Somewhat hypothetical: Prepping a W123 300D as a tow vehicle - PeachParts Mercedes-Benz Forum

KAdams4458 · #1 08-23-2013, 09:35 PM

I am considering using my '77 300D for some heavy hauling in the future. The trip would be round trip from Western Washington to Missouri, so there are plenty of mountains in the way. If I haul an enclosed trailer, the trailer and load will likely be around 2,500 - 2,750 lbs. Obviously, the trailer would have brakes, and the car will have a custom 2" receiver installed. (None of this bumper mounted stuff.)

My car does have a a turbo OM617 swapped in, as well as a four-speed manual, and 3.07 gears. I also have alarmed gauges to monitor engine oil temp, boost, and exhaust gas temp. For that matter, I have an intercooler that I could even install that might just come in handy climbing long grades, though I have no data on whether that would really be helpful at all.

I already know that some people will consider it insane. (And I might just be!) That is beside the point. Let's just discuss the feasibility of the idea. Ready? Go!

Mölyapina · #2 08-23-2013, 09:45 PM

This is something I'd like to do with my 300SD, too. I wonder if you could install SLS from a W123 wagon... I would have to use the W126 SLS, of course.

I believe that the 617 turbos were rated for 3000 lbs. of towing in Europe, so I would imagine you could tow 2750 all right... just don't try passing anyone

.

Mölyapina · #3 08-23-2013, 10:00 PM

Here's the post I was thinking of, and here is the thread that it started:

Quote:

Originally Posted by pdrayton

Thinking of putting a tow hitch on your Mercedes W123? Here's how I did it.

I've been looking for a tow hitch / tow bar setup for my 1984 300D for a while now. I got the hitch on yesterday and towed my almost 2000lb boat with it and nothing fell off, so that's a good start. It was a bit sluggish, but that is to be expected. I'm going to measure the tranny temperature after towing for a while and compare it to normal and see if there's any reason for concern.

I thought I'd share everything I learned so it's an easy road for someone who wants to add a tow bar to their baby.

Towing Specifications for Mercedes W123 300D 240D
First off, towing ratings / specifications for the mercedes W123 are 750KG (~1650lbs) unbraked, 1500KG (~3300lbs) braked, but only in Europe. Apparently if you use the same car in the USA you can't tow with it, according to Mercedes USA . There is some speculation as to why this is, but no-one really knows, at least not anyone I read from.

Tow Bar
After a lot of searching around, I found a towbar from Uhaul. It is made by Curt manufacturing curtmfg.com and is Uhaul part # 28033. This hitch is rated to tow up to 2000lbs and retails for $99.95 not including labor, so it's Class I -200lbs GVWR, 200lbs tongue weight. It attaches to the bumper and subframe. Takes 4 holes and about 30 minutes to put on, pretty easy actually. It's not a big Class II type hitch. I'll try get some photos up.

The wiring setup is easy, once you have it figured out! So here's it all figured out! First, you have to buy a 5 wire to 4 wire converter. It's a little plastic black box adapter available in a kit or by itself from Walmart or Advance Auto (probably other places, those are just the places I went to). Costs about $15 for the converter, or $23 for the kit, which you will need if you don't have any trailer wiring stuff. I had a connector left over that I used.

The wiring is actually very easy. The kit above comes with splicing stuff, so you don't have to cut any wires on your merc. All you have to do is take out the driver side plastic molding in the trunk - the interior plastic molding that covers the antenna electronics, etc. Once it is out, you can cut a stripe in the wiring that leads to the lights and access the wires that go to both sides of the rear lights. Then it is a simple process of matching the colors below to the colors of the merc wires. If you don't have an american merc, you might want to check the Haynes or Merc FSM for the correct colors, or use a multimeter to make sure you have the correct wires.

Towing wiring: Use 5 wire to 4 wire converter from walmart.
Wires: Black/White = Left
Black./Red = Stop light
Black/Green = Right
Gray/Black = Tail light

If there are any questions, or something I haven't covered, feel free to post.

Happy Towing

Here is a useful Benzworld thread where towing with a 300D is discussed. Govert presents a link to what looks like an official Mercedes-Benz site and a table there lists the towing capacity as 1500 kgs (3300 lbs). Someone else posts a page from the EPC where the capacity is listed as 1900 kgs (4250 lbs!).

Mölyapina · #4 08-23-2013, 10:07 PM

The table from Govert:

Information about the W123, 300 T-model diesel
General Overview
Engine
Chassis and power transmission
Transmission and Performance
Dimensions and Weights

Wheelbase 2795 mm
Track front / rear 1488/1453 mm
Length 4725 mm
Width 1786 mm
Height 1470 mm (with roof rails)
Minimum turning circle 11.29m
Empty weight (car weight) 1570 kg
Perm. Total weight 2190 kg
Perm. Front axle load 990 kg
Perm. Rear axle load 1200 kg
Payload 620 kg
Perm. Braked trailer load 1200 kg, 1980 to: 1500 kg
Perm. Trailer load without brakes 750 kg

The page from the EPC:

KAdams4458 · #5 08-24-2013, 01:08 AM

I thought about SLS to compensate for tongue weight, but then I realised I have the wrong head. So then I thought maybe rear air shocks, but those don't exist for a W123. Now I'm pondering if maybe a setup could be made using a set of wagon shocks with a manual adjustment system.

