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Purchased a 1750L - Questions/Driving/Parts

1282 Views 31 Replies 10 Participants Last post by  TRUKTOR
Hello Unimog BenzWorld!

Long time lurker and very recent Unimog 1750L owner. I recently made the plunge and purchased a 1988 U1750L. The truck is in amazing shape but of course there are a few things that could be fixed on any 34-year-old truck.

Questions:
Driving. I just got tags on the truck and can now drive legally. Taking it for a few spins around some low traffic roads I have noticed that I can't downshift from 8th -> 7th-> 6th etc. No issues shifting up through the gears (and very little griding and it gets better as I learn to drive it) but down shifting has been an issue. When I make a turn, I end up at a stop waiting to get back into 4th before I can move forward again. Is there something I'm missing? How do you downshift in a Unimog? It feels like it's me and not the truck.

Parts:
Parking brake lever. Does this bleed air when in park? I have noticed that air is leaking out of the (overflow?) port on the bottom of the parking brake. I can put my finger over it, and it will stop the air escaping. I figured it might need a rebuild kit, but it could also be a bleed down for when the truck is parked. I could not find anything in the manual about it.

Breakers. I have a breaker popping in reverse only. It's the dash light (IE parking brake indicator/4x4 indicator) breaker and I see the lights dim on the dash as soon as I put it in reverse. I can reset the breaker easily, but I know there is a short somewhere. I did not see any obvious wires on the bottom of the center console reverse lever, so I assume it hooked up to the gear box in some other way. Does anyone know an obvious reverse switch somewhere that might be bad?

Tachometer. I have noticed that the Tachometer stops working at speed sometimes. It starts working again usually when I stop but its happened a few times. Is there a common issue or do I need a new cable?

Speedometer. The speedometer seems to be accurate, but it bounces like crazy. It also seems to be making a clicking sound. This truck lived as a snowplow for many many years and likely did not get above 20mph. It might just need a new cable but maybe this is normal?


Lots of other questions but I don't want to overload this initial post. I can add some photos soon. Overall super excited about this journey. I am in NC and can also help answer any questions people might have about insurance and registration here in NC.

Thanks in advance!
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Hello Unimog BenzWorld!

Long time lurker and very recent Unimog 1750L owner. I recently made the plunge and purchased a 1988 U1750L. The truck is in amazing shape but of course there are a few things that could be fixed on any 34-year-old truck.

Questions:
Driving. I just got tags on the truck and can now drive legally. Taking it for a few spins around some low traffic roads I have noticed that I can't downshift from 8th -> 7th-> 6th etc. No issues shifting up through the gears (and very...
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· Unimog Moderator
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Welcome!

I'll work from the bottom up with whatever I can help with.

Speedometer, follow the speedometer cable down to the transmission, look for a "box", that might have what looks like two outputs on it, one might have a rubber plug on top, those need greased (like you have to physically open up the gear reduction box and put grease in it) sometimes and can be missing teeth, you may also need to lube the speedometer cable. They reduction box might not be there, if not don't worry not all trucks had them, just a place to start looking.

Tachometer. Not sure here, start chasing grounds I would say. Is it the kind that opens up (Tachograph) or just the standard tachometer?

Reverse switch is on the transmission, but check the wiring at the back of the truck and work your way up (after a quick visual of the wiring at the switch on the transmission). There might be a weatherproof box at the back up on the inside of the frame rail that has a bunch of wiring in it, worth a check.

Parking brake lever, no, shouldn't leak, nothing should really leak when just sitting other than the pop off valve once in a while popping off to maintain pressure when running. Rebuilt is likely in its's future.

Downshifting should be just as smooth as upshifting. When was the transmission fluid last changed? There are instructions in the service manual on adjusting the linkage for shifting, this can sometimes get out of whack if the cab has been tilted recently. Does the truck have the optional clutch pedal booster? Is it a double clutch or single clutch?

Also be sure to post a pic of the new beast!
 

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Discussion Starter · #3 ·
Welcome!

I'll work from the bottom up with whatever I can help with.

Speedometer, follow the speedometer cable down to the transmission, look for a "box", that might have what looks like two outputs on it, one might have a rubber plug on top, those need greased (like you have to physically open up the gear reduction box and put grease in it) sometimes and can be missing teeth, you may also need to lube the speedometer cable.

Tachometer. Not sure here, start chasing grounds I would say. Is it the kind that opens up (Tachograph) or just the standard tachometer?

