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Old 03-24-2014, 04:51 PM
Join Date: Sep 2007
Location: Canada
Posts: 759
m116 engine code diagnostic port location?

Hello, could someone please tell me where to read the troubleshooting engine codes for a 1988 420 SEL. Is there a link for this procedure? I have a friend that has a OBD2 code reader. Would it require an adapter for the Mercedes vehicle? Thank you.

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Old 03-24-2014, 05:10 PM
is thinning the herd
Join Date: Mar 2010
Location: Columbus, Ohio
Posts: 3,340
A 1988 is not an OBD2 Compatible car. OBD2 did not become the norm in the US until 1996.

Beyond that I cannot tell you how to use the diagnostic port on the car, but someone more knowledgeable will post and tell you how it is used.

If the car is OBD1 compatible, OBD1 is fairly limited in what it can tell you, its mostly emissions related codes. What issue are you having?
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Old 03-24-2014, 07:43 PM
Join Date: Sep 2007
Location: Canada
Posts: 759
Very long story....found two cam followers loose under the pass. Side valve cover. The cam was severely chewed up as well as the followers had the surfaces of them worn almost all the way through. We replaced the cam and all the followers from a used source on the pass. Side only. We can no longer get the vehicle started. We also had some bad spark plugs in the cylinders that had lost the followers that we changed. The car was running before this was discovered...actually surprisingly well. We also had it running after the new cam and followers were installed. It seemed to run well until the engine was put into gear to back it out of the shop. Limped it off to the side until the next morning and now it will not start.

I think it possibly has some issues with spark plug wires but would like to see if the computer could tell us anything about the fuel system. It seems as if the throttle body area is leaking from somewhere but not sure where. I may put some regular spark plug wires on to see if this engine is even viable to put any more work into. It almost seems like it might be a lost cause. Spark plug wires are expensive and if the engine is toast, we don't want to spend any more time/$ on it. After the cam change, when we briefly had it running, I put my finger over the hose coming off the valve cover (pass. Side) and the motor stopped running. My friends seem to think that this means this motor is finished? I tend to agree, but wanted to try a few things first before condemning it.
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Old 03-24-2014, 08:11 PM
JamesDean's Avatar
Electrical Engineer
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,034
The m116 is not compatible with ODB1 either.

It does have a duty cycle based diagnostic that may or may not be useful.

If you probe pin3(+) and pin2(-) at the X11 diagnostic port on the driver's side with your meter set in duty cycle/frequency mode the CIS-E unit will output a value.

Typically with the key on, engine off, it will output 70% duty cycle if federal, 85% if Cali.

When key on, engine on, it will output a % that after about 1 minute of running, should bounce around--ideally between 45-55 or so. If it remains at a fixed % then it it displaying a fault code.

That's all well and good if the engine is running but if the engine is off. I'm not sure if it can tell you anything.

To my knowledge CIS-E wont tell you anything about fuel, other than maybe that the EHA is bad.

Sparkplug wire run about $100. I picked up at set at AutoZone I think, no complaints.

Are you sure are getting spark at the plugs?
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Old 03-24-2014, 08:31 PM
Registered User
Join Date: Feb 2011
Location: Modesto CA
Posts: 3,243

Check compression in all cylinders first:
Remove all spark plugs.
Remove fuel pump relay (FPR).
Check pressures w/ open throttle.

Pull plugs from ignition amplifier (EZL) to prevent sparking.
Remove distributor cap, crank engine.
Is the rotor turning? If yes, bring #1 cyl to TDC, and check that the rotor points to the scribed mark on the body of the distributor.

Jumper sleeves 7&8 in the FPR socket. Do the pumps run?
Remove the return line at the fuel distributor (FD), and fit a piece of hose.
Run the pumps to check for return flow.

If these checks are positive, you can then proceed with additional analysis.
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Old 03-24-2014, 10:17 PM
Join Date: Sep 2007
Location: Canada
Posts: 759
O.k Frank, we will do what you said, next time we are with the car...unfortunately it is stuck at a friends shop where the cam was changed. We pulled a couple plugs and checked for spark...we were getting spark at those we pulled. They did spark sitting connected against a part of the engine in the open where we could see....whether it is good enough spark??? Also took the cap off and cleaned it and the rotor. The plugs we did pull looked slightly wet when we initially pulled them from the engine after it ran briefly, following the cam change?

My friend posed the question of camshaft compatability. He was wodering if they used slightly different cams for these m116 engines, from year to year. He was worried that maybe that replacement cam was perhaps not allowing the valves to close completely. The car we are working on is an 1988 420sel, and I am nearly certain the replacement cam came from a 1987/1988 or 1989 420 sel. Not sure if it makes a difference.

I should also mention that we had a complete cylinder head with us that we took the replacement cam and followers from. We noticed a couple of lifters (things screwed into the head with balls on the end) had collappsed on the car in qusetion so we changed them out with some good ones from the parts head.

One of the collappsed lifters actually came apart by pulling the ball part reveal a spring and a check ball.

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