Intermittent missing 87 560 SL

[ukrmh88]

My 87 560 SL with 132K miles has an intermittent miss. I just bought the car, and it has not been driven much in almost a year, so I thought it might have been bad gas. I ran the tank close to empty, changed the fuel filter, filled it up and added 3M Extra strength fuel injector cleaner. I put another 200 miles on the car and it still has this miss.
When it misses, all gauges continue to work normally, except the tach, which drops to 0 rpm. At highway speeds, with the tach at 0, the oil pressure gauge still registers at the highest mark, which leads me to believe the true rpm is at least 1000+ rpm, even though the tach shows 0. I went out to the car in the stealth of night and ran the engine to see if I saw any fireworks under the hood, but all appeared normal, so I think the plugs and plug wires are good.
99% of the time the car runs just great!
I took it today to a really good independent shop, and they spent a couple of hours checking it out. Based on the tach dropping to 0, they suspect it could be the ignition module. Unfortunately, they said the part was not in the US, but rather in Germany. To add to the bad news, they said it cost $1200! Hard to believe! I am really hoping this is not the problem. They said this would be pretty rare, but not unheard of, for this part to fail. Does anyone else out there have any additional ideas/ input about this issue? I am planning on driving this car 900 miles soon from VA to Kansas City, and I really would like to have some peace of mind while I am driving. Thanks in advance.
Bob

[450slcguy]

Originally Posted by ukrmh88

I took it today to a really good independent shop, and they spent a couple of hours checking it out. Based on the tach dropping to 0, they suspect it could be the ignition module. Unfortunately, they said the part was not in the US, but rather in Germany. To add to the bad news, they said it cost $1200!Bob

Suspecting and knowing at your expense is just like putting your money on red and spinning the wheel to see what happens.

Replacing the ignition module (ICU) takes about 20 minutes, a couple of screws and connectors.

It never ceases to amaze how shops won't lift a finger to locate replacement parts. These guy's know damn well they can get these parts for a fraction of what the new dealer cost is, but they choose not to or they tell you it's $1200 part from the dealer then buy a $250 rebuilt part and charge you the $1200. You'll never know the difference. The more money you spend on parts the more money they make. Some shops wil double even triple the parts cost in addition to their labor charge.

You can purchase rebuilt modules from many many sources, including this site under the parts section, as in:

C4040-49620 Idle Control Unit Programa IN STOCK $374.84 $254.90

Do some internet shopping and you will find what your looking for without much effort, and save big $$$$. Couple hundreds buck and a 10mm socket and you'll be done in under 1/2 hour.

[karburn]

Amen. I just wish I had the technical savvy (and guts) to tackle the electrical stuff myself. I took the car to an indep shop cause it kept dying unpredictably and the problem was a loose crank sensor. Months later I had a 'miss' come on suddenly, but now my 87 560SL runs great now after a trip to the local dealer (prev owner had installed wrong plugs, plus I needed a coil and they replaced the plug wires while I was at it.) The dealer gives a 15% discount on parts AND labor for MBCA members, so now I'll likely stick with the dealer. Good luck on the trip back.

[KCM]

Originally Posted by ukrmh88

When it misses, all gauges continue to work normally, except the tach, which drops to 0 rpm.

Based on the tach dropping to 0, they suspect it could be the ignition module. Unfortunately, they said the part was not in the US, but rather in Germany. To add to the bad news, they said it cost $1200!

Well, the tachometer does run off of the ignition module, and if the tach drops to 0 only when it misses, there definitely is a connection. Ignition modules do go bad. Last time I checked for an '89 they were more like $2000 for new, around $500 for rebuilt, and $100 on up for used. And all should be available somewhere in the US, though there are many different part numbers for these modules, so you need to get the one to fit your car.

But if it is the module, I would think that it would quit completely and not just miss, especially if the tach drops to 0. Maybe there is a short in the tach circuit or tach itself. If I remember right there is a junction block on the fender well where the tach wire runs to and can be quickly disconnected. I would definitely rule out the tach before replacing the module.

The only other part I can think of that might make the tach drop to 0 is the ignition pickup sensor on the flywheel, but then again, the engine would stop completely if it failed.

If you install a new or used module, don't forget to apply a coat of heat conducting paste, as well as the plastic foil, to the back of the module.