Reverse switch is on the transmission, but check the wiring at the back of the truck and work your way up. There might be a weatherproof box at the back that has a bunch of wiring in it, worth a check.

Parking brake lever, no, shouldn't leak, nothing should really leak when just sitting other than the pop off valve once in a while popping off to maintain pressure. Rebuilt is likely in it's future.

Downshifting should be just as smooth as upshifting. When was the transmission fluid last changed? There are instructions in the service manual on adjusting the linkage for shifting, this can sometimes get out of whack if the cab has been tilted recently.

Thanks Trev! I will check all these things you listed!

The cab has been up recently before I got it (another story I need to write up) and there could defiantly be issues there. It is smooth shifting both directions parked. The gates seem to work fine too. At speed is the only issue. I will check for grounds and lubing of the speedometer hookup. I will take some photos too.

Thanks for such a quick and knowledgeable response. I am excited to be here!

Thanks!
 

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U1450L DOKA
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Wait, does nobody get a woosh of air when they engage their parking brake ? Both on mine did/do it, and the air is exiting below the central shift plate, under the cab, not in it.
Just one woosh, not continuous leaking.
 

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U1450L DOKA
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Yes, normal.
I'm just wondering if that is what Jerrion is describing with his parking brake - a whoosh, or does he have a constant leak ? One is normal, one needs attention.
 

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Discussion Starter · #10 ·
I am in the US (North Carolina). Does anyone know the best place to get a rebuild kit? I am waiting on call back from expedition imports but I thought about calling the dealership to see if they can order parts.

Thanks!
Jerrion
 

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U1300L
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If you are going to try to order through a North American Mercedes dealer you will need to have exact part numbers obtained either from an online or old school paper parts book. Even then it is a bit hit and miss in my experience as most dealerships are not familiar with ordering these parts. - most of these trucks were never sold by Mercedes in North America.
 

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1988 U1300L RW1 Working gears Dual Tanks AC Rigged for Camping Plus: 91 F250 HD 4x4
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678 Posts
It's a constant leak. Not huge but noticable when the unimog is tuned off in park.

It has the normal woosh too when engaging or disengaging.

Thanks!
When turned off, and parking brake "On", there is no air pressure in the spring brake cylinders. The springs, a simple foolproof mechanical device that cannot leak down, do all the work of holding the brakes on.

Coupled with the "whoosh" when applying/ releasing the park brakes, this suggests that there is an air leak at some other point than the spring brake cylinders themselves. If they were leaking, it would be while in "driving" mode, and one probably could not hear the hiss. Also, the truck compressor would have a more frequent duty cycle (which takes experience with a "no problem truck" to be able to evaluate).

Time for a shop compressor, the spray bottle of soapy water, and a likely a lot of tedious searching. Unless, of course, you start at the last place you look, which is where one finds all lost items.

EDIT: re-read first post, and if there is a hiss at the valve, then the valve is likely gone awry. It would seem to me that that would bleed off all the time, unless the air supply line/ pressure is being bled off only when no air is supplied to the spring brake cylinders. Either way, first thing to find out is whether this a brake on and/ or a brake off problem.



Lee
 

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1986 U1300L; 1976 U900 Cabriolet
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Your tacho runs of the W wire on your alternator. Check that the alternator belt is tight and not slipping. A dying tacho that jumps into life typically means slipping belt.
Check the W wire from the back of the alternator to the block on the firewall. Mine broke somewhere inside the wiring loom and had to be replaced.
 

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Discussion Starter · #15 ·
Everyone is dropping knowledge faster than I can check! It is going to save me a massive amount of time and make the repairs better. Thanks so much for all this info. I will get some pictures of the issues and report back. It's raining today so I will take a look tomorrow!

I do have the shop manuals for the 437 series so if anyone needs a copy let me know. I need to scan them still, but they are OEM manuals (2 binders) and seem to be complete.
 

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Discussion Starter · #16 ·
In the meantime, while its raining. How do you maintain the "anti-freeze" on the air system - this thing:

Motor vehicle Automotive tire Automotive exterior Automotive wheel system Gas


The previous owner replaced it but did not add any Isopropyl Alcohol in it.

Bonus question: The manual mentions an air dryer, but I don't see one on this configuration. Would it be better to install a standard air dryer for a big truck or get one from MB?
 

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Very nice truck. Looks like a fun machine.

