jimstim rpm pots

Building and using the Stimulator to test your Megasquirt

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atthetrack17
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jimstim rpm pots

Post by atthetrack17 »

Hi, my rpm pots(when testing with jumper wires) on the jimstim used to show correctly on the leds when turning the pots clockwise, by blinking faster. Today when I turn the pots its all or nothing. leds are off to the left and turn on when the pots are all the way clockwise. I have removed the cpu and tested the volts and also checked the pots. All checks came out normal. Do I need a new CPU? thanks, Brad
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Re: jimstim rpm pots

Post by racingmini_mtl »

Are you sure you have all the needed jumpers and that you have selected a valid wheel mode? Also, check the voltage at the pots when you turn them; that should go from 0V to 3.3V.

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atthetrack17
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Re: jimstim rpm pots

Post by atthetrack17 »

Thanks Jean. Yes, all jumpers are off and wheel mode switches are all off.
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Re: jimstim rpm pots

Post by racingmini_mtl »

All jumpers off will not work. You need to check again because you need some jumpers and you need to make sure you're using the right ones for your specific setup.

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atthetrack17
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Re: jimstim rpm pots

Post by atthetrack17 »

Here is what I have been following. This is from the instructions for the jimstim. Thanks, Brad

Testing the tach signal

If the Jimstim voltage regulator and/or CPU are getting hot when power is applied, refer to the next section (Troubleshooting tach signal, CPU and 3.3V)..

Testing of the tach signal can be done using the LEDs. This test will also verify that the Jimstim CPU is working correctly. To do this you need to do this:.
Power off the JimStim and disconnect it from the MegaSquirt
Remove the pull up jumpers
Remove the primary tach jumper
Set the wheel mode to distributor mode (all DIP switches off)
Connect a wire jumper from the primary tach header (where the jumper usually goes) to an IAC led. Use the primary tach header pin closest to the pots (away from the PRIMARY TACH text) and the last pin of the 10-pin header (pin closest to the pots)
Connect a wire jumper from the other side of the IAC led to 12V. Use the next to last pin on the 10-pin header (just besides the pin connected in the previous step) and one of the pins on the 3-pin header besides the voltage regulator.
Turn the RPM pots all the way counterclockwise.

Once the connections above are done, power up the JimStim (with the MegaSquirt still not connected) and start slowly turning the RPM FINE potentiometer clockwise. You should see the led start flashing and it will flash faster as you turn the pot more.
Warning: With the JimStim not connected to the MegaSquirt, the bottom of the board will rest on a surface with all the exposed leads and solder pads. Make absolutely sure that the surface is non-conducting and that there is nothing that could make a short also on the surface. A short may cause permanent damage to the CPU and possibly other components..
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Re: jimstim rpm pots

Post by Dick »

When I do this tach signal test, the LED sometimes blinks, but is usually off. I turn the fine RPM pot until I find a spot where the LED blinks on and off, but turning the pot more turns it totally off. I'm not sure what the instructions mean when they say to remove some of the jumpers. Shouldn't ALL of the jumpers be removed?
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Re: jimstim rpm pots

Post by Dick »

I replaced the jimstim ic, but the problem wasn't solved. Usually I get no LED flash when I turn the fine RPM pot, but occasionally I get a blink. I checked the voltages at the ic socket, and they looked correct, and varied with the position of the rpm pot correctly.
I am using a wall transformer, 12vdc 500ma rating, which gives 12.9vdc open circuit, 11.1vdc with the jimstim turned on, 10.39vdc at the 12v header, and 3.03vdc at the 3.3v header pin. Two VOMs agree very closely. No heat on the polyfuse, ic, or voltage regulator. Both RPM pots respond similarly (i.e. NOT). ??
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Re: jimstim rpm pots

Post by racingmini_mtl »

Post a picture of your unit in test setup. Make sure all the jumpers and DIP switches are plainly visible in the picture. We may be able to see something wrong in your configuration. An exact description of your testing process would also help. Checking voltage on all pins of the CPU is also a good thing to verify making sure that the ones connected to the pots do vary when you turn the pots. Also make sure you don't short the pins when taking the measurement because it is easy for a probe to slip and create problems where there weren't.

Jean
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Dick
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Re: jimstim rpm pots

Post by Dick »

IMG_5554.jpg
IMG_5554.jpg (704.37 KiB) Viewed 6071 times
Test procedure is to turn off jimstim, attach two jumpers per photo, all dip switches off, RPM and RPM fine full counterclockwise, turn on jimstim, turn RPM fine slowly clockwise to full stop.
I've swapped to another dc power supply, no changes. Sometimes wiggling a jumper wire seems to help, but I've swapped jumper wires and header pins, can't find one weak link. Board appears to draw 30-50 ma, but measuring on 500ma meter, so can't tell for sure. Jumper obscures dip #4 in photo (sorry) but it's off. I measured voltages on ic socket before inserting chip, and they were correct. Thanks for taking a look.
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Re: jimstim rpm pots

Post by Dick »

photo must be too big. I'll try smaller...
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Re: jimstim rpm pots

Post by Dick »

jimstim test setup
jimstim test setup
IMG_5554 (1).jpg (39.34 KiB) Viewed 6070 times
racingmini_mtl
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Re: jimstim rpm pots

Post by racingmini_mtl »

That looks like a correct setup for testing the RPM. So if you have the fine RPM pot all completely clockwise and the coarse pot (RPM C) completely anti-clockwise, you don't see the LED flash?

