Automobiles Forum banner
1 - 20 of 23 Posts

·
Registered
Joined
·
130 Posts
Discussion Starter · #1 ·
Not related exactly to the ecu's operation however in my drag car I've wired up a relay to switch the voltage to the rev limit wire. Since my transbrake (which holds the car still on the starting line) operates off batt. voltage I split off that wire to turn a relay on which puts the 3.27 volts to the rev limit wire. The limiter funtions this way however at times I'm getting a delay in the release of the transbrake, you let go of the tranbrake button and the car does not immediately go. The engine is also staying limited until the car moves. My thinking is it's somehow getting "flyback" voltage from the reay which would keep the transbrake and rev limiter engaged. This is not what you want on a drag racing car. This doesn't happen all the time but it does happen, anyone know if this could be the cause? Thanks Joe
 

·
Registered
Joined
·
376 Posts
-Easy to check/test; most automotive relays have a flyback diode in them, and you can generally tell from the diagram on the relay. If there isn't one, stop by a FLAPS and pick one up

 

·
Registered
Joined
·
2,564 Posts
Based on your description, I dont think back EMF on the relay is the issue.
If you are putting 3.3v on the input and then just disconnect it (with a N/O relay)
the signal will not immediately drop in voltage since the input has a capacitor and
no discharge path.

If you take the input and give it a 1k resistor to ground it will drop immediately.
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #4 ·
90/90qs, thanks I'll look at the relay and see. Architect, the 3.3v side isn't the issue I'm having at least I don't think so. The problem seems to be some latent power staying on the B+ side of the transbrake circuit for some unknown reason power seems to be getting to this circuit even with the switch that supplies it released (open). In the relay on the left 85 would be B+ and 86 would be ground correct?
 

·
Registered
Joined
·
58 Posts
Are you saying the relay stays closed for a fraction of a second after opening the circuit to the relay coil? If so, try another relay. They age and the contacts can stick. Also the return spring for the contact inside the relay can wear out.
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #6 ·
Steve, no I believe the switched side of the relay is working properly, my theory is the power in the relay coil (being introduced by my transbrake switch B+) is somehow "backing up" into the wire from my transbrake switch and keeping the transbrake and the relay on after I've released the switch that powers it. My in car datalogger shows the release of the transbrake and the time between it seeing the release and the car moving never varies, yet there are times when I've released the button and actually been aware of "hey the cars not moving" then Wham it takes off. This of course has a detrimental effect on the reaction time. Since the engine RPM is still limited I've determined that it can't simply be an issue with the trans. or the transbrake solenoid. I don't know maybe I'm grasping at straws here, actually had my son drive the car as I thought maybe I'm just too old I can see his hand come off the button and a n obvious delay before the car moves.
 

·
Registered
Joined
·
376 Posts
The relay coil circuit and the load circuit should be independent of each other, unless there is something odd in your wiring. Seems to me there shouldn't be any back EMF reaching the solenoid at all, even without a diode in there. What are you using as a trigger switch?
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #8 ·
Relay control comes thru momentary contact switch B+ to relay term. 85 term. 86 goes to ground. 30 has 3.3v in 87 goes to rev. limit wire for limit 1 input. So when I push button B+ is directed to my trans. solenoid also wire T'd off going to relay term.85 which closes relay allowing the ECU to go to limit1 (3.3v). When I release button (momentart contact switch) B+ is interrupted to the trans. and relay, here's where I'm seeing an issue the car does not immediately move nor does the engine switch off limit1 though they both do eventually occur at the same time. Viewing this on my datalogger you can see the input (B+ to trans. and relay) go off then the car move (driveshaft moves accelerometer indicates movement) the time between these two things never varies ( B+ goes away, .035 seconds later car moves) though at times it's obvious that there is a delay from when you "physically" release the button and when the car actually moves. Again since I see no change on the datalogger time between button release and car movement it indicates to me that somehow the circuit is not "going dead" immediately when you remove the B+ input (release momentary button) this is why I'm asking my question. Sorry for the long winded post but trying to explain it as clearly as I can. Thanks Joe
 

·
Registered
Joined
·
376 Posts
I'd have to take a few minutes and draw up the circuit you're describing, but I don't understand why you are using the coil side of the relay for a load or signal purpose.
(If I understand your description correctly)
Your momentary switch and contacts 85 and 86 should serve a dedicated purpose: to open or close the relay. (-Nothing else)
Depending on what you need to happen when the momentary switch is released, you can use an appropriate relay NO, NC and SPST or DPST (are my guesses without drawing it out)
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #10 ·
Term. 85 is powered by momentary push button supplying B+ to term. 85, term. 86 is grounded, pushing button energises coil in relay closing contacts between term. 30 and term. 87 which allows 3.3 volts to be applied thru those 2 terminals to rev. limit1 wire in ECU. All works correctly until I release the momentary switch which should deenergise relay, opening circuit 30 to 87 as I've said I thought maybe I was getting some sort of back feed thru term. 85 which is connected to relay coil, keeping power on the circuit. At times there is an obvious delay between when I break the power to the relay (release momentary switch) and when the transbrake and rev. limiter releases. Releasing the button should make power on this circuit go away immediately and that doesn't seem to be the case.
 