[tecqboy]

The heat conducting paste can dry out over the years. If it has, the module may be getting too hot and causing the problem. It wouldn't hurt to do a "re-paste" with new heat conducting thermal paste. You can buy a small tube at Radio Shack.

[ukrmh88]

tecqboy, thanks for the tip on the reapplying the heat conducting paste, and where to purchase it!
KCM, in reference to the short in the tach or tach circuit, where precisely is the junction block on the fenderwell where the tach wire runs? Left fender by the module? Right fender? Does the tach wire connect into the ignition module itself?
The car ran fine today, except for a little "misstep" at a low speed for only a short distance. Last night, I found the part # of module on my car and found an exact match on ebay by part #, so I "Bought it Now" for $89, and it will be waiting for me in KC upone my return this weekend. I am willing to take a chance for less than $100 on this, whether it works or not. Thanks for all the input so far, and I will continue to update until I get this fixed.
Bob

[ctaylor738]

Suspecting and knowing at your expense is just like putting your money on red and spinning the wheel to see what happens.

Replacing the ignition module (ICU) takes about 20 minutes, a couple of screws and connectors.


First, you are quick to criticize a shop that the owner has some confidence in. So tell us in this situation what you would want to "know" that the ignition unit was the problem. These cars don't store codes, which makes intermittent problems hard to find.

Second, few reputable shops are going to spend (uncompensated) time on the web looking for a used, non-returnable electronic part that may or may not be the correct one, and may or may not work when it arrives, and may not last more than a week. Then they have a PO'd customer on their hands. All this while the car sits on their premises while they wait for the part, in the way, subject to damage.

One thing I've learned is that the world looks a lot different when it's what you do for a living, rather than a hobby.

[450slcguy]

Originally Posted by ctaylor738

[I]
First, you are quick to criticize a shop that the owner has some confidence in. So tell us in this situation what you would want to "know" that the ignition unit was the problem. These cars don't store codes, which makes intermittent problems hard to find.

Wasn't criticizing the shop, just warning the owner to be careful spending $1200 on a part that the shop "suspected might be the problem". I'd want to know what they would do if after spending $1200+ bucks the problem wasn't solved. Situation happens all the time, and the customer gets stuck with the bill for needless part purchases and labor charges.

Originally Posted by ctaylor738

Second, few reputable shops are going to spend (uncompensated) time on the web looking for a used, non-returnable electronic part that may or may not be the correct one, and may or may not work when it arrives, and may not last more than a week. Then they have a PO'd customer on their hands. All this while the car sits on their premises while they wait for the part, in the way, subject to damage.

Firstly, noone suggested a "used, non-returnable electronic part". I suggested a reconditioned part. The example given was from this site, Fastlane $254 vs $1200. The customer should be given a choice if he so requests or desires.

Secondly, any shop that specializes in older MB's shouldn't have to spend needless time searching for parts. The shop should already have a source of online vendors who they can purchase parts from besides the "dealer", this is not 1972 anymore. And who say's uncompensated time? The shop surely is already profiting dearly on the part sale, if not also charging for the time to source it. Most if not all part suppliers can get parts to you as fast as the dealer can, if not sooner.

Originally Posted by ctaylor738

One thing I've learned is that the world looks a lot different when it's what you do for a living, rather than a hobby.

If that's what they do for a living, then the "Professional" should be held responsible for correctly diagnosing the resolution of the fault. The customer shouldn't have to pay for parts and labor if the diagnosis was incorrect. Don't you agree?

And one thing I've learned is don't let the "professionals" take you the most expensive route when there are other part options available to them that are equally as effective and resolute.

[KCM]

Originally Posted by ukrmh88

KCM, in reference to the short in the tach or tach circuit, where precisely is the junction block on the fenderwell where the tach wire runs? Left fender by the module? Right fender? Does the tach wire connect into the ignition module itself?

On a 1989, it is on the left front inner fender well. There is a junction block with a screw terminal. I believe the same signal is also used to signal the fuel pump to run so there are more than two wires at the terminal. You should be able to trace one of the wires from the module to this junction block without too much difficulty

[ctaylor738]

There are four wires, and they should be all green/yellow.

In from ignition unit

Out to x11 diagnostic socket, tach, and idle speed/compressor/fuel pump relays

[ukrmh88]

The car made it ok from Roanoke, VA to Kansas City, as I expected. I arrived KC this afternoon. I did experience a couple of misfires, usually right after I would start the car, but the car "came out of it" ok after a brief time. As I said, I did purchase a used ignition module on ebay (item # 230215963177), and expect to receive it Tuesday, at which time I will install it to see if this very intermittent missing goes away. I am curious about the "heat conducting paste" mentioned by 2 respondents. I went to Radio Shack (RS) and guess what, they have 3 different types of this:
1. Thermal compound CR (ceramique) 3.5 g in a small gun for $6.99.
2. Thermal compound AS (arctic silver), same style container for $9.49, and 3. Silicone base heat sink in a larger tube for $2.99. The guy at RS said all 3 would work, so I bought the cheapest. Which one should I use, or does it make a difference??? Also, what is the plastic foil mentioned by KCM? Where do I buy it? RS?
Let me say that the shop I took it to in VA was Martins in Roanoke, and they came highly recommended by friends who live there. They were NOT trying to sell me the module because they knew I was leaving town in a few days, and besides, the part was half a world away. Their best inclination as to what this intermittent problem could be was the ignition module, primarily based on the zero rpm tach reading with other gauges still working during the misfires. They even showed me where it was on the left inner fender. I believe they have very high ethics and standards. They spent over 2 hours looking at my car, and only charged me for one hour's labor ($82), so I think that speaks for itself! If the module does not fix the problem, I will look at all of the connections to insure they are tight and corrosion-free. After that, I may just take it to the local MB dealer here in KC and see what they come up with. I will advise more later. Thanks for all the input!
Bob

[tecqboy]

The cheap paste will work fine, provided there is enough in the small tube to cover the surface area. The plastic foil is really a mica sheet which serves as a heat conductor and an electrical insulator. It must be salvaged from the original unit. It must be used. You can buy new silicon based sheet material from specialty outfits like MCM, but Radio Shack will not have it. I don't know if it's an MB part. Spread the paste in an even / thin layer on both sides of the sheet. Careful, it is brittle and fragile. You may also want to get a small can of spray contact cleaner to use on the electrical connections. And, I would still have a look at the crank position sensor. Be sure it is aligned properly, the cable is not deteriorated, and the plug is clean. In my many years of electronic repair experience, I have never known a silicon component to abruptly fail intermittantly. They either short or open just once and that's it. My first inclination is a bad connection or bad solder joint somewhere in or around the module. Good luck. Intermittents are the worst problems to solve.

[ukrmh88]

tecqboy
Where is the crank position sensor located on my car? I will check it out and look for any loose, brittle or broken wires. Thanks for the response!
Bob

[ukrmh88]

tecqboy
Thanks for the response. As information, I received the used ignition control module I bought off ebay, but before I installed it, I decided to follow your suggestion, and I cleaned every contact on the existing ignition control module, coil, and an adjacent contact board with emory paper and contact cleaner. Even thought the battery was new from the previous owner, I also cleaned both battery posts thoroughly, too. I put everything back together, and it seems to run MUCH better. In the past 5 days, I have not seen the tach drop to zero at all, and it only "stumbled" one time. So, hopefully, the problem was just dirty connections in/around the module, or bad contact at the battery. If this continues to act up, I will install the used ignition control module I purchased on ebay, using the paste and mica foil recommended in this thread. I will continue to look for the crank position sensor, too, and do the same with those connections.
Thanks to all for their input on this issue!
Bob

[ukrmh88]

Just a final note on this issue, as I posted the original thread. The car started to act up again shortly after I thought the problem was corrected, even after I cleaned all contacts, etc. Based on that, I did install the used ignition control module I purchased on ebay, and the car runs just fine now. No problem locating the heat conducting paste, but I had a problem locating the heat conducting mica silicon-based liner to put between the module and the inner fender. I could not find any in Kansas City, so I contacted MCM electronics in Dayton, and it was on back-order. It took almost a month to get it, but it finally arrived this Monday, June 23rd, at which time I correctly installed the module, and all has been well this week- no more misfires or intermittent missing. I was fortunate the seller on ebay sold me a good used one for $89. A new one, or even a rebuilt one, would have cost many multiples of that! Thanks to all for their input on this. I read the forum a lot and have learned a lot, too.
Bob