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Engine & DrivetrainDiscussions about the engine and drivetrain of your vehicle.
This is a discussion thread titled "4 Low signal to ECU", within the Engine & Drivetrain forum, part of the Technical & Vehicle Assistance Forums category.
For a few months I'd been wanting to test out the effect of the "4LO" signal to the Engine ECU & ABS Control from the 4WD ECU. This wire is grounded when the truck is shifted into 4-Lo. I theorized that it was there to modify the shift schedule (I already knew that when in 4-Lo overdrive is locked-out). Here are the results.
When the 4-Lo wire the Engine ECU is Grounded:
1. overdrive is locked out -- ie. it only shifts 1st through 3rd.
2. part-throttle shifts are FIRMER and occur a slightly higher RPMs.
3. WOT shifts are SOFTER (and accompanied by even more spark retard during the shift) and occur at GREATLY lower RPMs (like 3,500-4,000!).
4. WOT downshifts generally don't occur above 25mph from 3 to 2 and 10mph from 2-1.
#3 I expected, #2 & 4 I didn't. For now I'm going to leave the wire to the Engine ECU disconnected so the shifting is the same in 4-Lo as in the other driving ranges. I will also exercise caution since the engine output is no longer "artifically limited" as it is in the stock configuration.
Another tidbit: The 4-Lo Signal to the ABS Control causes the ABS to TURN OFF! So FWIW, when you're doing serious off-roading in Low Range the ABS is automatically disabled which is generally preferable. If you decide to interrupt the Low Range signal to the Engine ECU, do it at the Engine ECU itself to avoid interfering with this feature.
Just thought some of you would find this interesting.
I assume the torque converter never locks with 4LO grounded? It would be interesting to watch the signal going to the transmission line pressure control solenoid to see if the firmer shifting at low throttle openings is due to higher line pressure, or just because there is not as much load at a given shift point. Did you ever cause the check engine light to come on by changing the state of the 4LO ground without being in 4WD? I wonder if the top speed limiter still works with the 4LO signal grounded. although i think the limiter hits right around redline in 3rd anyway.
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Originally posted by JeffD I assume the torque converter never locks with 4LO grounded? It would be interesting to watch the signal going to the transmission line pressure control solenoid to see if the firmer shifting at low throttle openings is due to higher line pressure, or just because there is not as much load at a given shift point. Did you ever cause the check engine light to come on by changing the state of the 4LO ground without being in 4WD? I wonder if the top speed limiter still works with the 4LO signal grounded. although i think the limiter hits right around redline in 3rd anyway.
I didn't notice the lockup. I actually didn't think to pay attention to it at the time.
The firmer low-throttle shift must have been from more line pressure. While in 2WD I did two 1/4 throttle runs from a stop to about 30. The first was with the line ungrounded (normal). The second with the line grounded. When it was grounded the shifts were a little later and noticably firmer. I thought to myself "here's my tow-haul mode!" Then I proceeded to do the full-throttle tests and my ideas for a "simple" tow-haul mode switch vanished...
Check engine light never came on. I drove 2-3 miles with the 4LO grounded when it wasn't in 4LO.
Top speed limiter is based on SPEED, not RPM. Every vehicle seems to be slightly different, but in the case of mine it will hit the top speed limiter before redline in 3rd. If you hold the gas to the floor, it will stay in 3rd and 105-107 all day long. If you let up and let it upshift to 4th but continue to maintain 1/2 throttle it will stay in 4th an continue to hover right at 105-107. Based on the amount of pull in 4th from 100-107 at half throttle, I'd guess a V8 Tundra could hit 125-130 if it weren't artifically limited.
2. part-throttle shifts are FIRMER and occur a slightly higher RPMs.
That is an understatement, even with the FIRMER in all caps! JARRING would be a better word! Mine is so FIRM, that I usually just leave it in LOW and just creep. I've been afraid to let it shift, especially if I have passengers. I've often wondered what WOT in 4LOW would do. I've been tempted to try it, but with the truck off warranty, I figured it would probably blow the front end or something. There have been times when I have considered racing someone in this mode, since the truck is so slow off the line in 2HIGH. I suppose the gearing would be way too short for 1/8 mile drags? The other problem with this is that you would have to stop and get back into 2WD before you could do much turning. What would the top speed of the truck be in 3rd gear low range? It would need to be near 70 MPH for 1/8 mile drags.
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As an aside (and not meant to hijack this thread), it would be really nice to have the wiring worked out for the front diff sensor that allows the transfer case to shift into 4LO so that if someone where inclined to do a solid axle swap, they can do the wiring themselves so that 4LO will still engage without having that sensor around.
Tundraon42s had issues with this when he did his swap and ended up shorted out some stuff when he tried it himself and then had to have Toyota do the wiring to get it to work. It would be nice to be able to do this yourself.
The 4LO shifting is FIRM, but it is actually pretty smooth after you get into 2nd. The transition from 1st to 2nd is the hardest shift I've ever felt from an automatice tranny
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Originally posted by Dude Boy That is an understatement, even with the FIRMER in all caps! JARRING would be a better word!...
I've often wondered what WOT in 4LOW would do.
What would the top speed of the truck be in 3rd gear low range?
It is firm in 4HI with the wire grounded to "fool" the ECU into thinking it was in 4LO. When you compound the firm shift with the reduction of low range you end up with an even firmer (even jarring) shift.
As I stated, WOT in 4LO is greatly softened and much earlier RPM than in 2/4HI.
If you cut the 4LO wire to the ECU, 4th gear would be allowed in 4LO. I would estimate 65-70mph in Low range 4th gear.
Originally posted by TMS2U As an aside (and not meant to hijack this thread), it would be really nice to have the wiring worked out for the front diff sensor that allows the transfer case to shift into 4LO so that if someone where inclined to do a solid axle swap, they can do the wiring themselves so that 4LO will still engage without having that sensor around.
Tundraon42s had issues with this when he did his swap and ended up shorted out some stuff when he tried it himself and then had to have Toyota do the wiring to get it to work. It would be nice to be able to do this yourself.
The 4LO shifting is FIRM, but it is actually pretty smooth after you get into 2nd. The transition from 1st to 2nd is the hardest shift I've ever felt from an automatice tranny
I could design a circuit to eliminate the front axle disconnect if desired. I was already working on a mod to allow 2LO shifting on the stock setup. I don't know if anyone would be interested in that.
Originally posted by Dude Boy Did the "4-Low" light come on in the instrument cluster when you grounded the wire?
No. I cut the wire between the Engine ECU and 4WD ECU and grounded the Engine ECU side for testing purposes. Because the wire was already isolated from the 4WD ECU, there was no effect on those systems -- the 4WD system & indicators operated normally. However, now with that wire still severed, the transmission shifting and torque management is no longer "modified" in 4LO -- it shifts the same in 4Lo as in 2/4Hi.
that could be nice to have the 4lo gear on only the rear wheels and front wheels free, when towing. I would think that would work out nice in a lot of situations.
I could design a circuit to eliminate the front axle disconnect if desired. I was already working on a mod to allow 2LO shifting on the stock setup. I don't know if anyone would be interested in that.
anyone who wants to creep around on easy slickrock trails, or for some reason needed low gear on dry ground, would surely find that useful. you can get manual hubs, but they're one large rather than a couple hours with a few relays, toggle switches and a soldering iron.
So how would I go about making this circuit? I don't know much at all on the subject, this sounds like something I would like. I have an 86 toyota 4x4 and run low gear without locking the hub quite a bit.
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4 Low signal to ECU
Linear Mode