You will find that the air system in the Unimog Square cab is vital to maintain for all its various functions. Depending on the system it will likely be a High Pressure, Low Volume system compared to North American equipment that runs a high volume, low pressure, so parts will not be safely compatible. I've owned that style (uses a wick to pull alcohol up into the path of the air) and the dryer, the dryer is available heated, and just standard cartridge. Obviously the Heated is the best (but can't be mixed with alcohol for...obvious reasons), and the cartridge system works really well. The Alcohol system works just fine, if it's maintained like anything else.

Your first purchase if you don't have one should be the English owners manual (here is a snip from the German manual, all I have handy right now), and a sort of passable Google Translate of the Alcohol system:

1 filler hole and dipstick
2 adjustment handle (summer setting)
3 adjustment handle (winter position)

Before the onset of the cold season and frost,
check protector, clean and Top up with ethyl alcohol.
When the outside temperature is below 5 degrees C
switch over to winter operation.
Control and filling only at Sommerstel
carry out lungs.
Check fluid level daily if necessary.
refill. Antifreeze if possible
clean, because of increased
condensed water the antifreeze effect
get lost.

Font Parallel Paper Advertising Machine
 

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U1450L DOKA
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Here is the location of the switch for the reverse light on the transmission case.

I agree that the short is probably to the rear, in the wiring to the lamp. The wiring is usually routed in the tube of the bracket that supports the tail light and reverse light. The insulation can rub through inside the tube causing a short.

I can't tell from your photos, but most square cabs have a version of these tail light brackets.



Automotive tire Motor vehicle Hood Vehicle Automotive fuel system
Automotive exterior Wood Bumper Auto part Composite material
Auto part Automotive exterior Metal Composite material Bumper
Automotive exterior Bumper Auto part Metal Tool

Gas Composite material Auto part Metal Nickel

Plant Automotive tire Motor vehicle Vehicle Gas
Wood Auto part Metal Composite material Fixture
Tool Bumper Pliers Saw Slip joint pliers
 

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1988 U1300L RW1 Working gears Dual Tanks AC Rigged for Camping Plus: 91 F250 HD 4x4
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Going from memory, the switch on the transmission housing completes the circuit path to ground (coming FROM the lamp), and does not send power out to the reverse lamp. The reverse lamp always has power to it (with key on ??), just not the path to ground until and unless one shifts into reverse. This would indicate that if the breaker pops only after shifting into reverse, then there is no short to ground on the preceding wiring. I am speculating that there is a bypass of the load (lamp filament) in the reverse lamp, and once grounded, an excess current is sent to ground.

Lee

EDIT: and FWIW; even with (maybe because of) the factory WM wiring diagrams (it's the Teutonic Logic, man), it can be harder than hell to follow the current paths. This stuff has bedeviled me more than once, but just try to do the diagnostics without the manuals/ diagrams, and you will go bonkers for sure.

EDIT #2: might try a clamp-on ammeter and hit reverse. Also could test the above theory by putting a known load (lamp) into the circuit in lieu of the connection to the reverse lamp, then shifting to reverse. If breaker holds, then it is the lamp.
 

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1988 U1300L RW1 Working gears Dual Tanks AC Rigged for Camping Plus: 91 F250 HD 4x4
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There is another possibility here. IF another circuit/ current path from the same breaker source to a different load (like the 4x4 indicator light) is shorted to the reverse light circuit BEFORE THAT OTHER LOAD, AND CONNECTS IN DOWNSTREAM OF THE REVERSE LAMP ITSELF, then there would be a parallel bypass of the reverse lamp, but only completed when shifting into reverse. This would potentially be an injection of full voltage/ no load, direct to ground at the time of shifting. This is a bit esoteric, in my mind, but plausible. I think the best way to check this would be to go to the wiring diagrams, and pull connections to eliminate sending voltage to other devices, and check the effects by shifting into reverse at each disconnect.

Another curiosity is the lights dimming, but report is the breaker trips. In a straight short to ground, one would expect to see the brake pop immediately, perhaps with only a momentary flash of lights. If that is accurate, that suggests a time delay as something is heating up, changing resistance, and when resistance falls far enough, current increases sufficiently to overload the breaker. Ammeter could tell the tale here. This scenario makes the lamp itself the more likely culprit. Any chance someone installed the wrong bulb (like a 12 v in a 24 v truck) ?

Lee
 
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