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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Tnbill on August 20, 2025, 07:59:46 AM
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The first ride on my new to me k100rs was not a good one. After about a 35 mile ride we stopped for a few minutes it would not start. I could not hear the fuel pump so i know that was the issue. 4 hours later after i got it home it started right up and again i could hear the pump run. I know intermittent problems are difficult to diagnose and after trying to duplicate it with no luck. I did try putting a hair dryer on the fuel injection module it still would fire right up. I am leaning towards that module having an issue. My question is, is it known to have issues. I did read that it is a pretty robust unit.
I did a complete fuel pump, filter, hoses and a good cleaning of the tank as it was a mess with some real butchering of the wrong pump and piece of heater hose that was dissolving in the fuel.
TIA
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Heating the fuel injection control unit for proof of its function is a novel approach. Maybe that's an automotive diagnostic approach. On Bricks, usually heat is applied to the Hall Effect sensor. A faulty sensor often temporarily fails after an engine is fully warmed up and has been run for a while; furthermore, a heat gun—not a hair dryer—seems more effective at creating sufficient heat to generate a problem with a faulty one. Bricks come with some unique attributes and treatment strategies. Your attempting to reproduce the failure is a commendable strategy often neglected by others; nevertheless, it's story time.
I bought my Brick in Maine. The responsible adult who owned it enjoyed riding it regularly and stored it in a well-built, dry barn but he wanted a Ducati so there we were. He allowed me to take an unattended 20 minute ride on the roads around Boothbay. I bought it on the spot and trailered it back to Vermont that day. When I backed it off the trailer at home, I started the engine and moved it to its permanent parking spot.
The next day I started it and headed down the mountain on the three-mile dirt road that leads to the river and civilization four miles away. I turned left after the stop sign at the river, the bike stalled and I coasted to a stop on the road's shoulder. Zip, zero, nada. No start, no lights, nothing. I knew little about the inner layout of the Brick so I did what any red-blooded bike rider would do; I called my partner of thirty years and pleaded with her to stop whatever she might be doing and bring me the Rider's Handbook that came with the bike. She had mercy on me and delivered it along with a benediction and a piece of apple crumb cake, then she got the hell back home before the cursing and self-recriminations started.
I looked at the fuse layout diagram in the handbook then pulled each fuse. All fuses, their blades and socket receivers were clean and intact. Then I came across the drawing of the LE Jetronic fuel injection control unit in its box mounted under the seat. I deduced which direction to push the latch of the unit's plug down in the hole of the box and figured out it disconnected by swiveling it from the right, but first I noticed that the plug was not quite square into its housing. I pulled on right-hand the plug and it wasn't latched. I pushed it inward until it was latched and couldn't be swiveled outward, stood up, turned the key to On, pushed the starter button and she fired right up.
I didn't have another power problem until it stalled 60,000 miles later. That problem was caused by a dirty ignition switch and was intermittent. By that time, I'd read quite a bit about Bricks and their owners' frolics with them so I was ready. I was able to wiggle the key until it started, then rode it home. Cleaning it was the cure.
On a Brick being recommissioned, cleaning all electrical connectors, inspecting, cleaning and tightening all battery positive and ground connections, cleaning and tightening starter mounting bolts and connectors, and cleaning and tightening the main ground connection on the frame backbone should be happening early in the program. This problem is likely to be an electrical connection/oxidation problem. Intermittent fuel starvation from dirty injectors could be in the picture, too, but, as you know, electronics tend to run the whole show. Intermittent operation failures are tough but neglected Bricks can respond well to thorough tending to electronics cleaning, including the relays in the relay box beneath the tank.
With the given depth of your automotive mechanics experience, the reason for this condition is likely to be accompanied by an "Oh, there's the problem right there" solution at any moment. 112350
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As to using a hair dryer thats what i had and it did get it pretty warm. Yes heating a component to try to duplicate is one trick and another one is to cool down a component right as it fails. I did that on an old chrysler ignition trigger in the distributor to confirm it was the issue. I don’t think the Hall effect is my issue that would stop the fuel pump from running when I momentarily bump the starter button but i do think if was heat related with it starting after a long cool down. So again is the fuel injection module known to have any issues. I did find there a guy that offers a repair service for them.
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I had an intermittent fuel pump issue at one time and traced it to a dirty/loose connection on the FI controller. I pulled the connector, cleaned the contacts, greased them with non conductive grease to keep moisture out, and securely seated it back in place. I've had no further issues since.
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So again is the fuel injection module known to have any issues. I did find there a guy that offers a repair service for them.
40 year-old electronic control units certainly can have issues but corrupted signal processing can range from ignition timing to injection timing. Several riders have posted about fueling and ignition problems that ended when a replacement unit was installed, a search of the site will reveal that; however, grounding and wiring faults seem at the root of most such symptoms, like the one described above by Scott. There are LE Jetronic testing and diagnosis manuals downloadable from the Technical Directory of this site.
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I have not found the smoking gun yet. Checked the ficm connector and all was good there. It runs perfect before this and after with nothing coming on gradually. It was a failure to to restart and so far in the shop with multiple heat cycles it has always started. All i know to do now is to ride it and maybe carry a way to jump power to fuse 6 to see if it will restart if it does it again. I am not going to fire the parts cannon at it but i might replace the ficm relay next time i get into it.
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Another thing i found by studying the wiring diagram is the fuel pump relay is controlled by the ignition module NOT the fuel injection module so trying to heat it with a heat gun is not going to happen where it is buried. The parts bike does have the ignition module so i do have a spare handy.
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For those following along in this thread, there is an LE Jetronic training manual and also an LE Jetronics diagnostics manual available in the Technical Directory here. Scroll down the list in Service Bulletins (https://motobrick.com/index.php?board=61.0).
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I could not hear the fuel pump so i know that was the issue.
Very often and very typically the problem with threads like these was the four pin plug to the fuel tank, although I think there hasn't been one for a while, so we're overdue for a problem like this. Sometimes they're just dirty, sometimes the pins are loose. Often the plug is replaced. Most of my bricks have had this problem.
There were a few changes in 1985, the fuel plug was one of them, both plugs had problems. In the later style, the pins are often a loose fit. In the earlier style, the plug itself is also often a loose fit.
If wiggling the plug causes your brick to stall, you want to have a look at it. If you've plugged it in and out a few times lately, that itself might have fixed it.
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I have taken a dental pick to carefully close up the female pins on the chassis side of the fuel tank connector a couple of times. The first time the bike quit on me was going home after work (25+? years ago). Side of the road I took the left knee pad off and looked at the fuel tank connector- one of the original round types at the front of the tank. I had a small screwdriver with me and closed up the gap in the female pins for a firmer contact. That fixed it for years. Don't remember what led me to check that first.
Frank
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Ok i said i was not going to fore the parts cannon BUT the parts bike had an ignition module and fuel pump relay so i swapped them out and tightened the female pine on the pump connector at the tank. The bike once again started right up like always. I will try to heat cycle it in the shop a few times than take it out again and see what happens.
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Well sofar i have taken it out 3 times. Yesterday twice for a 45 mile loop and today for a 75 mike ride and all is well. Not once has it failed to start. Fingers crossed that problem solved.