·
Registered
Joined
·
2,564 Posts
V6 pony said:
Term. 85 is powered by momentary push button supplying B+ to term. 85, term. 86 is grounded, pushing button energises coil in relay closing contacts between term. 30 and term. 87 which allows 3.3 volts to be applied thru those 2 terminals to rev. limit1 wire in ECU. All works correctly until I release the momentary switch which should deenergise relay, opening circuit 30 to 87 as I've said I thought maybe I was getting some sort of back feed thru term. 85 which is connected to relay coil, keeping power on the circuit. At times there is an obvious delay between when I break the power to the relay (release momentary switch) and when the transbrake and rev. limiter releases. Releasing the button should make power on this circuit go away immediately and that doesn't seem to be the case.
Can you "hear" the late release of the relay?

Based on my concept of how you wired the 3.3v signal to the aux "rev limiter" input.
Without using a DPST relay or a discharge resistor, the signal will take some time before it goes from 3.3v to the lower voltage the ECU uses as the high/low voltage threshold for that feature.
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #12 ·
No can't hear anything in the car with engine reving, never could see an issue just trying the system while sitting still with engine off. Can't remember exactly but it seems I have a resistor on the 3.3v circuit as the voltage would slowly drop without it, looking at the monitor screen you see it change rather quickly. My issue seems to be on the B+ side of the relay, thinking as the field collapses in the relay the voltage is not immediately disipating and holding the relay on as well as my transbrake soleniod, as they both show on datalogger switching off at the same time (switch input goes away ) as seen by datalogger as B+ then engine goes beyond rev. limit RPM.
 

·
Registered
Joined
·
2,564 Posts
When you put a back biased diode across a relay (for back EMF protection), the field dissipates slower since the stored voltage takes a while to dump into the 1v or so diode drop.

Some relays use a carefully selected resistor to act as the passive "snubber" which puts the back EMF voltage at a higher level.
This dissipates the stored energy faster than a diode drop.

If I were you, I'd put a fast acting lamp (or LED with limiting resistor) on the field terminal so you can see whats happening and know if the field signal or the relay is the slowness culprit.
Better yet, put an LED on the ECU rev limiter input too.
 

·
Registered
Joined
·
5,037 Posts
Just a shot in the dark:
Try sending the +12v trigger to 86, and ground 85 instead. I have actually seen a couple of relays "hang" like this, until I switched the trigger wires.
May or may not work and IDK exactly why, but worth a shot
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #15 ·
OK, finally got back to track to test this, that problem is fixed...new switch though I can find no issue with the old one. However after running all year on this setup engine developed a miss, looked at all mechanical, compression, valve lash, springs ect. Engine runs normally at lower engine speeds, but when it gets up around 5500-6000 starts to miss, break up so looking at monitor screen I noticed the CDbug number changes when problem starts,(had this issue before and found it to be the type cam sensor I was running) goes from steady 53 to 112-114 then back. Changed sensor inspected wiring, verified shield is grounded back to ECU. Can find no issue, I have no idea how it went from running perfectly to this however since I've got 2 races left I'm afraid I'll once again have to revert back to my carb and MSD setup. Damn I hate doing that but after spending the aftyernoon trying to track this issue down I'm just worn out. Thanks for al the help on this and I guess I'll go at the CNP, EFI setup next spring. Joe
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #16 ·
Alright it suddenly ocurred to me last nite that prior to this CDbug issue I did make 1 small change on my car. I had wired the power for my coils to the same relay (40 amp) that powers my car datalogger, I had been seeing spikes on the traces and in the data, so I moved the datalogger power elsewhere. The spikes are now gone on the data however now I'm having this issue, the coils are grounded directly to the cylinder heads BTW. My belief was I was getting feedback from the coil feilds collapsing cause the spikes, since there are no longer any spikes, could it be my datalogger was acting as some sort of filter or capacitor and by removing it from the circuit has induced this issue? Really I'm at a loss as to how all of a sudden I'm once again seeing the CDbug issue. I'm figurinfg when the ECU is seeing this shift in the CDbug number it's resetting and causing the percieved miss. Does this sound possible to anyone? Then what would be the fix? 1 more thing I checked my shield on the cam sensor wire for continuity to ground it has it, should it be going to chassis ground, as I was testing from shield to the engine. Wasn't sure if the shields were put to chassis ground thru the ECU or not.
 

·
Registered
Joined
·
2,564 Posts
Dont use the engine as a conductor.
Your coils secondary is dumping current to your heads which is unavoidable.
Keep the relays away from the coils and give them a real path to ground "bat-".
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #18 ·
Well I'm not sure I'm using the engine as a conductor, the coils are grounded to the heads and the heads are grounded to the chassis. Is this not an acceptable arrangment? Up until last week it has worked fine however I'm up to a rewire if that's what is required. What would you suggest, running a dedicated ground from the heads to Batt-? A long run since my batteries are in the trunk but can do that. What gauge wire would be required?
 

·
Registered
Joined
·
2,564 Posts
Its not ideal, particularly when the coils dump their secondary current into the same node where you are getting your relay grounds.

At the very least pull your own actual grounds to the relays (dont use the engine block for the return its noisy as hell).
 

·
Registered
Joined
·
130 Posts
Discussion Starter · #20 ·
Ok so the relays are grounded to the chassis, basically so is the engine. So one of them needs to go back to the batt - correct merely seperating the 2 ground paths? I thought maybe grounding the engine directly to the batt- would be the best since the batt would serve as a bit of an absorber for the current?
 
1 - 20 of 23 Posts
Top