My baby's been mostly an around-town car for a long time, but now I am preparing it for a 1600 mile drive from Florida to Maine. It's got new brakes, fuel tank was just flushed (lots of gunk was in there), fresh Rotella full synthetic diesel engine oil and new filter, and I'm preparing to drain the rear end and put in Mobile 1 75W90 synthetic. I don't know what's in there now or how long it's been there, but it doesn't smell new. Your sage advice about what else I might do in preparation for my trip would be appreciated.

Today I took it for a drive to check it out at speed and found at 60mph the engine was turning at 3000rpm, which seemed excessive so I didn't take it any faster. Isn't the peak power of a diesel engine around 1800rpm? Looking online I see people talking about closely related models at 75-80mph turning 3100-3300rpm, 70mph @3000rpm etc, which suggests my perception of excessive rpm of 3000rpm at 60mph is accurate. My tires are P195/70 R14s inflated to 44psi. I can tell all four gears are working so that I'm running in 4th at 60mph. The automatic tranny got new oil and a professional filter change a couple of years ago, and when I checked it today it was down about half a quart from the bottom line on the dipstick BEFORE I took it on the road, and I added a half quart I had so at least the oil level was at the bottom line with the engine and tranny warm from a couple miles around-town driving and measured carefully several times with the engine running, before I took it for its run. The transmission seems to shift okay but rather early I think, it hits 4th gear well before 40mph, maybe even 35 or less, I didn't pay close attention to that. But maybe 3000rpm @ 60mph is normal for this car and I'm concerned about nothing! Anybody know for sure?

 

You guys are awesome! Is it possible to look up my car's VIN on the Web or do you have some special access? Either way here it is:

WDBAB53A9EA174620

 

Did you check the transmission level with the car running? It should be checked with the car running otherwise it will not show the accurate fluid level. Additionally it is possible that the clutches are worn and it may be causing the engine to rev out a bit if the clutches are slipping, but you would notice that on the shift. Additionally most merc diesels top out at 4k rpm. Here is your data card I pulled from WIS. Don't mind the VIN change, its just something the WIS program does.

 

Now that you mention it, I did have a voltage problem last year that I fixed with a kludge. My battery would occasionally discharge into a low voltage state (11V or so) insufficient to crank the starter motor, but it tested fine after being recharged. The voltage output of the alternator was usually between 3.1 and 3.4V when I tested it at the battery terminals with the car stationary. I thought it might have a bad voltage regulator (worn brushes maybe making only intermittent contact when I happened to be testing) so I replaced it and there was no change. I installed a voltmeter on my dash so I could track it while driving and lo and behold I found that every now and then the voltage would be 'stuck' at around 11.4V and just stay there. Not all the time, just some of the time, sufficient to explain the battery draining down. I finally tracked the problem to the glow plug relay sticking on sometimes, but since I didn't want to spend the $100 for a new relay right away I just started disconnecting the relay plug after starting the engine and that "solved" the problem. That was about this time last year, then I spent the next ten months out of state and when I returned the glow plug relay had "healed" itself and no longer sticks. Of course I don't expect it to stay "healed" and will eventually replace it, but I am concerned about my charging voltage pretty consistently being in the 3.1 to 3.4 range while driving, a bit lower than it should be. I did see it hit 14.something once when I first got back to Florida and drove it this year, but that one time was it. I figure it's still going to maintain the battery charge minimally well at 13.1V so long as I don't overload the system with accessories, but now I am facing a long drive north which will involve considerable night driving. Is this something I should worry about or not? I suppose the real answer is replacing the alternator but maybe there's just a high-resistance point in the wiring somewhere that I don't know about? I am able to monitor the voltage continuously now while driving so that at least will alert me if a new problem arises.

 

Thanks for the info, you are FAST! What is WIS? I don't see anything in the data though that would give me any hint as to what my rpm should be at 60mph...

 

You are welcome. WIS is an abbreviation for Workshop Information Systems. Its what us techs use to look up vehicle information or repair procedures. I am trying to find your ratios but its an old enough vehicle I may have to track down the actual paper book that has that info. As for your other post about the alternator, you want to see it charge between 12.4-14.1 volts depending on driving conditions and accessories being powered. Being the age of the vehicle it may be a little less. I would imagine the brushes are worn if the alt has high mileage on it. Some people will recommend replacing the regulator first to see if that fixes it, but I almost always recommend a whole alternator on higher mileage or age vehicles due to the domino effect of replacing one part and it causing another part to fail. I would replace the alternator before your trip if you can, I would hate to be stranded because of that.

 

did you cheat and use google? haha

 

 

Actually a few years ago I made an Excel spreadsheet for the purpose of calculating road speeds at specified rpms given tire sizes and rear end ratios and gear ratios, but I can't find it now. Using the site at http://www.crawlpedia.com/rpm_gear_calculator.htm I find that I should be running 2500rpm at 60mph, not 3000rpm. So the possibilities are:

1. Inaccurate speedometer
2. lots of slip in the transmission (doesn't feel like it, shifts firmly)
3. somebody changed a rear end gear that i don't know about (there is what looks like non-factory white silicone sealant on the pumpkin cover, but why?)
4. somebody swapped a transmission into it with weird gear ratios that I don't know about (seems highly unlikely)

Does this calculation look right?

 

moved to W123 forum...

 

Does moving a thread mean that nobody sees it again?

 

These guys say their Mercedes speedometers are inaccurate!

http://www.peachparts.com/shopforum/...eu-repair.html

 

I know my W123 speedo is inaccurate anywhere above about 45mph. 55 indicated is about 58mph actual, and 75mph indicated is pretty much 80mph.

 

That's probably the best explanation, but that doesn't mean I'm happy with it. Thanks for your help, I wasn't expecting my speedometer to be off!

 

Your speedo is accurate. The lower RPM applies only to 85 models with the 2.88 differential ratio.

Peak power of your engine is at 3800rpm. Peak torque is at 2400rpm.