Crank speed sensor - '93 300TD

balge59

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1993 300TD 1988 230TE
hi
been doing a bit of digging, suspect my tach/tickover problem is down to a faulty speed sensor, but I couldn't find the connector (L3x), or the sensor itself :Oops: so tested through the diagnostic port - presumably the resistance should still be same-ish, 2 k Ohm?
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http://www.ps2cho.net/downloads/MB CD/W124/w124CD1/Program/Climate/83-570.pdf


The pics all show left hookers, should I be looking behind the battery or the fusebox for L3x?
Is the sensor easier to get at from below or do I need to take off the airbox or something maybe?
I will be digging in again over the weekend but any tips in the meantime appreciated.
Also, couldn't find a part number on EPC, anyone got an idea how much these run out at?

cheers!
 

television

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If the engine runs, then not that,, I have some info somewhere about the tach on the diesels
 
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balge59

balge59

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hi
yes, engine runs, doesn't seem to be any control of the tickover, there's no change in speed when you disconnect the pump, even pulled the relay but it still runs.....
Initially I suspected the ELR relay, but as I couldn't get a resistance for the tach I need to test the wiring and sensor first:confused:

cheers!
 

Number_Cruncher

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The connector will be in the same compartment as the brake cylinder and the fuse box - you should be able to see where all the wiring comes across from behind the sound deadening panel behind the engine.

The sensor will not stop the engine running, it will stop the idle speed control and the rev counter.

Have you checked fuses and OVP?
 
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balge59

balge59

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hi
yes checked OVP fuse, also swapped with OVP off my 230TE - assuming they're compatible??? Didn't help anyway.
Turned and cleaned all the fuses in the box as well - are there better fuses available at all? These ally ones are just nasty:shock:
Connector near the fusebox -check! - you mean the removable panel thing? But not behind that, right?:D
(not good with car terminology lol)
Do you know the easiest way to get at the sensor?

cheers!
 

Number_Cruncher

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I wouldn't spend much time getting to the sensor itself, because it's only at its connector where you can check it. IF you follow the bundle of wires which come up past your oil filter, you'll see they disappear into the sound deadening panel at the back of the engine, and come out again near the fuse box - there should be a connector there where you can test.
 

television

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On the diesel engines the tach is picked up on a separate sensor added to the front of the engine
 

Number_Cruncher

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It's on the flywheel ring gear, as per the drawing in the OP.

On some older diesel engines in W123s, yes, there was a sensor on the crank pulley.
 

television

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In the 124 250d it was added on the front as an extra in place of the clock,, my info seems to show the same for the 300
 
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balge59

balge59

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1993 300td w124

Excellent stuff, got it now
so - get in again, find the connector, test the sensor - 1.9 kOhm, +/- 02 kOhm at the connector, if not test the wire continuity and then its down to new sensor - check?
If it is borked and needs replacing - down from above, or up from below? Any idea how much, and where I can get one (can't see em at ECP or GSF...)


cheers !
 

Number_Cruncher

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Yes, that's the way to check it.

I would also follow the wiring back the other way, and find the idle speed control ECU - that's where the raw signal from the crank sensor is turned into a signal that the rev counter understands. Check to make sure there is power and a good earth.
 
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balge59

balge59

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:confused::confused: ok, I was doing so well then...do you mean trace up to the relay when you say ECU? Mine has an 'ELR' relay, which I think controls the low speed running via the 'actuator idle speed control Y22'?
I have an ECU but I thought it was just for ABS?

Hate it when I don;t know the right name for things

cheers!
 
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balge59

balge59

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hmm - that's turbo though isn't it? Don't think I have any of that EGR stuff either, and I totally don't recognise anything like that 'electronic diesel system control unit'....Is that a US spec diagram?
I have been using a diagram, but its 2MB, is that too big to upload here?
 

Number_Cruncher

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>>Hate it when I don;t know the right name for things

Every different workshop manual, and every different manufacturer has their own name for things, so, mixing names up is very common. ECU or relay, it's a box of electronics.

A shocking number of faults on cars aren't the sensors, or the electronics, but are the simple low-tech stuff like connectors becoming corroded and dirty. Fitting a new part will often cure the problem, as making and breaking the connection to fit the new expensive part provides a new clean contact.
 
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balge59

balge59

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ha ha, I hear that!
I separate and clean up every connection I see, now - scarey number of them crudded up so far - will have to 'chase' the whole harness eventually...just starting to hope it might just be a poor connection now - so it's probably going to turn out to be the most expensive possible fix instead lol!
I learned what little I know about electrics on old Brit bikes, this Bosch stuff is luxury after 6V Lucas carp!

cheers!
 

Alex Crow

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good advice from number cruncher, as ever, and good point about breaking/making connections curing faults! SRS is particularly prone to this secnario with airbag igniter sensing - for obvious reasons.

i will bet my house on it being the ELR or OVP units. i have seen plenty of these with no idle increase and no tacho and it is always one or the other - OVP is favourite. what colour top does the OVP on your car have? if it is not red like the one from your 230te, or if it is but has more fuses then it is not a direct swap! and both the ELR and OVP are worth checking for dry joints.
 
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balge59

balge59

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hi
well I initially thought relay (after reading heaps of posts) but if I get a fault with the sensor it doesn't mean the relay isn't borked as well...:shock:

Can't recall the colour of the OVP for sure, but they both have only one fuse - plus last time I had an OVP problem the ABS light was on all the time?

Do the relays open up fairly safely then? Might be worth a quick squint inside you think?

cheers!

edit @ Malcolm sorry missed one - mine has clock AND tacho? Seems to lots of changes round this MY....
 

Bolide

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1993 car - which engine?

You say it's a 300TD - if it has a 603, AFAIK there is no electronics to control idle. If it has a 606 it has electronic idle speed control via an ECU / relay-type box

I assume it's a RHD UK car with no turbo

Nick Froome
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