1973 MB 450SL Throttle Switch- Should Injectors Fire?

I have a quick question regarding the TPS on a 1973 Mercedes 450 SL. I am familiar with the Bosch DJet system, however, I'm curious as to whether all eight injectors fire at the same time as the throttle opens. I am aware that the injectors fire in pairs: one and five, four and eight, three and six, two and seven. It appears that in my case, when the engine is off and the ignition key is turned on and the distributor is pointed at cylinder number one, the two immediately following injector pairs fire (four and eight, three and six) as the TPS is opened (I, of course, hear the graduated 10 clicks). Should I be hearing all eight fuel injectors fire?

Also, as a separate question, when I test the ignition trigger delivery from the distributor to the ECU (back probing the five wires at the distributor, in fact, the PO inserted five bullet connectors for ease) I cannot fire an individual set of injectors twice in a row. I suspect it is by design, when I connect the number 12 pin to each of the four pins the 4.8 volt reading goes to zero after connecting each pin. In fact, as I cycle through all four sets, it appears that I only have 4.8 V at two sets at a time.

Hi ???,

bith effects are by design. The ECU can fire on acceleration pulses without trigger contact closing. And on trigger contacts input block there is a mechanism to prevent boucing contacts from double firing injectors. Otherwise 123 ignition distributors for D-Jetronic who have the error that they pulse twice on each contact would create double pulses.

Thank you very much for the very prompt response. Forgive me, my name is Kevin Ritter it's an absolute pleasure to join the forum, thank you for taking the time to maintain it for all of us. Another question if I might, as I indicated, I verified that I have continuity and power to each of the four wires and number 12 ground associated with the triggerpoints/ As I turn the crank/distributor, each of the four triggerpoints appears to stay closed for roughly 100 to 110°. It would seem to me that I could "wire in" a dwell meter in an effort to obtain a more precise value for each?

Also, just to clarify, do all eight injectors fire at the same time with each incremental movement of the TS? Or simply the two sets of injectors ahead?

Hi Kevin,

yes you can use 4 dwell meters to measure trigger points. I prefer a 4 channel oscilloscope or my igntition distributor test stand. Then I can see how the are all shifted by 90° and have same dwell and do not jump.

Regarding acceleration contacts induced injector pulses. They are independent of trigger contacts and are normally meant to prelong the injection pulse. But they can happen independent of trigger contacts. Whether all 8 fire then at the same time? I would have to read wiring diagrams before I can answer with 100%. PWM will definitely al fire, but then I would have to verify how drivers would approve these pulses to drive injectors. To be homest, it seems a pretty theoretical question and I am too lazy to do now.

Thank you again for the response, perhaps you are correct that may prove to be a theoretical nicety.

More practically speaking, I have a 1973 Mercedes 450 SL that is currently operating with vacuum of 12 inches, extremely steady with no needle deviation...that is obviously extremely low. I'm currently making my way through the DJet checklist and using the online wiring diagrams with the index that you provided… the index is EXTREMELY helpful as the online interface is "very muddy". However, the wiring diagrams are not year and model specific. For instance, when verifying the resistance value for coolant sensor T2 at pin 11 and 23, the diagram does not address the fact the blue and white wire at 23 travels through a hot start relay, the changes to a green and white wire before connecting to the sensor at the passenger side rear cylinder head. I am obviously savvy, not sure that a newbie would be. It any rate, back to the low-vacuum issue, which I suspect maybe well beyond a simple leak. Let me provide some additional metrics:

-when the engine is running, I can remove and replace each fuel injector connector one by one on the driver side, and the vacuum drops exactly 3 inches for all four cylinders. However, when performing the same exercise on the passenger side, the volume drops by exactly 1/2 an inch.

-compression test was performed on the passenger side with all eight spark plugs removed, however, the throttle plate was closed (forgot to open...all values may be skewed low) and the starter was cranked seven times. Compression was 140 PSI for all four cylinders one, two, three and four on the passenger side

-eight new spark plugs, eight new wires (verified resistance in each), cap and rotor in good shape. Also, repaired the green insulated wire between the distributor and ignition amplifier. Good solid spark to all eight cylinders

-verified the performance of the four triggerpoints as indicated above, however, keep in mind that the injectors are fired in pairs one and five, two and seven, three and six and four and eight. Candidly, I find it extremely unlikely that the triggerpoints are responsible as the injector impulses appear to be being delivered flawlessly on the driver side

-all 8 injectors had values of 2.9 ohms and were firing as verified by noid light with the exception of number three on the passenger side. This injector will not open and close i.e. click when connected to a 12v source. Currently trying to source a replacement injector.


My intuition would lead me to believe that it is a camshaft timing issue on the passenger side. However, when removing the valve covers and verifying the relationship between the two camshafts and the crank, everything is in PERFECT alignment at TDC i.e. The PO obviously replaced the timing chain with IWIS verified by the presence of a MasterLink. The camshaft vmark lines up with the mark on the camshaft bearing cover. I should mention that the timing chain tensioner is in dire need replacement, and yes I clearly understand the pitfalls of a suspect timing chain tensioner…that is on order. In the meantime, I prime the tensioner by turning over the engine several times with no fuel and spark. In fact, I installed a separate button to facilitate this practice as I plan on continuing the protocol after installing the new tensioner. I simply do not like the design and prefer the ratcheting style.