420SEC Diagnostic Codes.

[OxfordSteve]

420SEC (W126 046)

In an attempt to solve some erratic revs problem on my 420SEC 1991, I have constructed a basic code reader tool and plugged it in to my diagnostic socket.
HOWEVER:
The 8 pin diagnostic socket on my car has only two terminals, 1 and 7 connected!!
Also I get the codes 12, 30, 31 and 32. But according to the only code reading chart I have code 30 and code 32 are not used!!
Does anyone have any experience reading the codes on a W126 046 and can they help me with this??
Thanks,
Steve

 

[gbjeppm]

Does post 8 in this thread help?

Homemade Code Reader - Mercedes-Benz Forum

 

[grober]

http://www.mercedes-club-bg.com/Mercedes Benz Help PDF/Merecedes-Benz-Manuals/cs1000_mb[1].pdf

with an 8 pin diagnostic socket there is often no 12v supply with the ignition on- you have to supply this from the battery positive terminal directly pin 1 is earth and usually pin 3 is for the CIS injection system pin 7 is for airconditioning usually. If pin 7 is connected to the aircon then you are screwed unless you can piggy back on to correct terminal on the ECU plug-- not a job for the inexperienced.

If your car has the cylindrical motorised idle valve then the OVP RELAY is a likely candidate for your problem. However irratic idle can be cause by many things including inlet system air leaks which might not show up in a diagnostic.

try this for wiring diagrams
http://www.benzworld.org/forums/w12...6-need-diagram-ecu-test-proceedures-same.html

 

[jaymanek]

Never known anyone to successfully read codes from a W126.

Erratic idle is normally down to idle control valve, idle control relay, OVP relay or an air leak.

 

[OxfordSteve]

Thanks for the info.
The tester I made was just to terminal 1 (Negative) and to 7. Do I need a connection to positive as well directly from the battery??
I seem to be getting a code read out - even though i cannot find what they mean!!!
Thanks,
Steve

 

[balge]

According to the test sheet linked -

pin 7 is for a/c, and

code 12 is engine coolant temp sensor short circuit
code 30 is circulation pump fault and
code 31/32 is duo-valve faults.

presumably the sensor fault might give an idle control issue?
hth

 

[OxfordSteve]

Thanks 'Balge'
Where was the test sheet linked? I would appreciate seeing that code reference sheet as none of the ones i have seen so far recognise a W126 046 .(420SEC 1990/1991) and none seem to have a output for 30 or 32.
The erratic revs are only when the engine is warm/hot and between 1200 and 1900 revs- Start well and usually idlwey OK but sometimes likes to stay at 1100.
Thanks,
Steve

 

[balge]

From Grobers link

http://www.mercedes-club-bg.com/Mercedes%20Benz%20Help%20PDF/Merecedes-Benz-Manuals/cs1000_mb%5B1%5D.pdf

Models Model Years​

124.026 124.030 124.050 124.090 124.051 124.230 124.290 - 1988-95​

126.024 126.025 126.035 126.039 126.045 126.134 126.135 - 1988-91

Spotted, it doesn't actually list a 126.046. Could be a typo... What is the difference between 045 and 046 spec, anything relevant?


I think it might refer to left/right hand steering?

 

[OxfordSteve]

Thanks,
The 46 was the SEC coupe , the 35 SEL LWB and the 34 the SE . (I think)
However, I think they all had the same 420 engine - but I wouldnt necessarily think Mercedes would use all the same electrics!!!!
Even so, where did you find these code referencies in your attached file. I have looked a couple of times but must be missing somethin.
Continually grateful,
Steve

 

[balge]

Thanks,
The 46 was the SEC coupe , the 35 SEL LWB and the 34 the SE . (I think)
However, I think they all had the same 420 engine - but I wouldnt necessarily think Mercedes would use all the same electrics!!!!
Even so, where did you find these code referencies in your attached file. I have looked a couple of times but must be missing somethin.
Continually grateful,
Steve

Page 10 for the pin-out and 87 - 89 for the fault codes.

From what I've seen with 124s MB use the same harness for lots of different models across any platform, just adding or removing bits depending on market/options. I would assume the same engine uses the same sensors etc too?

cheers

 

[OxfordSteve]

Thanks again.
However is that readout for the 124 not the 126?
Also this seems to be just for the AC circuit. If so that ties in with the fact that the A/c does not work - another thing on the list to fix later. I am not sure how those problems might affect the engine revs.
But thanks for your support.
Steve

 

[gbjeppm]

I would go back to Jay's post, OVP, idle control valve or relay or vacuum leak. I had similar problems on mine when i first got it, OVP sorted it out, as well as a small vacuum leak.

If its not been changed in your ownership, its worth doing, its about 70 quid from MB, and you know you are good for the next 10 years or so.

 

[balge]

Thanks again.
However is that readout for the 124 not the 126?
Also this seems to be just for the AC circuit. If so that ties in with the fact that the A/c does not work - another thing on the list to fix later. I am not sure how those problems might affect the engine revs.
But thanks for your support.
Steve

D'Oh...it's page 83 not 87 Page 10 is right for the pin-out at least

A/C​

​

​

​

Models Model Years​

124.026 124.030 124.050 124.090 124.051 124.230 124.290 1988-95​

126.024 126.025 126.035 126.039 126.045 126.134 126.135 1988-91​

Connect wires of Scanner as follows​

Scanner Data Link Connector 8-pin​

Yellow Socket 7​

Black Socket 1​

Red Battery (+)

Temp sensor failure could perhaps be cause idle-control issues?