Explain Limp Home mode

edfiero1

Member
Joined
Apr 6, 2017
What sensors can go bad and then be bypassed by limp home mode?
For example, I know the car will run with the MAF disconnected. Does that put you in Limp Home mode for ALL sensors?
Would the car run with the CAM sensor disconnected in a similar way? If so, why does Caspers sell that CAM FIX Module?

Also are there different kinds of limp home mode. For example, a sensor going bad, might give you one result, but what about if the entire ECM dies, is there a different limp home mode?

My car starts, and immediately dies after 2 -3 seconds. Unplugged the MAF and the result is basically the same. Trying to understand what I can unplug vs what I can't as a process of elimination.
 
If the entire ecm dies the engine will not run.
 
I don’t think “limp home mode” is a real thing


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its real also referred to as bypass mode
if the ign module opens the internal circuit the white wire will go open and will code 42..this will put ecm in limp mode (bypass) and then it runs the car using the smaller calpak chip ,,if you are on stock or close to stock injectors the car will be able to be driven lightly , if injectors are larger it wont run well due to being excessively rich and should not be driven

cam sensor failure after start will put fuel into batch fire mode firing several injectors at a time similar to how a throttle body injected engine works ..if the cam signal is lost while engine is running the motor will still operate in batch mode but once its turned off it will not start again without cam signal..the caspers gives a signal to allow starting but its only temporary and will operate in batch fire mode so it should not be installed unless theres an issue
 
Limp mode is when the ECM can't run from the PROM. There is a redundant Fuel Device (RFD) chip in the ECM that will use the CALPAK (the small chip) calibration to run the fueling. The spark advance is run from the ICM. In the case of our cars it will be at 10° BTDC (not ideal).

In limp mode the ECM will flash/flicker the SES/CEL light. To simulate this remove the PROM and start the engine. It will be in limp mode. And the SES/CEL will flicker to alert the driver that something is wrong.

If you are running larger then stock injectors, limp mode will be very rich.

Big Note: limp mode is not the same as default modes. This is where if the MAF takes an error code the ECM goes into N-Alpha mode. Creating an airflow based on TPS and engine RPM.

Same if the CTS sensor fails. The ECM will use a default value.

There are defaults for other sensors. Such as not going into closed loop if code 13 (O2) is active.

The bottom line, limp mode is only invoked if the ECM isn't running from the PROM.

RemoveBeforeFlight
 
It should be noted that these modes are just to get keep it running enough to bring to a shop or home for diagnosis.
 
I had the same Problem after my rebuild. I talked to RC and if I remember correctly he said the usual cause was a bad ESC module. On initial startup timing is controlled by the ESC and then there is a handoff to the ECM. I believe this is what he said but I can't swear by it.

On mine it happened to be I had forgotten how to disable Anti Theft function on my Extender chip. Once I found the correct way to do it the car ran fine.
 
***Tech Alert***

One of the biggest fears for the stock ECM is if you should get into the bypass (limp) mode using big injectors; you'd wash down the cylinder walls and hurt the engine in no time.

The Calpack chip is nothing more than a series of resistors in an unusual matrix. Each value gives the ECM a "starting point" for each closed loop sensor which should allow the engine to run without closed-loop sensor activity. There is a resistance value that's somewhat matched to your stock injectors (28lb). If you can change that resistor value to one that's more appropriate to your larger injectors, your bypass mode will be much safer for your engine should your PROM chip fail. It would be a much safer environment for your engine.

So.

Some years ago, I decoded the chip. Various resistance values input an equivalent value to the processor to "fake" the existence of sensors. One of these resistance values controls pulse width on your injectors - and your challenge is to find it and modify it. You'd have to access the pins of the chip and externally modify its value with resistors or trimpots. The ECM schematics are out there and a bit of research would get you to the correct resistor. A bit of experimentation will allow you to fine-tune the injector pulse and trim it to your setup. Richard C, if I'm not mistaken, you should have a benchtop rig to analyze ECM's, which would be a great starting point.

Take a look at the decoded chip layout here along with an explanation:
http://www.installationinstructions.com/FYI/16036503_calpak_tech.pdf

Techie geeks, have at it! If you decide to take on this project, all I ask is that you share your research, as I am doing, with the community.
 
I know you can go fast on a stock ECM, I've gone 130+ mph in the 1/4 thru a stock MAF sensor, lifts the tires at the track off the footbrake, but today no way , not going to ever go back to a 80's ECM to control my engine.

Case in point, I lost the line to my MAP sensor in the burnout in 2015. My car goes from 0 boost to 25+ in less than a second off the line. Not enough time for me to do anything to lift other than the pop pop pop sound I heard after it went waaaaaaay lean because the alky didn't spray (no map reference signal) and the stock ECM with TT 6.1 chip could only add 30% fuel at best to try to save it after the AFR's were lean.

That's why I ditched the 30 yr old wiring harness and 30 yr old ECM. Now I can control everything from one ECM instead of piggyback on piggyback on top of ancient technology. After alky control, the Holley EFI with Cruz harness is the #2 best thing I've ever done to the car for street/strip duty.

I've had the car for 18 yrs and had to swap stock ECMs on the side of the road before, BTDT, I always had working spares.
 
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