We have an error 80 conveyor overload. Overload detection (jamming, belt overload). Spring on the overload mechanism tensioned insufficiently.

Quite a few things can cause this, check the pulley for any nicks or rough edges, check the pulley alignment with each other, make sure of the correct belt tension. power steering pump binding causing overload. wrong belt for pulley, A section belt V section pulley , V section belt A section pulley.

If the belt is coming off it is misaligned or the tensioner pulley unit has failed and is providing insufficient spring tension to retain the serpentine belt. The pulley arm itself may also be misaligned. Over time these tensioners weaken and need to be replaced.

If the belt tensioner has not been replaced for many years I would have it replaced and then see if you still have the belt throwing off.

Belt tensioner failure signs

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its gears are bad.
or is jammed and the motor clutch slips.
the track must run free. or the motor will overload.
its just a simple motor and simple mechanics.
i remove motor and move parts by hand to see if jammed.
if not jammed its motor new time.

Not exactly. The Mustang of that series had leaf springs and "overload" springs were usually a set with an extra leaf or two. Overload shocks were shock absorbers with an additional coil spring.
I don't think you could special order overload springs since the car was not designed to haul any extra weight like a truck or station wagon.

Timing belt installation

1. Ensure locking bolts located correctly [4] & [5] in the picture below:

CS = Crankshaft
CA = Camshaft
IP = Injection Pump

2. Ensure timing marks aligned [10].
3. Fit timing belt in clockwise direction, starting at crankshaft sprocket.

NOTE: "ISUZU" lettering on belt should be readable from front of engine.

4. Slacken tensioner pulley bolt [9]. Push tensioner pulley against belt.
5. Ensure belt is taut between injection pump sprocket and tensioner pulley as well as crankshaft sprocket and camshaft sprocket.
6. Position tensioner lever [8] against tensioner pulley housing.
7. Remove locking bolts from sprockets [4] & [5].
8. Apply a load of 10 ... 12 kg to tensioner lever. Use spring balance [11].
9. Tighten tensioner pulley bolt to 80 Nm.
10. Turn crankshaft slowly 45 degrees anti-clockwise. Use spring balance to tension belt.
11. Repeat tensioning procedure at 45 degrees intervals for one anti-clockwise turn of the crankshaft.

NOTE: DO NOT turn crankshaft clockwise during tensioning procedure!

12. Fit tensioner lever to original position. Tighten nuts [12].
13. Install: Injection Pump flange [6], Camshaft sprocket flange [7], and water pump pulley.
14. Install components in reverse order of removal & refill coolant.

Hope that helps. and good luck out there in the country side.

Tensioners should be replaced about every 80 thousand km when u let of the gas it relieves tension on belt tensioner if the spring inside is wearing out chances are there's ur squealing sound would sound like a belt squeal not a bearing

Removal & Installation

1. Rotate the drive belt tensioner clockwise and remove the drive belt.
   WARNING Never suddenly let go of the tensioned idler pulley. The force of the spring pressure suddenly released may damage the idler pulley mechanism. Always release the spring pressure gradually.
   
   
   

   Fig. Rotate the belt tensioner to relieve tension on the belt-4.6L engine shown, others similar
   

To install:

1. Route the belt over the pulleys making sure all the grooves in the pulleys and the belt line up correctly. Refer to the accessory belt routing section of this manual.
2. Rotate belt tensioner clockwise and slip drive belt over idler pulley on tensioner.

1. Rotate the drive belt tensioner clockwise and remove the drive belt.
   WARNING Never suddenly let go of the tensioned idler pulley. The force of the spring pressure suddenly released may damage the idler pulley mechanism. Always release the spring pressure gradually.
   
   
   

   Fig. Rotate the belt tensioner to relieve tension on the belt-4.6L engine shown, others similar
   

To install:

1. Route the belt over the pulleys making sure all the grooves in the pulleys and the belt line up correctly. Refer to the accessory belt routing diagrams above.
2. Rotate belt tensioner clockwise and slip drive belt over idler pulley on tensioner.

If for any reason, the tensioner is crooked, replace it!!! Also, install the belt with the writing facing you like you are trying to read a book. This WILL (a trick I learned at Mercedes) prevent even a new belt from squeeling!!

Not familiar with your Merc. This is general info. My porsche has a mecanical belt tensioner down low under the engine off the ground. Toyota has a spring charged piston pushing a belt tensioner wheel. Most other cars I've worked on had a pivoting alternator. If your car has a flat serpentive belt then it probaly has a spring loaded tensioner. If it has Vee belts then something sets the tension mechanically at install. Good luck!

The job of maintaining belt tension falls on the Belt Tensioner. It's time to replace it. They're pretty reasonable in the $25-$35 range.

Removal & Installation

1. Rotate the drive belt tensioner clockwise and remove the drive belt.
   
   WARNING Never suddenly let go of the tensioned idler pulley. The force of the spring pressure suddenly released may damage the idler pulley mechanism. Always release the spring pressure gradually.
   

Rotate the belt tensioner to relieve tension on the belt—4.6L engine shown, others similar

To install:

1. Route the belt over the pulleys making sure all the grooves in the pulleys and the belt line up correctly. Refer to the accessory belt routing section of this manual.
   
2. Rotate belt tensioner clockwise and slip drive belt over idler pulley on tensioner.