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Mitsubishi FCB 25 electric forklift truck, not starting repair
During summer time of 2015 I have been working as an electronics technician, on a base of a limited time contract, for our Public Power Corporation. My working place was located in West Macedonia, the northern part of Greece, which is also the place I was born and grown up. The reason for the delay to publish this article is that I had kept the relevant photos in a memory stick which I could not remember where I had left it, until I recently came across it while searching in my things for something else…!!
Anyway, my tasks up there were general electronics’ repairs. The most common incoming equipment for repairs (apart from the countless computer SMPSs) was welding machines of various rated power. I had obtained a very useful experience on them, about which I had no previous experience at all.
The range of the equipment I had repaired there at that time was ranging from very small portable inverter type welding machines, units rated 150 to 200A max, up to gigantic 600A, thyristor operated three phase monsters, which were capable of melting welding electrodes of 10mm thickness within seconds… I still remember that during their operation the melted electrode iron was running like tap water forming a small pool on the welding spot to be filled with metal, without any spilling of it here and there due to the D.C (unidirectional) welding current. This was always an amazing spectacle for me, which I enjoyed every time after finishing a new welding machine repair, watching its operator working with it during testing its performance, before accepting the repair and putting it in normal operation.
One day, among other repairs I had to do, I was asked to take a look on an electric forklift truck which had some problems and perhaps help the case if I could. My curiosity about that case was flown sky-high! I had never seen such a circuit before. Anyway the supervising foreman of the department I was working arranged our visit at the place where this truck was used for the next day in the morning.
You can see the forklift truck below:
The next morning a car driver appeared at our department to pick us up. After a short (ten minutes) travel we were at their premises. Another foreman, who was expecting us, came to welcome us and gave us some explanations about what the problem with this forklift was.
Well, the forklift, before its last condition, had starting difficulties. Turning the key to the ON position it should hold a main relay energized. But this was happening once out of twenty to thirty attempts or perhaps more than that. In the past a technician was called from an authorized service based in Athens. He came here and tried to fix it without success. He informed the administration that the truck had to be sent in Athens for further inspection and repair. The administration had no other option but to load the forklift on a transporting truck and send it to Athens. After about twenty days, as I was informed, the forklift was sent back after being repaired but it worked only for a few days and then stopped completely. Nothing was happening with the key ON any longer. This was its condition when I first saw it.
The operator of the forklift was more specific. He told me that its speedometer was long ago stuck, showing a permanent speed of 60Km per hour even when the truck was standing there and also a triangular-shaped red light annunciating danger was permanently flashing, indicating a trouble to be solved. When they sought relevant info about that signaling from the authorized service in the past, they received the answer: “don’t bother about that. This is a non important function anyway”… and he finished saying: “during the last days, before it stopped completely, the forklift was behaving weirdly”. But he didn’t give me any further info…
So I began troubleshooting the machine, starting from the easy and obvious things first. I checked the battery array voltage first. It had an array of 24 huge lead acid 2V/750Ah flooded type cells connected in series, making a battery of 48V nominal voltage. Indeed all cells were fully charged and in good shape. I measured around 52V across the array. So I had to move now to the controlling main board which was located at the back side of the forklift, enclosed in a watertight cabinet.
First of all I wanted to locate the fuses of the system and check them afterwards.
Below you can see the huge 675A main fuse which I found burned. I had never seen a fuse like this one before. If one is not familiar with it, it is not obvious at all that this is a fuse…!!
Furthermore I removed the power transistors’ drivers and the two huge electrolytic capacitors rated 20000μF/100V in order to test them in the laboratory. Locally I tested the four power output transistors (modules or “bricks”) included therein, which were Darlington connected as you can see in the relevant drawing, stuck at the back door of the cabinet of the electronics, shown further below. Fortunately these parts had no problem at all. They were manufactured from the Japanese company Fuji. I could not find exact data of them, as they seem to be custom made, but I assume that the fuse rating justifies their collector current capabilities…
Here below you can see one the (two) driver modules, after I removed them and washed them thoroughly with soft soap and a lot of water using a tooth brush. They were almost totally black-colored because of the coal dust coating all over their surface before I cleaned them. No parts could be seen…
In the below photo the drivers are the two vertical PCBs and the capacitors are located at the right side of the circuit. The copper bars you see feed the transistors’ collectors. You can now imagine the magnitude of the currents flowing through them…
In the below photo you can see a part of the basic circuitry for elementary troubleshooting:
As there was no spare fuse available I asked the local foreman to help me out by properly modifying an ordinary “knife-contacts” type 50A fuse used in industrial electrical installations, attaching to it two copper bar retainers according to my instructions.
