1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4.5V8

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1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4.5V8

Postby FioranoCars » Mon Jul 03, 2017 7:41 pm

Hi all, a bit general, but some help would be useful, a long post!

1974 Mercedes-Benz 450SLC (107 series)
Chassis WDB10702422006###
Engine 4.5L / V8 / Petrol - Type M 117.982
Bosch D-Jet Efi

Having some issues getting accurate technical data for this D-Jet system, although very familiar with later K/KE-Jet, as used by more usual for us Ferrari Testarossa/328 etc that we see, but this D-Jet is causing some issues, not least the lack of reliable history, so I'm reaching out and uploading some captures that might interest people more generally, as illustrates some more universal principles, though not to the depth of Steve Smiths recent 4xWPS post!

So I guess unless you know D-Jet or have factory or other specs for Mercedes M117.982 4.5 V8 pre 1975's (ie pre K-Jet!) then this might not be the time to chime in, as don't need any guesswork, hunches or hip-shooting please, hard facts and personal experience with this system only please ... unless you have comments on the captures, then freely ask/postulate away :-)

For those not familar with D-Jet ... it is EFI rather than CIS (K/KE-Jet), and predates the (K)continuous Injection with the injectors being positive triggered (in pairs for the 8 cylinder versions) from a lower set of lobes on the dizzy shaft, so Dizzy operation is pretty critical.

The car arrived directly post new owners purchase, with little prior history except a lengthy storage (years and years, no more detail!)

It ran, but lacked power and was regularly rough. The client wanted a thorough service too, plus some hope of future reliability and ease of maintenance, rather wanted to get things right now and then enjoy ...

Visual inspection noted that the fuel filter and pump had both been replaced recently but no paperwork to support/give further information. The tanks might be known to suffer from rust internally, so far not gone there, but might drop the tank or endoscope it to confirm internal state.

First port of call was to smoke test and MultiVac, and a few issues found. Fixing those rectified part of the problem but not completely and lack of power persisted but less consistently rough.

Next was new HT leads, dizzy cap, rotor arm, points, spark plugs as they were all down within the service element due to aged and perished etc. Sadly, no miraculous improvement, so 4Gas out and fuel delivery/pressures checked (as new duff pump/filters we've seen before affacting pressures/flow etc) and these appear to be good/OK:
Fuel Pressure - 32psi (K-Jet is 5bar, this appears in spec)
Fuel Delivery - 3.8 ltrs/minute
HC's - 100PPM
CO - 5% (when running ok) - need to recheck as figures blurred with oil :-(

The Coil was replaced as king lead monitoring showed weak spark, sadly that capture not saved, but with new coil a capture shows the first revelation, we managed to get a Picoscope during a rough period post coil replacement, and this that showed high KV and short burn time on secondary for 2 pairs ... the injector trigger was only firing at 2 of the 4 positions, so only 4 of the 8 injectors were firing this was then verified at the injectors later. (thanks to STC to suggest looking at King lead rather that each lead one by one or 4 by 4)

Peak Kv and burn time.png
Peak KV and Burn time

At this point decided to send Dizzy off for a rebuild to a specialist as we don't have the parts to do it (not obtainable either) as it has a failed diaphragm too (dual diagpham - 1 for advance 1 for retard), but also due to fluctuations/possible shaft wear, so possibly needing new bushings.
During removal of the dizzy showed the injector trigger wiring resin at the junction entering the dizzy housing was falling apart, so in fact will need complete new injector trigger (rather than a rebuild kit) ... but the spark condenser was showing 220nf (it is mounted internally to Dizzy, NOT externally).

Dizzy is still away, so can't do more tests at the drop of a hat, hence current reflection time and more research.

Injector during is about 5ms at idle (on those that work!) measured at injectors
Injector bank 2 only 2 firing 5ms.png
injector duration

so where's the questions ... here!

1. Our Dizzy has a Condenser, but the specialist rebuilding the Dizzy is adamant our dizzy for an 1974 car did not have one. Mercedes are less than helpful, I've found reference to 2 different ratings (from memory 180nf and 280nf), our dissy hosuing clearly has a cut out for the cable ... and then many people are saying cut the wire anyway as it caused more issues than it fixed.
a. Is the dizzy we have right for the car? It's a Bosch 0 231 403 004 (TFUDX8).
b. The condenser is Bosch 1 237 330 203 - is this the right one,
c. and what is the correct spec (nf)? and should we have one!? Should we disconnect it (aka cut the wire, but in a more polite and reversable way!)

2. What is the correct Dwell (hearing 30 and 36, no I'm not splitting the difference, I'd like a correct or best spec please!)
b. how much should it wander (told no more than 2deg factory limit but 1deg ideal others say 3deg)
c. at what revs should it be measured (or should it remain pretty constant from idle to x,000 rpm)?
d. with or without Vac connected?

3. What is the correct ignition/dizzy timing, as again have 2 different sets of figures, I think 30 and 36 from memory (not at the shop, so not to hand). Our Rebuilder has been unable to confirm, so a tad worried, and would like to get some independent facts. Are there 2 sets of data ... for idle & 3,000 rpm? So Degrees BTDC, at what RPM, Vac on or off

4. Do our fuel figures seem ok? Delivery taken pre-rail in effect (rather than post damper/pressure regulator).
b. The fuel pressure drops to ZERO instantly you cut the pump - is this right? Are they are supposed to hold some pressure for a short while ... maybe the fact there is no mention then they don't, but might be pointing to the wrong pump being fitted or another fuel issue.

5. We seem to have huge (ok, what's huge, not measured it sorry) rocker cover pressure, enough to whistle if bunged (after doing smoke tests I left the RH breather bung in, my bad). Will measure it but is this usual, should a cylinder leak down be done ASAP or is there a possible PCV type valve we've missed (no parts manual to check against - any pointers where to get one?)? Anyone experience of issues with this, what would be normal for this engine?

6. Compression appears good, not done leak down test (rocker cover pressure means we might have to), see enclosed (using a Fluke PV350 rather than a WPS500, but hey it did the job) - Compression test - 150psi to 160psi (10.5-11bar aprox) ~ 5% differential worst to best
mercedes SLC450 1974 in-Cyl #5 compression.png

7. Other parts ...
a. Both ballast resistors are cracked but while not causing a fault will be replaced as client wants things to be spot on for future reliability.
b. The Fuel pressure regulator and fuel damper while appearing old are not from fuel figures an obvious issue, should they be replaced as part of a major service/potential to cause less clear issues and need specific testing protocols to prove their serviceability? Any specific issues we should look for?
c. Owner supplied 8 new injectors, so they were all fitted along with Dizzy quite early on.

8. Since testing post Dizzy cap/leads/plugs etc, the Bosch plugs have picked up lots of carbon, so was going to change to an NGK ... is there a "best" spark plug for this engine? Others say the Bosch WR7DC is a must?! Comments?

9. What should the Vac be at idle? Been told without a minimum of 15inches at idle (7.3psi) then there are problems, sadly can't check it as dizzy out, but from memory think we had 15psi, so no shortage, any comments on this?

10. Been told valve clearances on these pre-hydraulic tappet cars can be a big issue and need to be reset every 10,000miles, with very fine tolerances, but not been able to get a figure for the clearances. Anyone have the exact data, a workshop manual section might be handy : -) ... inlet and exhaust ...

11. Need a Wiring diagram for the Fuel/D-Jet elements please! Ideally a ECU pinout would be great so we can confirm the wiring harness is intact, as currently testing at each component rather than what the ECU is seeing (Ok, have a 4cyl generic diagram for a 25pin ECU, but wanted a more specific one if possible).

So this is a reflection of the feedback offered so far, any other comments, further tests, general constructive feedback most welcome ...

