Fuel but no spark

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Machbuffet
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Joined: Tue Sep 11, 2018 4:28 pm
Location: Northern Ireland

Fuel but no spark

Post by Machbuffet »

Hi guys
I know this is flogging a dead horse, but I'd appreciate any guidance you could offer.
Car is a 1985 e24 635csi UK car.
Car has been off the road for a number of years but I regularly turned it over, the last time it fired up was a year ago.
Recently, it was sent for restoration and for the first time in 19 years, it wouldn't start.
I've just had it back and I've been troubleshooting.
So far
Car was always garaged in a modern dry insulated garage.
I've checked the main and fuel relays on the fuse box.
I've bypassed both relays to check, no start.
The fuel pump runs.
Coil checked and readings within spec.
I've tested resistance on the reference sensor and speed sensor.
Reference sensor (grey plug)
Pin 1-2 1060 ohms
Pin 1-3 nothing
Pin 2-3 nothing

Speed sensor
No readings from any of the pins.

I'm a little confused. I understood that the reference sensor prompted spark timing and that the fuel pump would only run during start, after which it would be governed bu signal from the speed sensor.
Given the reading from the reference sensor, and the absence of any reading from the speed sensor, I expected it to fire up and then die, due to faulty speed sensor.
However, one guide said that even though the reading of 1060 should be ok, there should also be 100k ohms on the other pins, mine have no reading.
They're expensive from BMW and I don't want to buy and then turns out it's not the issue.
I've found these on eBay ( item number 332357817168) and as I understand, both sensors are identical, just perform different tasks. They seem reasonably priced and they're Bosch. At that price it might be worth replacing just to rule out. What do you reckon? The other option would be to get a pair of known serviceable sensors and swap.
Trouble is, I have to work from home, as the car isn't yet roadworthy, so I need to have a go at fixing it myself.
Any help or advice would be really appreciated.
Thanks in advance
1985 635 csi
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Ralph in Socal
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Re: Fuel but no spark

Post by Ralph in Socal »

Just recently had a No-Spark issue with a swapped M30B32 into an e28. Speed and Reference sensors are the exact same part but provide different function readings to the ecu They both have to provide their proper feed to the ecu to effect spark.

The absence of one sensor feed prevents the ecu from having the information necessary for providing spark.

In our case, both sensors were good and provided good Resistance readings at the bulkhead connector. However, the readings were absent for the Speed Sensor when checking for Resistance at the ecu plug. Continuity test on the wires from the bulkhead connector to the ecu plug showed a line break. We ran an external set of wires from the ecu to the bulkhead connector and bypassed the problem.

In your case, try to find someone locally who has a spare, good sensor for you to try out. Otherwise, buy new as they often break at the connector

I have seen others strip the sensor connector completely and just tape the wires and car harness connector together. And get the car to run that way!! Good luck.

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
Machbuffet
Posts: 11
Joined: Tue Sep 11, 2018 4:28 pm
Location: Northern Ireland

Re: Fuel but no spark

Post by Machbuffet »

Thanks Ralph, very much appreciated.
1985 635 csi
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Brucey
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Re: Fuel but no spark

Post by Brucey »

IIRC both speed and ref sensors measure ~1000 ohms between the outside pins. If open circuit there (at the manifold brace connector) the sensor is definitely NFG. If OC at the ECU, it might be the wiring harness at fault.

having spare sensors in the boot is a good idea, in case of a roadside problem, so best to get some spares anyway if you are in it for the long haul.

Note that if the sensors are good but the connections are reversed (which is easily done, there are wrongly labelled photos on the interweb.... :roll: ) then the car won't go.

Note that most of the fuel for a cold start comes from the cold start injector, so make sure that is working.
Note also that you will get a crank but no-start condition if the ECU doesn't see the 'start signal' on pin 4.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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randall977
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Re: Fuel but no spark

Post by randall977 »

Hi Machbuffet, I started restoring my car just before Christmas - it hadn't run for 9 years. I had a very frustrating few months trying to get the car started and went through everything 10 times. In the end I can't be sure why it started but replacing the cold start valve might have been the key. As mentioned the Bosch sensor are quite cheap on eBay - about £45 each - I replaced both. Have you tried Easy Start into the throttle body? I thought I was getting fuel when actually I probably wasn't. If you have spark then it will start with Easy Start.

Look at my threads here, see the fun I had - might be of some help;

viewtopic.php?f=2&t=29437

viewtopic.php?f=2&t=29428&p=211191#p211191
Machbuffet
Posts: 11
Joined: Tue Sep 11, 2018 4:28 pm
Location: Northern Ireland

Re: Fuel but no spark

Post by Machbuffet »

Thanks Guys. I really appreciate the feedback.
In my 19 years of ownership, neither sensor has been replaced and they may be there from new, so to eliminate them, I'm replacing both sensors.
I'm hoping this'll help.
I'll let you the outcome.
Thanks again
1985 635 csi
Machbuffet
Posts: 11
Joined: Tue Sep 11, 2018 4:28 pm
Location: Northern Ireland

Re: Fuel but no spark

Post by Machbuffet »

Hi Guys
Just a quick update.
I've ordered two replacement Bosch sensors and before fitting, wanted to test the resistance readings.
Both sensors were bench tested and although measuring correctly at 966 ohms, there's nothing at all between pins 1&3 and 2&3
The books say they need to have 100k ohms minimum.
Does this need to be measured with the sensors installed to get the correct readings?
Thanks guys
1985 635 csi
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Brucey
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Re: Fuel but no spark

Post by Brucey »

you should have good insulation between one of the pins and the others. This is because there is a screen over the cable to prevent interference, that is not connected in any way to the other two pins (between which you should measure about 1K ohm). A lot of multimeters read in a slightly misleading way if the resistance is higher than the selected range, such that (especially if you don't use a DMM regularly) you might think you are measuring 'nothing' when actually you are measuring a very high resistance. From what you have said I think your sensors are OK.

There is a second check you can do on the sensors which involves measuring their voltage output when they are installed and the engine is cranking. However if the sensors measure the correct resistance and are installed correctly above a flywheel with its teeth /timing peg present, it is extremely unlikely that the sensor output is incorrect. I'd check everything else first and then I'd plug a an oscilloscope in to check the output waveform, when I'd run out of good ideas.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Machbuffet
Posts: 11
Joined: Tue Sep 11, 2018 4:28 pm
Location: Northern Ireland

Re: Fuel but no spark

Post by Machbuffet »

SORTED !!!!!!!!!
Firstly, thanks to everyone who contributed, it was very much appreciated.
After all the faffing, it turns out to be the Speed Sensor on the bell housing. There was no reading on pins 1&2.
During the testing, what was throwing me was the readings between pins 1&3 and 2&3.
According to the inter web, I should have been getting at least 100k ohm between these pins.
Three different multimeters, including a loaner from a trade acquaintance, read absolutely nothing at all, across any of the scales.
Two brand new sensors read exactly the same and this confused me totally. However, Brucey set me on the right path with his helpful explanation and guidance on what to expect from these readings. The more resistance the better and the 100k ohms is just a minimum requirement, the higher the resistance the better.
Even though the Reference Sensor was showing a health reading on pins 1&2, I bought two new Bosch sensors as a precaution and I'm glad I did. When I removed both sensors, the wires were so stiff and brittle.
Replaced both and fired up on the first turn of the key.
Thanks so much to everyone again, I'm so grateful.
Barry
1985 635 csi
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