Given that the main fuse of such a high rate was found burned, I just wanted to identify the magnitude of the possible disaster or, on the contrary, to make sure that this burn of the fuse was due to its aging or to a momentary overload of the truck and thus the main motor and the power transistors were intact.
On the other hand, in case of problematic function, with the test fuse being now of much smaller rating, it would blow immediately revealing the real status of the circuit. In fact this is what I was expecting to see happening…
This was the testing strategy I had in mind at this stage of the procedure. The modified fuse I put there for the test is shown below:
And below you can see it installed in the place of the original, ready for the test.
Right after installing it, I asked the operator to try to start the forklift and if we had a normal power up to perform all the basic functional tests on it. I instructed him to be extremely careful with the “gas” pedal for the motor, taking care to press it very gently and only as much as required in order for me to see if the motor can move the truck normally in the forward and reverse directions without any instant blow of the test fuse.
The operator understood my request and started the procedure. With the key switch repeatedly operated, the main relay, which is the one you see above in the left side with the test fuse on it, was always energized but without being locked in that position. Anyway, the operator told me “don’t bother, I am used to this behavior. Let’s see the rest of the tests when it locks”… Then he tried to move the fork assembly up and down which was successful and then he continued with the very-very slow front and back direction movements of the vehicle. These functions were also quite normal. The truck responded, slowly of course, but exactly as I wanted it. It was working perfectly. No fuse blow whatsoever. This was good news for the motor, the power switchers and the main board controller as well. After that I informed the local foreman to order a new fuse for restoring the normal function of the truck and left the place.
Nevertheless this burned fuse was bothering me. It was still an open question in my mind. There should have been a good reason for blowing a fuse of such a huge rating…
Anyway the new fuse arrived next day. I revisited their premises again, installed it, checked that everything was OK and left the place again.
Two days later I had a new call. The forklift was energizing the main relay, the operator could hear the characteristic “click” of it, but locking of the relay in its active position was not possible. It was permanently refusing to lock the relay…
This was a confirmation for me that the feeling I had in the first place about the nature of this weird failure was standing on a real base…
I revisited the place again. My plan was now to check any likely protection circuit activation. Fortunately the forklift had a general schematic of its electrical system printed on a plastic foil which was stuck on the back side of the cabinet closing door.
A part of it is shown below:
By the way this photo above shows the electromagnetic speed sensor of the forklift. I took it when I was proving that the relevant connections were in good condition.
Finally I did a verification of all inputs-outputs from and to the main board. I spent the day there and the entire wiring proved to be in perfect condition. There was only one exception…
Under the operator’s seat there was a safety switch. Trying to verify the condition of the wiring and the incoming signal of that switch informing the CPU that “the driver is seated”, I found that the switch was not sending the “contact closed” signal when the driver was seated. What a safety device I thought! No driver seated, no starting of the forklift!
So I removed the seat in order to remove and inspect both the switch and its wiring.
I dismantled the switch assembly, removed the switch and cleaned its contacts with a contact lubricating and cleaning spray. It was really very dirty. Then I tested it, reassembled it on the seat base and put the seat back in its place. You can see the switch below:
Thereafter I asked the operator to test the forklift again. With the exception of the usual behavior it had lately, the main relay locked after some attempts and the forklift was working normally. After the verification that everything was in order, I left the place.
Next day, at around 10.00’ o’clock I had a new call. The forklift started normally and after around two hours it stopped at the middle of the warehouse and refused to start again no matter the countless attempts on the part of the operator.
Well, this time my eyes started glazing with this weird behavior…Anyway I was convinced now that the seat switch was not directly related with any protection inhibiting the starting of the forklift and most likely I was confronted with a “non typical” failure once again.
I revisited the place again. All I took with me this time was a freezing spray only. I found the forklift in its usual parking place. I asked for the operator to come and assist me. The man came in place and I asked him to sit there and wait for my instructions. I removed the top cover of the watertight enclosure of the main board and I had the entire main board exposed.
You can see it below:
At a first sight of it I didn’t see anything wrong in it.
So I took the freezer spray, attached the plastic extension straw on its valve head and directed it first at the power semiconductors’ side. I sprayed the area and after a while I asked the operator to check the “key ON” function. There was no positive result.