I didn't figure I would be passing anyone... Well, maybe if I come across a 240D towing a trailer, I could show the driver who the real boss is. LOL! That said, my stick converted car is surprisingly loaded with torque; Cruising at 30 MPH in fourth gear and then mashing the accelerator pedal reveals no hesitation or lugging, but rather a smooth pull up to speed. Maybe I just got lucky with the parts I combined to build this thing, but it feels like it could rip down large trees with very little effort.

Stretch · #6 08-24-2013, 01:33 AM

They sell stiffer "taxi" springs for the rear here - wanna link?

But I'd just do a service and check your tyre pressures - it sounds like you'll learn that you're not the slowest person on the road after all.

JB3 · #7 08-24-2013, 02:27 AM

Quote:

Originally Posted by KAdams4458

I am considering using my '77 300D for some heavy hauling in the future. The trip would be round trip from Western Washington to Missouri, so there are plenty of mountains in the way. If I haul an enclosed trailer, the trailer and load will likely be around 2,500 - 2,750 lbs. Obviously, the trailer would have brakes, and the car will have a custom 2" receiver installed. (None of this bumper mounted stuff.)

My car does have a a turbo OM617 swapped in, as well as a four-speed manual, and 3.07 gears. I also have alarmed gauges to monitor engine oil temp, boost, and exhaust gas temp. For that matter, I have an intercooler that I could even install that might just come in handy climbing long grades, though I have no data on whether that would really be helpful at all.

I already know that some people will consider it insane. (And I might just be!) That is beside the point. Let's just discuss the feasibility of the idea. Ready? Go!

I towed a vw rabbit from MA to MO once with a 1981 240 4-speed pulling the rabbit on a tow dolly. I think your engine will handle it fine, the concern id have is tongue weight on the unibody. Even with the light tongue weight tow dolly attached to a custom 2" receiver on my 240, it was dragging a$$ for 1500 miles, a normal enclosed trailer like you are talking about may have you scraping along.

cooljjay · #8 08-24-2013, 03:53 AM

Agreed on the scrapping a$$ comment, the early w123 naturally sit lower in the back as it stands....I would put stiffer springs in and have some spring shims handy to twist in the coils or find some way to bag the rear end......also it seems like you could find someone to convert an old pair of shocks to air shocks....

Mölyapina · #9 08-24-2013, 06:11 AM

The W124s had the SLS run off the PS pump. Could that be retrofitted into the W123?

Skippy · #10 08-24-2013, 06:20 AM

I have no doubt the engine and transmission can handle the extra load. I wouldn't even worry about an intercooler if you're fuel and boost levels are stock. I am a little concerned about the tongue weight. I suppose you might be able to procure a wagon steer trailer (fairly common in the military) with essentially no tongue weight. Word of warning: Trying to back up a wagon steer is an exercise in weirdness.

gatorblue92 · #11 08-24-2013, 06:53 AM

I have seen a W123T towing a boat in some of the old promotional videos that are floating around the web so it can be done. I dont know how I would feel about launching a boat with a W123 though.

DieselPaul · #12 08-24-2013, 09:27 AM

To the comment about not towing with USA versions, when my father worked for Mercedes Benz they said due to the way the motor is insulated for sound purposes, the cooling system had been optimised for those conditions. So additional loads like towing will easily overheat the motor.

I agree that sounds strange and you're welcome to disagree with it but that's what my dads salesmans literature said in 1992.

pawoSD · #13 08-24-2013, 09:48 AM

Towing 2,700lbs with a 300D + 617 is going to be slow going.....and probably dangerous. The engine is going to be at 4,500rpm in 2nd gear on hills to lug all that, and then its going to overheat while getting 7mpg.

GregMN · #14 08-24-2013, 10:15 AM

Drive by your temp. gauges. Crack open the sunroof and turn on the defrost for extra cooling capacity.

Do not take long inclines at WOT. Find a gear and rpm that allows pulling the grade at less then WOT. No matter how slow that is.

Remember that comfortable speed you used to go up the grade and don't go any faster down the grade.

That tail is gonna' want to wag the dog. Don't get behind on your steering.

I saw a 10% reduction in mileage when towing a SEC with a SDL from AZ to MN.

kerry · #15 08-24-2013, 10:55 AM

SLS struts with a manual hand pump would be an interesting swap. Don't know if the mounting points would allow it but you could avoid the leveling and swap in the original shocks when you're done. HF sells a hand hydraulic pump.