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I have a slk320 that drives fine until you accelerate hard…

Customer Question
I have a slk320...

I have a slk320 that drives fine until you accelerate hard then it stumbles and throws a p0100 code. I have replaced the maf with a genuine bosch I have replaced cam angle sensor , o2 sensors, reprogrammed an ecu, resoldered the k40, replaced map sensor. Bypassed the maf wiring, looks all seem perfect. The mercedes agents on Christchurch nz have spent 14 hours diagnosing and come up blank. I have noticed a reported iat of 65degree when ambient is 17 Celsius

Mechanic's Assistant: Have you located all of the vehicle's oxygen sensors? Are the wires leading to and from each intact and in good shape?

They are oscillating perfectly

Mechanic's Assistant: What year is your SLK320?

2001 m112 v6

Mechanic's Assistant: Are you fixing your SLK320 yourself? What have you tried so far?

I am working with the agenrs. The question I would ask is. Does the iat / maf sensor earth through the k40?

Submitted: 8 months ago.Category: Mercedes
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Answered in 36 minutes by:
11/26/2017
Mercedes Mechanic: Matt, Engineer replied 8 months ago
Matt
Matt, Engineer
Category: Mercedes
Satisfied Customers: 23,459
Experience: Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Hello

OK to get that code and the fact that you have a 65 degree IAT reading points to either a faulty MAF sensor or a fault with the wiring or a fault with the ECU

as the IAT sensor is built into the MAF

when you say you've bypassed the MAF wiring can you tell me more about this? as the MAF is the main load sensor into the ECU ?

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Customer reply replied 8 months ago
We substituted the transponder chip into a same part number ecu and it still did the same. So we have gone back to the original ecu. I have resoldered the k40 board but am wondering the high reported iat maybe as a result of a bad earth. I am aware the iat and maf are in the same unit. My understanding is a high reported iat could be a bad earth or low supply voltage. We have tried 2 new bosch mafs in it
Customer reply replied 8 months ago
I am about to replace the k40 with a new one tomorrow. Can you please tell me if the maf circuit is supplied and earthed from yhe k40?
Mercedes Mechanic: Matt, Engineer replied 8 months ago

Hello

I'd agree that a new K40 unit is a good idea as the overload protection unit is a well-known cause of starting, idling, and engine stumbling problems.

This unit provides voltage to all of the control modules and electrical components, including the fuel pump, electric air pump, and horn. When the K40 malfunctions, these components don't get their required voltage so leads to all sorts of errors

once its removed I'd also check the relay side of the board for obvious problems: charred or loose connections, etc. as the connections can sometimes be damaged too

a good squirt of contact cleaner into all the K40 sockets is also a good idea

and finally I'd do a continuity test between the MAF socket and ECU socket to ensure the wiring is undamaged

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Customer reply replied 8 months ago
THE mercedes agents here have no answers and have charged me $2100 for that information. The problem started immediately after I stall launched it to get a 0 to 100 time. The engine will maintain high rpm in a lower gear but the moment you accelerate hard and load the torque on it stumbles.
Mercedes Mechanic: Matt, Engineer replied 8 months ago

OK

its possible the hard launch has shaken loose a connection ?

I feel the stumble is either down the MAF signal being corrupted or the ECU not getting the right voltage

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Customer reply replied 8 months ago
I have replaced CAS, o2 sensors, 12 plugs, 12 HT leads, fuel pump, fuel pressure reg/filter. The rail pressure is constant throughout the stumble. The fuel trim is normal until it stumbles then goes to full rich
Customer reply replied 8 months ago
maybe the k40 will fix it?
Customer reply replied 8 months ago
Also the lockup clutch is not engaging when it should
Customer reply replied 8 months ago
When running the real time data shows implausible cam angle signal too
Customer reply replied 8 months ago
Does the k40 supply the voltage to the ecu?
Mercedes Mechanic: Matt, Engineer replied 8 months ago

Hello

Yes the K40 does supply the ECU

definitely replace it and clean / inspect the socket as next course of action

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Customer reply replied 8 months ago
I'm guessing that would explain the CAS LOCKUP and Mag If the voltage was low then
Customer reply replied 8 months ago
MAF
Mercedes Mechanic: Matt, Engineer replied 8 months ago

yes thats right

if the ECU voltage is low or has voltage 'spikes' it really upsets the whole function of it

so you get all sorts of random faults

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Customer reply replied 8 months ago
Low voltage will throw a a high IAT too? and set the hot film to 135 degrees if it's showing 65 degrees and then would the MAF struggle to maintain that temp under load?
Customer reply replied 8 months ago
If the voltage supply to it was already reduced?
Customer reply replied 8 months ago
Is there a specific pin on the k40 that supplies the ECU or is it supposed by more than one?
Customer reply replied 8 months ago
Supplied
Mercedes Mechanic: Matt, Engineer replied 8 months ago

hello

this forum post shows the pin out for the K40

http://www.benzworld.org/forums/r170-slk-class/1263021-k40-relay-circuit-associated.html

looks like

pins 6 ,7 & 5

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Customer reply replied 8 months ago
Should there be a letter attached to those pins to tell me what connector?
Customer reply replied 8 months ago
Sure but they are grouped alphabetically too for each plug
Customer reply replied 8 months ago
I guess it's the end one E
Mercedes Mechanic: Matt, Engineer replied 8 months ago

hello

if you look at the pin out diagram it looks like the ' K' group of pins

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Customer reply replied 8 months ago
K40 didn't work but I think I have found it. I turned the ignition on and put the volt meter from the centre earth pin to the negative battery terminal and got a reading of 1.7 volts. Bad earth. So I took the harnesses off the ecu and looked carefully with a magnifier into the female contacts and about 10 of them are blue with corrosion. A quick spray with contact cleaner and replaced brought the IAT to 24c and the car started perfectly without stumbling. It accelerated hard twice then stumbled. So I got my gas tip point files out and started to clean the contacts. But I think they are too far gone. A lot of crunching when the file went in. After persevering I reassembled it and it went worse the ever. Now that's a good thing because I believe I have pinpointed the cause. I might see if I can get the harness attacher/plug repinned.
Customer reply replied 8 months ago
That was the centre pin on the MAF
Mercedes Mechanic: Matt, Engineer replied 8 months ago

Well done for finding something concrete

I'd agree that replacing that sensor connection sounds like the way forward

possibly easiest repair is to buy used harness off ebay and splice the replacement MAF connector onto your harness

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Mercedes Mechanic: Matt, Engineer replied 8 months ago

Hi
do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! If you're not pleased, also let me know because rating bad does not result in a refund, there are better options. Rating now will not close this post nor prevent further replies
Thanks Matt

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