Bosch GRA53 36V Lawnmower - no motor

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

does the motor try and turn at all ?

Well, as they would say in The South... I guess I owe y'all an apology!

The motor DID spin freely when I first examined the mower a month ago when it stopped working but since then it has totally siezed and will not turn now unless heaved.

I suspect a bearing has gone bad, rusted, triggered the fault due to the speed not matching the current supplied and shut down. Over the past month the mower has sat unused, the bearing has dried out and has rusted solid.

Fortunately I do know how to get into the motor for a bearing replacement since I have had to do it once before. There is a domed cap concealing a circlip which thwarted me for a while last time.

So, consider it solved for now until I see how bad the damage is. If I can redo the bearings then all is well.

Thank you for the advice and suggestions of troubleshooting.

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

motor controllers can be picked up for under £10 from china - sold for building electric scooters

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

I have not checked the motor supply or behaviour this morning yet but I have managed to identify through Google what the mystery two square modules are for that I asked about in the first post.

According to the Parts Diagram it is "91 - Diode Module" so presumably I can check if they are working correctly somehow? My multimeter does have a Diode setting. Does it have to be done in circuit or out?

Oh, and the potted modules are simply listed as either 887 or 888 - Electronics Moldule and come with a big pile of bits for about £200. Of course finding anyone that has one in stock will be... interesting.

Parts List : https://www.mtmc.co.uk/Bosch-GRA-53-..._p-159836.aspx

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

I will check the mower tomorrow as it is late here. I appreciate the advice on not applying battery voltage direct to the motor.

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

3 wires will be brushless motor i would think .
not recommended to connect power to it . damage might happen . motors can be tested by spinning them and check the voltages at the phases .they have to be all the same .
does the motor try and turn at all ?
one thing you could try is warm the control box with hot air then see if it starts up .only thing doing that though is may have a thermal sensor inside it .

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

Thank you. I will remove the blade, disconnect the cable from the motor control box and see if 36V is present at the control side. The motor does spin freely and smoothly.

If there is power being offered to the motor then I will briefly apply 36V from the battery direct to the motor side of that connector and see if there are signs of life.

There are two of these potted control boards. The (upper) one deals with drive power and drive feedback as well as accepts the cable from the handebar that requests drive and speed. The (lower) other one only handles power to motor, tach feedback and the power bar from the handle. It also has the 50W 2.2 ohm power resistor across it. There is also a small wire jumping from one control board to the other.

For the main motor power cable, which of the black, yellow and green leads are positive and negative do you think? I assume it is a DC motor?

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

its usually the box with all the wires coming out of it .. potted so hard to try and repair it . first check is see if power goes to the motor upon clicking the start switch .. another thing is the motor free to turn ? it may feel lumpy but that's normal .
that's going off the ones i have worked on .none of them branded bosch but all pretty much the same parts .

[DanielCoffey]

Hi folks. I have a Bosch GRA53 36V lawnmower that has developed a fault and will not spin the blade.

I should say for the purposes of guiding what how you can explain things to me that that I have only modest knowledge of electronics, own a basic multimeter and soldering iron.

The mower is four years old, out of warranty, discontinued and the brand has been sold to Honda who have iterated on the design a couple of times and now sell their own 36V mower. My mower was effectively a prototype when Bosch tried the 36V Bosch Professional range of garden equipment which all used their proprietary 36V 4.5Ah, 6Ah and 9Ah batteries. It never sold well and was discontinued very shortly after I bought it along with two of the 9Ah batteries.

New the mower cost around £1600 and the 9Ah batteries were £400 each. The mower uses two of the 36V 9Ah batteries in parallel for 36V and 18Ah capacity.

Last year I began to notice a deterioration in the battery capacity. I discovered the range was discontinued but found it had been sold to Honda who were producing their own tools using the exact same battery format. I bought two more 9Ah batteries for £300 each and resumed using the mower.

About a month ago the mower cut out during use but resumed correctly. I finished the cutting, washed under the deck and put it away.

The next time I went to use it, the mower would not start.

The "child proof safety key" is in place. This interrupts the main battery positive.

The blade spins freely and does not feel unusual.

===

FAULT DESCRIPTION

When holding the "start" button on the handlebars and holding the bar against the handle no power is sent to the motor. There is just a high pitched whine for a couple of seconds, a pair of faint clicks one second apart then nothing. The fault is consistent with either both batteries or just one battery in either slot.

===

Having opened the top case of the mower I can see all wiring connectors appear normal. Nothing appears out of place or burned. I can understand what most of the cables are for and have identified a few of the components. While there are red indicator LEDs on the two boards, all components are buried under black potting compound (of course). There are some test or programming pins protruding from the compound.

I have been following how the mower has been designed and can see the pair of parallel battery positives go to each side of the child-proof switch. Each side then goes to a pair of square components which I do not understand. I have not removed them to see if there are any markings on the other side. If they are fuses then I am confused as to why there are two of them running in series.

Because there are a pair of wires running from each battery direct to each potted board, the boards sense the batteries and power up, showing a steady red LED as soon as the safeties of child lock, side button and handle bar are set.

So, first question, does anyone recognise the symptoms and what are the two square modules I have highlighted in the second image please?