It is important to note here that the entire PCB was covered with a very thick layer of plastic coating in order to protect the circuit against corrosion and also vibration problems.
I continued spraying sequentially all over the place without result, until I sprayed the microprocessor itself. After the first spraying on it, the main relay locked at once. I waited for some minutes and asked the operator to restart it again, this time without any intermediate spraying on it. The main relay locked at once again. Then I told him to go to work with it, with myself waiting there to see what happens.
The forklift worked quite normally for about 10 minutes. Then it stopped again in the middle of the warehouse refusing to restart. I went there and sprayed the CPU body again. It started normally. I asked the operator to bring it to the parking place.
Now I had to identify if I had a defective CPU or a problem with cold joints of the CPU terminals with the PCB foil traces. I waited for about half an hour and asked the operator to restart it. No result. I asked him to keep trying. While he was trying to start it I could see the movements of the main relay core. Then I sat down, in front of the exposed main board and pressed gently the CPU with my thumb. As soon as I pressed it, the relay locked immediately. Now I had the complete picture of the problem. It was cold joints of the terminals of the CPU with the PCB traces.
I informed the local foreman that I will remove and take the main board with me, at the repairs’ laboratory, in order to fix it. Then I removed it and left the place again. It was late in the afternoon and I still remember I was very tired. I left this job for early in the next morning, after a mind-cleaning coffee break…
Next morning, after the coffee break of course, I had this main board in front of me on my bench. It was indeed a very high quality PCB, both in design and implementation. It reminded me the typical Japanese quality I was used to see in electronics, although I had no info about its origin. Anyway what I am saying is based exclusively on my personal experience.
I started to work with it removing gently the plastic coating around the CPU terminals. In order to soften the thick plastic layer I used acetone and then I scrapped the plastic mass gently with a sharp cutter. The CPU was a MC68HC11K1CFN3 of Motorola origin.
Below you can see the result:
After cleaning the CPU surrounding area, I used a self made needle tip for my soldering iron, made out of the core of a thick single stranded copper cable, which I shaped properly using a file. Using this soldering tip I refreshed all the joints of each individual terminal of the CPU with its respective joint trace. We had no hot air soldering station available and therefore I was forced to use my imagination in the most effective way in order to help myself to overcome this difficulty.
After finishing this rework, I used a plastic spray which had an intense bluish color. I applied several layers one after the other in order to secure a thickness of it similar to that of the original application.
You can see the result below:
This work took me about three hours. I left the PCB in a clean place waiting for the sprayed coating to dry up. When it dried, I called the driver of that department to come and drop me there in order for me to complete the repair procedure.
In less than a quarter of an hour he was in our laboratory. So I grabbed my tool bag again and we left. When we were at their premises I started reinstalling the main board in its place. After 10 minutes I had finished with it. Then I asked for the operator to assist me again. He came there and I told him I was ready for testing the vehicle. He jumped on the seat and started it. The relay locked at once. I told him to repeat the “key ON’ step several times, while I was watching the relay. He did so and the relay was successfully armed and locked every single time. No failure in locking at all.
I told him to go to work, while I intended to stay there and watch further its function.
I remember that after a first round he came back to me with a strange smile in his face and asked me:
“ Hey Paris, what kind of magic did you do to this forklift?”
“Why do you ask that?” I responded.
“Because it not only works perfectly and starts with the first keying, but also its speedometer which was stuck for years displaying that 60Km indication is now working normally. I can see the real speed of it now and it really reminds me the first day I sat on this seat… and that red light with the danger sign in that triangular symbol is not flashing any longer”…
That was for me the happy final confirmation that the repair was successfully over. And the warmest of the thanks I had from many people thereafter; directors, division directors, foremen etc, the most touching ones having the high-charge of emotion, were those of that operator…
“Up to this day, I spent all my working life on that seat and I just love this truck. That’s why I thank you so much for this repair” he told me closing the conversation there. Right afterwards he pressed gently the “gas” pedal and left to continue his work. His emotional state was impressively obvious. The picture of happiness in his face at that time is stored in the “non volatile memory” of my brains…
As I had myself already spent three working days in total with this forklift in order to restore its original performance, I realized that I started loving it in a similar way this operator loved it…Contagious feeling? Strange, but nevertheless true…
I hope you enjoyed this repair adventure of mine as much as I did when working with that truck…This one included all the characteristic elements of an adventure indeed…
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Note: Please check his previous repair articles in the below link: