Injector Resistors are getting very hot

[dean924s]

I was bench testing my system left it to run for 20 min or so. I have 10R 25 Watt resistors in the Injector circuit due to me running Low impedance injectors. My setup and the resistors mounted to a 1/8" aluminum plate that is the back plate for a gutted ECU (DME) that I am using to wire the MS in to the existing wiring harness on my 924s. ANyway after 20 min or so I notes that the resistors and the alum plate that they are mounted to was very hot.

What I suspect is that I need to finalize the grounds and the power supply circuits. Currently they are wired with gator clips for testing. It makes changing things simple.

Am I correct that it is just telling me To wire it properly / permanently or is there another reason or setting that could cause this?

Here is a photo of my gutted ECU (DME) With the resistors mounted. I just added all the black wires today. They still need to be soldered in place.

[jsmcortina]

What temperature is "very hot" and what were your test conditions? e.g. simulated rpm and injector duty cycle.

James

[dean924s]

What temperature is "very hot" and what were your test conditions? e.g. simulated rpm and injector duty cycle.

James

So hot that I could not touch them. I could barley touch the alu plate that they are mounted to. I was running it through various RPMs with the JimStim (wired to the pip and saw leads of the MS) everything else was testing through an OE 944 the harness. I was cycling RPMs and the TPS along with changes to the MAP.

[racingmini_mtl]

At 100% duty cycle you should have around 15-20W of heat generated in these resistors depending on your injectors and battery voltage. That is going to mean quite a bit of heat. However, if you run at 10% duty cycle, you should have one tenth of the heat generated which should result in only warm resistors with your setup. Give it a try to see if that's the case.

If you always get hot resistors, that would likely mean the injectors are shorted open for some reason and you need to solve that.

Jean

[Peter Florance]

Shouldn't there be 2 more resistors?

[cmonref]

I was running it through various RPMs with the JimStim (wired to the pip and saw leads of the MS) everything else was testing through an OE 944 the harness.

EDIS with two plugs per resistor?

Brian

[dean924s]

I was running it through various RPMs with the JimStim (wired to the pip and saw leads of the MS) everything else was testing through an OE 944 the harness.

EDIS with two plugs per resistor?

Brian

No the resistors are for the injector circuit. The stock injectors for a NA 944 are 2.2-2.4 R. The resistors are to bring the R's up to what the MS needs to make it think it is wired to high R injectors. The EDIS PIP and SAW leads from the MS were run to my jimstim The rest of the MS was wired through a gutted DME that is connected to an OE wiring harness that's all the sensors, injector and TPS connected. This allows for bench testing of the MS in as wired in the car conditions. On a side note it is cool to have the injectors clicking away on the bench. If it maters the injector stayed cool during the testing.

[dean924s]

Shouldn't there be 2 more resistors?

I dont beleave so. I did the math and came up with this I also spoke to Matt at DIY about this and he concured that these would be a "Safe" chioce.

Here is the math behind it. I posted this in my rambeling thread about this build over on Clarks Garadge.

R= resistance in Ohms
R1 = The resistance of resistor #1 or in our case the resistance of an injector.
R2 = The resistance of resistor #2 or in our case the resistance of an injector.

944's we have two banks of two injectors that are wired in parallel. Since in our case we know that the standard resistance of a porsche injector in my application is 2.1-2.4 ohms (spec versus measured) To keep the math simple I used 2.2 ohms (if you use 2.4 the result is only changed by .1 Ohms)

Total R = 1/ 1/R1+1/R2 So our equation is

R=1/ 1/2.2 + 1/2.2
This equates to R= 1/ .45+.45
This equates to R= 1/.9
This equates to R= 1.11

So from this I have approximately 1.11 Ohms of resistance in each injector circuit.

So now what? Well we have to add enough resistance to make it seem like the MS is actually powering high ohm injectors some where between 10-12 ohms. I think this is a safe level but I will have to confirm this in the mega manual.

This means that we now have to look at resistors in series. This is much simpler. You literally add the resistance together of the resistors that are in series and you get the total resistance of the circuit. Or in this case the resistance that the Megasquirt will "see"

So we have calculated that the one injector circuit in a standard 944 (8v non turbo 2.5l) is 1.1 and I would like to have the MS "see" a resistance of between 10-12 ohms. The answer is put a 10 Ohm resistor in between the injector circuit and the MS unit. This will result in a total resistance of 11.1 ohms (10+1.1) This is well with in the spec that I would like and what the MS is designed to be outputting to.

[Peter Florance]

I was running it through various RPMs with the JimStim (wired to the pip and saw leads of the MS) everything else was testing through an OE 944 the harness.

EDIS with two plugs per resistor?

