D-Jet Theory - PeachParts Mercedes-Benz Forum

350SL4spd · #1 06-30-2007, 05:49 PM

It's my understanding that ye olde D-Jet ECU can read only one sensor at a time (at least as far as fuel mixture/load goes), checking the input of each, one at a time, in a cycle. Correct?

Next question: Name all of the sensors that the ECU reads to control mixture, etc.? Pressure Sensor (MAP-thingie), Throttle Position Switch, Water Temp Sensor, Air Temp Sensor and....um... What did I miss? (I have no EGR/Emissions of any kind).

Final question: Is it therefore logical that when my euro 350SL is "bucking" under load (full gas, no gas, full gas, etc. when you push the pedal in) that two of the sensors are giving the ECU divergent information? ie: One tells it full throttle, the next says 'whoa, too much gas'? Obviously meaning that one of them is faulty, because these sorts of arguements seriously impeed forward progress...

That or it's the ECU that is having an existential crisis?

Or at least someone tell if my logic follows...it's been a rough weekend already...

Carson357 · #2 06-30-2007, 06:10 PM

do a google search for D jetronic, i remember reading some good info in the past

Matt L · #3 06-30-2007, 06:12 PM

The D-jet is an analog system, so there is no "sampling" of any of the inputs. The inductance of the MAP sensor determines how long it takes to charge a capacitor, with the other sensors providing appropriate trim. The throttle position switch merely tells the system that the throttle is opening, closing, at idle or WOT to provide additional trim.

Your conclusion is therefore not logical.

wbain5280 · #4 06-30-2007, 07:38 PM

I remember reading a web site of someone who reverse engineered the D-Jet box. It's completely analog and the main input signal is the trigger points. I'll try to find the link.

whunter · #5 06-30-2007, 07:41 PM

Fuel Injection
Fuel Injection

Have a great day.

wbain5280 · #6 06-30-2007, 08:50 PM

OK, here it is:

http://members.rennlist.com/pbanders/ecu.htm

psfred · #7 06-30-2007, 09:21 PM

Bucking under load can be a couple of things, including the ignition.

A shorted closed throttle switch will cut the fuel above 1500 rpm, but that will severely limit speed, too.

Bad trigger point cause surging as the fuel cuts in and out (trigger points "float" or get fouled with oil temporarily, more fuel is added due to higher manifold pressure, points work, leans out, etc).

Big vacuum leaks play merry hell with D-Jet.

Flaky wiring (check the big ground on the firewall, all the injectors ground there, and if it's corroded, intermittant injection results).

Loose wires on the air temp sensor

Bad ECU

Peter

ABDow · #8 07-01-2007, 04:13 AM

Quote:

Bad trigger point cause surging as the fuel cuts in and out (trigger points "float" or get fouled with oil temporarily, more fuel is added due to higher manifold pressure, points work, leans out, etc).

This advice conforms with my experience of oily trigger points. After I rebuilt the distributor on my M116, and overenthusiastically oiled it, the car used to jerk and temporarily die and then run ok, and then jerk etc etc.

350SL4spd · #9 07-01-2007, 12:03 PM

I've been reading to many injection books lately (I got one covering all Bosch FIs and have been boning up...). I'll have to go back over the D-Jet section...

The bizarre thing about my bucking condition is that there's a 'sweet spot' in the accellerator travel where she runs fine and pull up to speed just great. You have to find it again, through much bucking, after you shift gears, but then it pull great again.

I found a cheap Throttle sensor and put that in this weekend w/ no success. And I just bought (but have not yet received) a cheap, supposedly working ECU. Now, I'm not one to throw money at a problem, but I'm running out of things to diagnose.

I replaced all my engine vac lines, propane-checked the intake mani for leaks, cleaned/replaced all grounds, cleaned the trigger points, upgraded to an Ignitor & new coil, new timing chain (that was necessary), and a bunch more stuff. She runs about the same after warm up whether the air sensor is plugged in or not.

I was getting kind of wonky vacuum at the MAP even after I changed the big vac line to it, but no one could confirm what that vac was supposed to be (in/hg) at idle or what it was supposed to do when you blipped the throttle. Any input there?

Could a cabin vac leak "play merry hell" with the runability of a D-Jet system? Being a non-A/C non-emissions euro I have very few vac lines under the hood.

So, I'm basically down to the MAP, ECU, or the trigger points, unless there's some strange electrical gremlin that is shorting out every where except in that sweet spot...

Anyone have a spare MAP I could borrow just to rule it out? Hell, I'll rent it, pay shipping and buy you a six-pack.

Thanks for the input. And I do plan on posting a thread on the solution if & when I find it...

alabbasi · #10 07-01-2007, 01:33 PM

Sounds a lot like a throttle switch,
Here is a D-Jet diagnostic check from our friend David on the W116 forum.D and K Jet diagnostics

Tony H · #11 07-01-2007, 02:33 PM

I had a similar condition with my 3.5 coupe. I installed a Pertronix ignitor and cleaned my trigger points. Problem completly gone.

Tomguy · #12 07-01-2007, 03:28 PM

The WORST hesitation my 4.5 ever had was when I broke the carbon brush in the distributor cap.
The second worst is when my trigger points get fouled. The foul rate is more frequent, it seems, when I'm doing more city driving.

It could be the TPS, ECU or MAP in that order. I do believe I have a spare TPS somewhere. I know I have a 3.5 MAP (opposed to a 4.5 one). Check your PMs.

350SL4spd · #13 07-01-2007, 04:19 PM

This is why I'm starting to scratch my head raw over the little bastard

.

New TPS, New Ignitor, Cleaned the points, etc. etc. etc.

Will testing the points like the quote below tell me whether they replacing?

"Trigger points:

1)With the engine not running and ECU harness disconnected from the ECU,check the resistance between
ECU harness 12 and 21,then from 12 to 22.One should show a low resistance and the other should show an open circuit.

2)Rotate the engine 360 degrees and check the resistance.They should have an opposite reading now.

3)If not,perform the same test at the trigger points themselves.This will tell you if it a component or wiring fault."

PMed you back Tomguy. Thanks.

Larry Delor · #14 07-01-2007, 04:41 PM

Did anybody ever save WDurrance's web pages?
He had some very good info there.

Tomguy · #15 07-01-2007, 08:48 PM

Quote:

Originally Posted by 350SL4spd

Will testing the points like the quote below tell me whether they replacing?

"Trigger points:

1)With the engine not running and ECU harness disconnected from the ECU,check the resistance between
ECU harness 12 and 21,then from 12 to 22.One should show a low resistance and the other should show an open circuit.

2)Rotate the engine 360 degrees and check the resistance.They should have an opposite reading now.

No.
You already know the trigger points work because the engine runs. If they're drenched in oil (SO easy to check that if you have not yet done so, you need to. Now) then it'll run like crap. There are 4 circuits on the trigger points, the test you provided says how to check 2 of them but not the other 2. Which is fine if you only care about 4 cylinders.

Pull your distributor out (takes 10 mins MAX) and then pull the trigger points. Clean them with a paper towel to get the heavy oil off, then use an index card between them. The hardest part is the retiming of the engine on reassembly, which is still very easy. It's less time consuming than loosening the ECU fastening screw, pulling the harness off, checking resistance, rotating the engine 360° (especially if you pull the fan to do so) and then re-checking.