1987 300D A Couple of Odd Issues
 

First of all, occasionally the car will die if I suddenly let off the accelerator.

In a parking lot or coming to a red light in town when I let off the accelerator the tach will drop all the way to zero occasionally actually dying.

This usually happens when the engine is pretty warm, like over 100C.

Second, when I first start the car I have no warning lights on however the first time I use the brakes I get a pulsation like ABS is coming on and then the Antilock light comes on and stays on until I kill and restart the car.

The pulsation only happens the first time I apply the brakes.

Any ideas?

Check your alternator.

Hmm common thread here is the OVP relay. The ABS and EDS (idle speed control) computers take conditioned power from the OVP relay. The car will run with the EDS failing but will not idle correctly.

If that is good then you'll need to go into the diagnostics for ABS and EDS which are in the factory service manual. Go to w124-zone.com or startekinfo.com and look for MB workshop resources, page through to about the 3rd page for the 124 chassis service information.

Something caught my eye. You shouldn't be hitting 100c on a normal basis. 80 to 90 is the normal range in my experience. I would be looking at the fan clutch to make sure the fan is working. When it is hot, have someone kill the engine while you watch the fan (or hit the stop lever on the IP). The fan should stop instantly and not coast down. If the fan coasts down the fan clutch is shot.

I would also remove the aux fan from the front of the condenser and check for leaves and crud buildup. Shine a light through the radiator/condenser combo and see if light passes.

Also make sure you have proper coolant - G05 from NAPA (or genuine MB if you have the $$) and distilled water. If your coolant is green do a flush and fill post haste.

My fan clutch was bad but I replaced it this weekend.

Then I blew a fuse for the aux fan.

I just finished replacing the fan and fuse but I haven't got it to come on yet.

It did get over 100C however and it has consistently since I bought it a month ago.

My idle seems fine at about 6-700 rpm.

My only issue with it dying is when I rev it up to get around a parking lot and suddenly let off.

I also noticed this situation when I would occasionally pop it into neutral while coasting down a steep hill.

In that instance it will drop all the way to zero or near it and then come back to a normal idle speed.

My coolant is definitely green.

Also, my oil warning light has started coming on the last week or so.

I know from when I owned a 1993 300D that this is probably a bad switch.

However, with all this happening at the same time could it be the alternator?

I'll do a search for the OVP relay to see what it's all about.

Blown Head Gasket?

Start with fresh fuel filters, get the alternator tested at a mcparts store, and report back

I hope not.

There isn't any blowby and the coolant isn't boiling over.

I haven't driven it on a long trip since installing the new fan clutch but when I drove it over long periods about 90 mph the heat would get up there close to 120C when running the A/C.

Outside temps were over 100F at the time.

Since installing the new fan clutch it will get pretty hot when idling.

I tested the fan by jumping the contacts on the sensor on the top radiator hose and it spins like crazy when I do that.

I'm still not sure what the hell is keeping the electric fan from coming on but I really need to get it sorted out.

I know that in the recent past it was working but since I blew the fuse I haven't witnessed it coming on.

Something's wrong and I think it could be the alternator/electrical.

There are too many warning lights popping up for no apparent reason.

It's probably not your alternator. You need to replace the OVP relay. There's really no way to test it, and you have a couple of typical symptoms.

Car is running too hot, and it's not the color of the coolant that's the problem. Is your new fan clutch actually working? I've had bad clutches right out of the box.

Ditto the advice on the OVP relay...those can lead to a ton of nasty symptoms when they get twitchy. Also have you checked your thermostat? Could be sticky/not opening all the way. Sometimes the simple things cause the biggest headaches.

The ABS pulsation and then light coming on means there is a problem in the system and it has shut itself off after the initial test. Most likely one of your abs sensors have gone bad. If you are lucky, cleaning the wheel hubs may fix this (magnetic shavings build up in the teeth that the ABS read), but in my experience the wiring on one of the front sensors is shot and causing the system to turn off. You can always unplug one of the sensors if you don't mind living without ABS..that will still cause the light to come on but will eliminate any of the shudder when you first brake.

Get an OE OVP. The Kaehls are not all that great, the last one I bought was flakey. Ended up with a #3 jy replacement which works like a champ. ABS traces back to OVP as well.

You should not be hitting 100C. I rarely get over 80, in 100F I will hit 86 which is where the tstat opens both ways IIRC.

That is exactly what my OM603 87 300TD and my OM606 98 E300DT do.

Gauge hovers around 80C in both cars but will peak at around 85-90C in severe circumstances - like a hard highway run then pull off and sit in traffic on a 90+F day. As soon as I get some motion the temp drops right back to 80C. Obviously this is with the AC on.

I second the notion of stuck t-stat based on OPs latest description.

Electrical - try the OVP, a junkyard OE OVP is better than new aftermarket, just make sure it's from a Diesel.

Out there question - you do have the belt threaded on the pulleys correctly, right? If you have the pulley turning the WP threaded backwards your cooling will be for naught.

I will check the how the belt is threaded.

I have noticed that it seems a bit loose but I didn't verify it was properly installed.

What/where is the OVP relay?

What does OVP relay stand for?

A bit of an update:

My new fan clutch is definitely working. I checked it manually and I can actually hear the fan noise when driving.

I tested the alternator and it is consistently at 13.92-13.96 at all rpm ranges.

I had a brand new Continental battery in my 1984 190D that has a oil burning problem so I swapped that battery into my 300D.

In the process of moving the cables out of the way I ripped out the main cable to a junction.

I wasn't forceful or anything so I suspect the connector was already weakened or cracked.

I put a new connector on it and reinstalled the junction.

I'm now going to take a drive to NAPA and get the Zerex G-05 coolant and I will flush the system with distilled water and then fill with a 50/50 ration of G-05 and distilled water.

I also noticed that my front shock mounts are pretty cracked so I'll order those soon and maybe get an OVP relay and thermostat when I do.

Thanks for all the help.

OVP=OverVoltage Protection. It's a surge protector. Behind the battery there is a plastic cover... the OVP is behind that, along with the EDS electronics. I don't remember how the cover comes off on a 124, on a 201 you have to pull up the weatherstripping.

The relay is a complicated thing, but what it does is shunt high voltage surges to ground. It has a fuse on top, but even if the fuse is good, it doesn't mean the OVP is good. The working component is an avalanche diode, I think the breakdown voltage is around 20V. The diode can be totally compromised without burning the fuse or relay. Because of this, there's really no effective test. So it's not a bad idea to replace it every once in a while, and certainly when you have symptoms.

The reason this function is needed is that when a coil is electrically powered, it retains a great deal of energy. When power to the coil is cut, that stored energy is dissipated in the form of an inductive surge. These surges can reach several hundred volts, depending on the size of the coil. If you're wondering where you have coils in your car, they are all over the place: every electric motor (windows, seats, wipers, washers, sunroof, starter), every relay, every solenoid, even the alternator itself have powered coils. Every one of these devices can produce a destructive power surge under the right circumstances.

In the days of very simple cars, a subsecond voltage spike could safely be snubbed by just the battery. But modern (even 1987) electronics is very sensitive. The OVP only protects a few key circuits, specifically the EDS which controls idle and feeds a speed signal to the Klima and the tach. It also protects the ABS system. So if your symptoms include tachometer, idle and ABS weirdness, then the place to start is with the OVP.