Brian

No the resistors are for the injector circuit. The stock injectors for a NA 944 are 2.2-2.4 R. The resistors are to bring the R's up to what the MS needs to make it think it is wired to high R injectors. The EDIS PIP and SAW leads from the MS were run to my jimstim The rest of the MS was wired through a gutted DME that is connected to an OE wiring harness that's all the sensors, injector and TPS connected. This allows for bench testing of the MS in as wired in the car conditions. On a side note it is cool to have the injectors clicking away on the bench. If it maters the injector stayed cool during the testing.

Why not just run them as low Z injectors and get rid of resistors? I'm still doing that with 6 low z

[jsmcortina]

If you run injector resistors you must run one per injector.

James

[Peter Florance]

If you run injector resistors you must run one per injector.

James

They are getting too hot because they are dropping too much voltage.

[dean924s]

Why not just run them as low Z injectors and get rid of resistors? I'm still doing that with 6 low z

I was trying to keep it simple. Eliminate one more thing I needed to fiddle with to get it running.

[Peter Florance]

Why not just run them as low Z injectors and get rid of resistors? I'm still doing that with 6 low z

I was trying to keep it simple. Eliminate one more thing I needed to fiddle with to get it running.

Jumper 'em out and run PWM mode


Make sure your harness is wired like this (possible sensor ground is also grounded to block)
to avoid any noise. Low Z injectors got bad reputation for noise but it was really due to grounding problems.

To test:
"If you unplug your MegaSquirt box and still have continuity to ground from your sensor commons, you might have an grounding scheme issue."

[dean924s]

This is interesting because back when I was talking to Matt at DIY he was running 3.3 R resistors (1 per 2 injectors). I thought I just saw a post from someone stating that there are 2 5R resistors in the stock set up and that I should be using something in the 3.3-5R resistance. This also would explain why my injectors seem to stop working over 3K RPM when bench testing. The resistors are to high limiting the amount of voltage to the injectors and getting to hot. Make sense? So should I go get some lower R resistors?

[Peter Florance]

This is interesting because back when I was talking to Matt at DIY he was running 3.3 R resistors (1 per 2 injectors). I thought I just saw a post from someone stating that there are 2 5R resistors in the stock set up and that I should be using something in the 3.3-5R resistance. This also would explain why my injectors seem to stop working over 3K RPM when bench testing. The resistors are to high limiting the amount of voltage to the injectors and getting to hot. Make sense? So should I go get some lower R resistors?

I would jumper them with 16ga wire and try it in PWM mode unless you can wire 4 resistors to 4 injectors.

[dean924s]

Although I hate to add another thing to fiddle with I am agreeing with you Peter it may add another bit of tuning but it will eliminate the resistors and the variables associated with them. I will give that a try tonight. I have to get off to work now. I think I am also going to pick up two lower R resistors as well that match what DIY had done in there 944. That way I can try both tonight.

[Peter Florance]

Although I hate to add another thing to fiddle with I am agreeing with you Peter it may add another bit of tuning but it will eliminate the resistors and the variables associated with them. I will give that a try tonight. I have to get off to work now. I think I am also going to pick up two lower R resistors as well that match what DIY had done in there 944. That way I can try both tonight.

I can post my parameters if you want; they've worked with every low z Bosch setup I've tuned.

Easy Button

[dean924s]

From the DIY web site in the DIYP&P for the 944 section:

Low Impedance Fuel Injectors

The MicroSquirt Module based DIYPNP will control two low impedance injectors normally (without the Sequential mod allowing for four), or a large number of high-impedance injectors. This vehicle uses four low-impedance injectors and as such you can do one of two things. You can add the sequential module and wire the injectors sequentially (a bit of wiring required). Or you can stay plug-n-play and add resistors in line with the injectors. You will need to use two resistors of about 3.0-3.6 Ohms and rated at 10-20w each. Put one resistor in series with each of the two injector outputs and from each resistor back to connector board pin 17. You may mount these resistors inside the DIYPNP case if you'd like. We found some ceramic 3.3ohm 10w resistors at Fry's that fit nicely.
•
If you are controlling the injectors as the stock ECU does in batch fire, you will need to make sure you set your injector staging to "simultaneous" in the engine contestants screen of the tuning software.



This makes me think my math was wrong some where. Before jumper the resistors I am going to do as DIY said above and see if that works. I think that the 10R were to safe. They protected my injectors as they staid completely cool but are to much protection preventing the injectors from running correctly and thus generating so much heat.

[dean924s]

I can post my parameters if you want; they've worked with every low z Bosch setup I've tuned.

Easy Button

That would be very helpfull if you could do that.

[dean924s]

I think I see the problem!!! The DIY setup uses only one injector circuit. Note that in the description there are two injectors per resistor. Then they state to wire the two resistors back to pin 17. This means that all 4 injectors are firing one one MS circuit and thus there system work. I have WAY to much resistance. I need to cut the resistance back to between 3.3 and 5R for each resistor in my set up as I am using the two separate injector circuit on my MS. Make sense?