liam.

[Liam O]

By the way it was the top sensor on the alloy mounting plate.

Out of curiosity does anyone know which one that is.

Liam