The problem could be the small transistor (Q1). Check that the solder joint are fine and you might want to post a close up picture of it from the underside. I assume you have already checked that the LED works by itself.

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Dick
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Re: jimstim rpm pots

Post by Dick »

Checked voltages at ic with ic in place. Pins 1 and 3 at 3.04vdc, 13 varies with fine rpm pot 0 to 3.03vdc, 14 is 0vdc with pot ccw. Thanks again for checking on this.
jimstim underside.  LED checks out OK.
jimstim underside. LED checks out OK.
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racingmini_mtl
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Re: jimstim rpm pots

Post by racingmini_mtl »

There's one pin on the transistor where the solder joint doesn't look very good but that's hard to say on a picture. Carefully re-applying the solder iron to it might be a good idea. Also cleaning the board from any flux residue is not a bad idea.

Having said that, it would be useful to check if the circuit is good by bypassing the CPU. If you remove the CPU and power up the unit, you could make a connection between pins 15 and 3 to power up the circuit. If you open and close this connection, you should see the LED turn on and off. You can use a small length of wire or a paper clip to make the temporary connection (be careful not to make any unintended connection). You can also use the 3.3V pin header instead of pin 3 of the CPU to power pin 15 if that's easier with what you have in hand (for example a female-male jumper wire).

If the LED turns on and off without the CPU then the CPU is problematic; the way it is inserted in the socket could also be the issue as well as the socket itself (less likely if the test above work well). If you can't get a solid on/off response without the CPU then the issue is either the transistor circuit or the LED. Using the other LED could also help diagnose this (pins 7 and 8 of the 10-pin header instead of pins 9 and 10).

Jean
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Dick
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Re: jimstim rpm pots

Post by Dick »

This test showed the LED having a solid response, so I suppose the CPU is bad. That's a shame, because I already replaced it once. The original and replacement CPUs responded identically. I can't see any problems with the socket, but will check it again before I order another replacement. Thanks again for the support.
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Re: jimstim rpm pots

Post by Matt Cramer »

One thing I've found that can easily kill JimStim CPUs if if you've accidentally set the JimStim down on something conductive while under power. Make sure you're not accidentally putting it on something like a bare aluminum MS case (I've done that before myself), or a workbench with a metal surface.
Matt Cramer -1966 Dodge Dart slant six running on MS3X
Dick
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Re: jimstim rpm pots

Post by Dick »

Thanks for the tip. My "workbench' is an old wooden office desk, with a plastic artboard covering the surface. No metal around. However, maybe I'll remove the artboard for this next cpu.
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Re: jimstim rpm pots

Post by Dick »

Installed 3rd CPU, with no success. Result similar to previous CPUs, maybe slightly better. Youtube video can be seen at https://www.youtube.com/watch?v=v8K6hpK1Jqk, or search youtube for JIMSTIM TEST. Note the intermittent LED response, which I had to "search" for with the fine RPM pot. It only responds at all near the lower end of the pot scale. I also have video of unpacking and installing CPU, if there is any interest. Hooked the rig up to the 120V outlet in the house vs garage, no change.
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Re: jimstim rpm pots

Post by racingmini_mtl »

That definitely looks like you have a partial connection somewhere. Put the coarse pot full counterclockwise and the fine pot roughly between a 1/4 to a 1/3 its range and don't move them from there. That would give you a fast flash that should be clearly visible. Check that you have 0V for the coarse pot at pin 13 and around 1V for the fine pot at pin 14.

Now try wiggling your jumper wires to see if that makes any difference. If possible try different jumper wires. Also push down with your finger on the CPU. Another thing to do is to switch all the DIP switches from on to off and back a few times to make sure they are all actually in the correct position (in that case that should not make much of a difference because they are all off but that's just an easy step to check). You can also check that pins 5 to 12 (the pins connected to the DIP switches) are all at 3V when the CPU is there and the JimStim is powered up (being careful to not short anything with the probe when taking the measurement).

Really at this point the only reliable method would be to use an oscilloscope to check the signal at the CPU pin and then follow the signal throughout the circuit. But I assume you don't have access to one which is understandable. A rough estimate would be to measure the voltage on the output signal CPU pin (pin 15) and check that you have roughly 1.5V; using higher RPM (turn the coarse pot half way up) should give your meter a better chance of showing this voltage. If you cannot see anything reasonable on the output pin then the issue is either the CPU power, the pot inputs or the DIP switch inputs. I assume the CPU is fine since you've tried many.

Jean
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Dick
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Re: jimstim rpm pots

Post by Dick »

Verified 0vdc at 13 and 0.95vdc at 14 with coarse ccw and fine at 1/3. No flash. Verified pins 5-12 at 3.02vdc with CPU installed. Cycled DIP switches twice. Switch 6 caused a lot of flashing on LED. Returned all to OFF. Swapped jumper wires (again) and pressed CPU. No joy.
Turned coarse pot 1/2 turn cw. Voltage at pin 15 was never 1.5vdc, was typically low mv range. When I'd get a DVOM spike to around 1vdc or so (infrequent), LED would flash.
Please excuse my electronic ignorance, but since Pins 13 and 14 seem to respond correctly to the pot inputs, doesn't that make the DIP switch inputs most likely?
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