I've enjoyed being a lurker in these forums for a while now, but a recent issue has cropped up on my wife's 2001 ML320 that I need input on.

A few times over the past year, the truck wouldn't start up; it would turn over but not catch/start. Waiting at least 15 minutes and trying again would do the trick - the engine would fire right up. The check engine light never comes on, and on previous occasions the dealer has not been able to find the problem; I assume that no error codes were reported by the engine.

This afternoon, after filling up at the gas station, the truck refused to start again. However, this time the engine would turn over and almost catch (causing the truck to shake slightly), but then fail. A friendly mechanic driving by looked at it and thought that the mass air sensor needed to be replaced, noting that he's seen that particular problem often. After trying for an hour and a half, we called AAA for a tow - only to have the truck start fine once the tow truck arrived .

From what I've read in these forums, it seems like the crankshaft position sensor is the culprit; if so, I'll be ordering one and attempting it myself. However, the mechanic's advice about the mass air sensor makes me wonder - does the mass air sensor only need to be suspected if the check engine light is on? Is there any other things I can try to help narrow down and troubleshoot the issue?

Thanks in advance for any input you all can provide.

typically when an air mass goes bad, it will turn the light on, mostly because self-adaptation values have hit their limits. I don't think a bad air mass will keep it from starting.

I had one awhile ago that would crank but not start, I think the AAM was the culprit but I am definitly unsure, it was waaaaay back and my memory is not as good as it should be. Jumping the contacts for the (starter?) relay while cranking with the key would get it to start.

I've also had ones with corrosion on the ground strap (directly under the starter to the frame) causing too much of a voltage drop for it to start. I think the starter would only click in those instances though.

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

Without proper diagnosis, it's hard to tell.

I would venture a guess that it's the crank position sensor. A bad MAS won't prevent the car from starting, but on-road performance suffers.

Although when mine had hard start issues, eventully the fuel pump died..and the light came on. The pump and MAS had to be replaced!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

No one mentioned the key problem. I have read here that there is a common failure of the radio device involved with the key. Could be the car, could be the key. If the computer doesn't get the signal it expects from the key, the starter turns, but no engine management activity. Seems there was a pretty easy way to diagnose this one, but I don't recall it. Try the search feature.

While that is true (It will crank but not start, or crank, run for a sec and die) the instrument cluster typically will flash "error" where the mileage reading usually is.

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

The ML key transponder problem is VERY easy to check! TURN the key to the on position & wait 5 seconds then try cranking, IF the starter works then the key IS sending a signal & isn't causing the problem!
IF the starter doesn't work (after the 5 second wait) then the KEY or transponder is the problem!

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MERCEDES Benz Master Guild Technician (6 TIMES)
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MB technical information Specialist (15 years)
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Betcha it's the CPS (crankshaft position sensor). Those are classic symtoms, the intermittent no start and dying.

Easy to do and only like $125 for part from dealer.

Search for my writeup.

Dickie
00 ML 320
97 328i
87 325is (FS)

Had the same symptoms last week on my '99 ML320 (90,000 km/55,000 miles). Got a new CPS and it hasn't let me down since. One thing though, 2 days after the sensor change, I got a CEL, and have reset it 3 times now, same code 170 and 173. It runs great, just sends a code every second day or so. I am told it's the MAF, but wouldn't that cause it to run /start crappy? I have no issues about performance. I am also told it's an O2 sensor, but wouldn't I get a different code? How do I know what part to buy or how to check?

Hmm...I've done some forum searching, and it seems like the fuel pump would normally be the prime suspect - except the fuel pump seems to have been redesigned in MY2001 to fix this issue.

I think my best bet is to try replacing the CPS, since it's also the cheapest possible fix. If the problem rears its head again, then maybe an expensive trip to the dealer for fuel pump replacement may be in order.

Thanks all for your advice - I will report on any progress on this issue, so maybe someone else with this problem might find it useful.

Hello

I have a 99 Ml320 with a bad CPS . I have been looking for the CPS unit for over an hour and still can not find it. I have also look at other posts on this site. Can somebody help me take a picture of it under the hood.

Thanks,

To get a good look at it easily, do this:
1-Open hood 90 degrees, thats all the way
2-Go to drivers side of vehicle with flashlight
3-Shine light between top of brake fluid reservior and that black throtle positoner unit above it.
4-Look down between the top of the reservior and the positioner at about 45 degrees and you will see it.

You can buy it on this site. It seems they call it a slightly different name than CPS. I would call and verify the part and part number Crank shaft position sensor

Thanks for all the help . Found the sensor and picking up a new one at the dealer tomorrow.

Ok - after having replaced the crankshaft position sensor a few weeks ago, there's no more sign of trouble. I'll probably report back in a few months.

One thing I wanted to mention for those of you wanting to try this...apparently there are two designs for the CPS for the 2001 model year (not sure about the other model years). A Mercedes part dealer is supposed to get the correct CPS by looking up your VIN, but this is not always correct; our friendly neighborhood discount OEM MB parts dealer said this is the 4th CPS that turned out to be the wrong one. The best thing to do before purchasing the CPS is to pull the plug out first and inspect it; you can search this forum for step-by-step instructions. The plug will either have a single catch on the straight part of the cylinder, or two catches, one on each curved end; make sure you get the correct corresponding CPS or you'll have to do the whole thing all over again (like me). Sorry, I didn't write the new part number down so I don't know which is which - maybe someone else who had this problem can post here and fill us in.

I have a 98 ML320 and have come to know the common problems with all the sensors.
1) check engine light on and the engine hesitates. (You step on the gas and it takes a second for the power to come on). Replace the Mass air sensor and reset it by disconnecting and re- connecting battery. Make sure you know your radio code to get your radio working again.
2) Check engine light on and car seems to run fine. Make sure gas cap is tighten properly. If not, fill it with gas and tighten cap. If light stays on after a few days replace the oldest one of the 2 oxygen sensors. Replace both if you have never replace any one of them before. The light will shut off without reset. The male and female connectors are tightly fit together with a rubber gasket. The wire harness sits inside a metal cave. Pull the whole thing out prior to disconnecting. You will need a fine (eyeglass) flat head screw driver to lift 2 plastic taps and a large flat head screw driver to gently pry loose the male from the female connector. My right O2 sensor was replaced under warranty at 17,160 miles in 8/02. I just replaced the left one today and I have only 33,000 miles now.