We have scoured the web and forums, online technical manual, Autodata etc, sure I've probably missed some stuff, any links most welcome...

I have got from a wiring Diagram that seems to be a good match (page 4-3 segments A:20 to H:25) covers the Ignition System, and starting, but not the "Fuel Injection" or "D-Jet ECU" (injector triggering to enrichment, TPS and other sensors etc), so I'm quite keen to get those if anyone has them. Oddly the Cylinder numbering/locations are printed on this wiring BUT are WRONG (contradicting the engine label) having the banks reversed! SO hard to be sure how accurate it is ...

Interestingly, there is a socket mounted on the engine near the distributor, to allow easy plug-in and diagnostics with a "Crypton" type tune machine, which even has access to a TDC sensor which the engine does not use!
Pin out for this socket, based on wiring diagram we have but not yet verified, but might be handy for some future person:
1 = Tachometer Input (PRM) from Transistorised Ignition unit
2 & 3 = wires to 760 Ohm TDC sensor (only used for this diagnostic socket from what I can tell, not the ECU)
4 = Live (+15)
5 & 6 = 0.1 Ohm Cyl#1 firing pickup
7 = Ground

I'll try to bring all the data into a simple and readable format, for future D-Jet troubles and then anyone in future looking can find it quickly. I'll also upload to the library the captures.

Next checks/resets once Dizzy returns and is setup
A. Enrichment Adjustment
B. Idle Adjustment

But clearly would rather have some feedback on maybe the bigger picture ... fuel pressure leak down, positive crank case pressure, and of course get some technical data confirmed and wiring info while we have a lull in proceedings ...

As always, thanks in advance

Richard Lukins
Mercedes-Benz_SL-Class_1974_Petrol_Compression Test.psdata
Relative compression
(122 KiB) Downloaded 4 times
Mercedes-Benz_SL-Class_1974_Petrol_injectors bank 2.psdata
Source for injector duration
(2.15 MiB) Downloaded 4 times
source file for peak KV and burn time
(1.34 MiB) Downloaded 5 times
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Re: 1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4

Postby STC » Tue Jul 04, 2017 9:51 am

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Re: 1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4

Postby KimAndersen » Tue Jul 04, 2017 8:38 pm

Hello Richard

It's an exciting car you got in your workshop and being able to work on some of the first fuel injection systems must be very unique experience.

This Mercedes-Benz document covers the engine model 250S and 280 SE - built from 1967 to 1972, and equipped with D-Injection system an early version - if not the first one (I guess here).

I know it's not a 450 SLC (V8) and 250/280 SE are in-line 6-cylinders, but there's no such difference between the two fuel injection systems at the time - I think, but I could be wrong !!!.

Mercedes-Benz 280 SE D-Injection
(1 MiB) Downloaded 21 times

Kim :wink:
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Re: 1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4

Postby Technician » Wed Jul 05, 2017 1:37 pm


here is the D Jetronic technical data for your vehicle;

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Re: 1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4

Postby Technician » Wed Jul 05, 2017 4:03 pm

Hi Richard,

Been looking at your first scope traces.

With reference to the ignition traces.

Cylinder 5 kv low, could this be either the resistance of the plug lead and or the spark plug being heavily carbon up? Oil burning in that cylinder or an injector poor spray?

Traces 3, 4, 6 & 8 may also be under suppressed. The sloping spark line in cylinder 5 could be the result of excessive volt drop which occurs as current flows between the spark plug electrodes.

What type of plug leads are fitted?

Should that model have resistive plug leads or low resistance copper leads of the day?

Cylinder 7 appears similar to cylinder 5 to me!

Spark plug condition, heat range, quality of plug leads, cap and rotor kv, we used to say the kv across the rotor tip should be less than 5 kv.

Just some thoughts for now.
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Re: 1974 Mercedes-Benz 450SLC (107 series) D-Jet M117.982 4

Postby STC » Sat Jul 08, 2017 2:57 am

Kv traces often reveal more with Snap Throttle
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