Why does the ABS light come on?
 

For some time now, the ABS light in my 94 E420 has been coming on in an intermittent fashion. It usually occurs after I have been driving for 20 minutes or more, and does not seem to correspond with applying the brake pedal. I'll just look down, and it will be shining brightly.

I assume that one or both of the ABS sensors are sending erroneous signals, which triggers the light, but just want to make sure there are not any other causes for the light to come on before I drop a big chunk of change for new sensors.

Thanks in advance for any helpful suggestions.

I think the OVP can trigger the light if faulty as well. Check the fuse, or fuses, on the OVP too, might be blown (the fuse(s) in the OVP are the only blade type fuses I've seen in a Benz).

There is no OVP on the V8 engines, if I am not mistaken. Is there anything else which can trigger the ABS light, other than the sensors?

If I recall correctly, there is no OVP on this model.

Have the charging system checked. The ABS system requires high voltage values to operate properly, and a drop in voltage will cause the ABS light to come on.

It sounds like your alternator might need repair.

There actually is a fuse in the computer box, I assume your E420 uses the same as my 92 400E.

That fuse (F1) protects the ABS module and throttle control unit (cruise control), if that fuse is burnt, ABS won't work, you will have ABS warning light all the times, cruise control won't work and engine rev won't pass 5500 rpm, I had all these problems on my 400E, replacing that fuse cured all.

Since ABS warning comes on intermittently, I would guess FUSE is OK, cleaning the ABS sensors will be the first thing I would suggest, also it's a good idea to check the battery voltage at idle, it should increase after you start the car, from something arround 12.3 to 12.8 Volt.

Thanks for the tips. Unfortunately, the ABS is intermittent, although my cruise control can also fail intermittently, so it may be worth while to change out the fuse either way to make sure, particularly since the light can come on without any obvious provocation. I have already cleaned off the sensors, which were quite dirty, but this did not seem to help. Other than the fuse, is there anything else I should consider before swapping out the sensors? Thanks.

By the way, did you mean a fuse in the fuse box, or a separate fuse where the computer sits on the passenger side?

It's one of the fuses on base unit in the computer box as you mentioned. you need an Allen wrench to open the box, there are 4 10 Amp fuses (3 if no ASR option). F1 is the first one counting from left if you stand in front of the car.

Can't remember what other fuses do, you might also pull out them and check.

I've experienced this problem several times. Every time it was an indication of a charging system problem. Have it checked.

Well, I had the mechanic check the charging system, and it read a steady 14.4 volts. When he put a load on the system, the ABS still did not kick on. He is getting a new sensor reading tool today which does a thorough check of all of the electronic sensors, which may tell me what is going on. I'll keep everyone informed.

My independent MB mechanic just purchased another piece of diagnostic equipment which monitors all of the circuits in the car while rolling. Finally, he was able to isolate my ABS problem to the driver's side ABS sensor. He replaced it last night, and so far, no more ABS light. I am planning on bringing the car back in to have him run through all of the systems, since I still have a very intermittent, very strange idle problem which thus far has gone undiagnosed.

Despite my mechanic's best efforts thus far, the ABS light still comes back on, albeit on a less frequent than before basis. To date, this is what my mechanic has done:

1. Cleaned both front sensors;
2. He checked the charging system:
3. Replaced both front sensors, and cleaned the rear sensor.

His diagnostic equipment suggested that the front left sensor was bad, but it continues to come on with the new sensors. He believes that there must be a short in the system somewhere.

After he does a repair, I drive the car around for a few days to see what, if any, difference there is. As noted above, the ABS light does not come on as frequently as before. Also, when it comes on, it stays on until I stop and re-start the car, at which time it resets. It seems to come on at low speeds, such as city driving, and very rarely at highway speeds. Is there anyone out there who has experienced this type of ABS light behavior?

By the way, after charging me for one front sensor, my mechanic has basically told me he will continue to work on the car for free until this problem is solved. I have been working with him for well over a year, and know that he is not ripping me off.

As mentioned in an earlier post, there is no OVP in the E420. Also, I replaced all of the fuses, both in the fuse box and the ABS/Cruise brain, but it still comes on from time to time. Any other suggestions?

Base Module is OVP
 

in your car. Check my web page under menu #8 to see what it does. Also ABS Module, menu #10.

Use a scanner to read codes in those two modules.

Problem might be in the ABS computer itself. Bad solder joints ??

You and me both, i have the exact same problem with my 300E, although i've noticed that when i corner fast the light will come on...Now my mechanic has checked everything also and his last word is to get a whole new ABS Control unit which is fairly exepensive, so i'll leave you with that...

Good Luck :D

On advice from Benzmac, I'm going to try a new stop light switch. However, in speaking with my mechanic, he swapped one off of another car on his lot, and it did not seem to fix the problem, but I suppose that there is a possibility that they were both bad. Mine is only coming on every two or three days, usually when the engine is cold and only at slow speeds. I'll post again a few days after the switch and report on any progress.

My 600SEL (1992) DOes it too??
 

My 92 600SEL does teh same thing and am still to find the fault.

I also have an intermittent SRS light. Any ideas there??

Vinalf,

Do you also have an intermintent horn accompanying the SRS light? Possibly worn slip rings, have seen several post regarding intermitent SRS warning lights and the cure was slip rings. I had no horn on the 560SEL I'm fixing up and found, you guessed it, worn slip rings.

There's a TSB out on this. Service bulletin DDAF54_30-U-2107A April 27 1998.

Alternator lamp comes on but only very dimly. Check altenator output.

I guess a malfunctioning alternator can cause this to happen. It applies to all vehicles with ABS. Maybe the following link will work:

http://library.alldatapro.com/alldata/LIB~C31925~R0~OD~N/0/56775861/62706861/62706888/62706889/34853741/34850750/42063452/82241336

Kuan

This kind of the intermittent problem is extremely difficult to diagnostic.

But if the diagnostic equipment says ABS sensor is bad even with a new sensor, I will guess either the cables that connect the ABS sensor to the computer module are intermittent or the computer module itself works intermittently.

For my 92 400E, the front speed sensors are connected to the ABS control unit through two coaxial connectors, the cable colors from left front wheel are Grey/Green, the ones from right front wheel are White/Pink, the ABS control module can be easily pulled out of the computer box.

You can try to locate those connectors first, see if the connection is firm, then inspect the cables.

SRS Light
 

Hi Mike,

Thanks for the reply.

No the horn does not come on at all. I have noticed that fiddling with the seat belt attachment will cause teh lightto go on and off though.

Is there a sensor down in the mount there??


Regards
Vince Alfonso

Like apparently a lot of people here, I too have an intermittent ABS light in my 1994 E420. Can you guys tell me where to find and how to clean the sensors? I haven’t yet checked any fuses but no other system is having a problem so we’ll see.