I am looking for help to understand why my shift light is coming on at 4500 rpm when the accelerator is fully depressed. I'm thinking I'm not getting the response I should because the engine isn't registering a wot condition. I've searched but not found anything relevant.

 

Mine does the same thing. I think that the shift light is to shift when it is most fuel eficient to do so. (to save gas) probably it is just not efficient past 4500. My oppinion I do not have anything to back this up.

 

this is the fsm test for shift light operation on 86-88



hugues -

 

The shift light comes on at any RPM above 2600 (except in 5th gear) when a higher gear will give you better fuel economy.

 

Icemark

If I floor it in my car the shift light does not come on until like 4500rpm it only comes on at 26-2700 if I am driving normaly.

 

Icemark:

what about in reverse above 2600 ?

Does it come on ?

Hugues -

 

Any performance advantage to disconnecting the 5th gear switch?

I know I brought up this question a long time ago and no one could say either way.

 

Yep, that is exactly right, again as long as you are not in 5th gear.

The shift up light is designed to help fuel economy, Just like in the 944. It is not designed to help you drive better (say, at what shift point or whatever) only to help you get the best mileage.

 

Mine flickered in 5th gear so I removed the bulb.

 

Only a loss of gas mileage at freeway speeds.

The 5th gear sensor (or "over the top" switch) sends the signal to the ECU, which advances timing slightly while leans mixture (if the throttle position is less than 50%) for better cruising/highway mileage.

 

That means your 5th gear (or over the top switch as Mazda calls it) needs to be fixed or replaced.

Just taking out the bulb is a micky mouse way to fix things.

 

I don't think so, but I suppose it's possible since the ECU does not have a reverse gear input.

 

Well, if someone were to wire in a switch cutting the signal from the "over the top" switch. Wouldn't that essentially remove the computer handicap from 5th gear and make going insanely fast more possible?

That's what I had in mind when asking anyways. Allow the comp to do its thing when cruising but be able to have the power settings for top end runs.

 

Maybe I wasn't clear.. the ECU is only uses the 5th gear sensor (or "over the top" switch) to advance timing slightly while leaning out the mixture if the throttle position is less than 50% for better cruising/highway mileage.

So if you are WOT (or even at 51% of WOT) the ECU doesn't change a thing.

So removing the switch or bypassing it would do nothing except to throw away a couple of extra MPG only while just cruising.

 

Thanks, I appreciate the help. I am going to check the switches so I can say I did it. How many times can you say you worried about something needlessly on a rex? I am a stickler for the details though.........

 

Sorry Icemark, I read that pretty quickly before.

However, I do have a slight discrepancy with that concept. My S4 TPS is at 80% or greater while cruising. So the over the top switch is pointless regardless of whether or not you bypass it by what you've just stated.

Any comments on this minor problem in reasoning?

 

I don't understand what you’re trying to say... if your throttle is at 80% you are not cruising, and therefore you wouldn't have the advance and the leaned mixture... so why would you want to remove the switch??? Or ...

If you wired a switch as you say, it wouldn't do a thing; driving as you say you are, as the ECU is already is ignoring the over the top signal (except to keep off the shift up mileage helper indicator).

Bypassing the switch or disconnecting the switch, wouldn't change anything if you are driving at 80% on throttle.

Again maybe I wasn't clear. 5th gear only affects the timing and mixture at less than 50% throttle. At over 50% throttle the ECU reverts back to standard full on normal operation (as long as the TPS is set correctly). The 5th gear sense is used to help increase highway mileage. This happens unless you are driving hard.

Maybe you mean jumpering the switch on when you’re driving not in 5th gear??? I suppose then you might gain a miniscule increase in gas mileage, if you could keep your foot pressed less than 50% of the way down. Of course I wouldn't want to try that on a NA since there is no knock sensor and the lean mixture driving with less than 50% throttle might have other problems.

 

Tsk, tsk, tsk. The most important use of the Over The Top Switch has not been discussed. It enables the Split Air Solenoid to deliver more air to your catalytic converter.

You do have your catalytic converters in tip top shape? Right. humor.

 

i have never seen my shift light come on! I guess that means the sensor is disconected. Is that possible?

 

S4 TPS registers 100% @ 1/4 throttle. 50% of throttle travel is well beyond the diferentiation of the TPS as it has not been in contact with the linkage for quite some time by then. This is precisely why I take issue with your claim of the <50% activation. In 5th gear even on a stock engine the car requires nearly 1/4 throttle to maintain cruise speeds.

 

actually no, according to the 87 and 88 FSMs, the S4 TPS registers 100 percent on at 18 degrees of rotation, since the throttle only fully travels about 40 degrees, that makes it pretty close to that 50% throttle.

Are you saying that yours is activating 100% at only 10 degrees of throttle rotation?

 

a shift light ?? WTF !

is this only on US models and NOT UK and JAP ones ?

where is it ? is it in the dash etc... ?

and having looked at the piccies above - where abouts is it - i can have a look to see if i have one ??

thanks

G.

 

grinder

The shift up light is only on series four cars and is towards the bottom of the tach. Just hold your car at about 3k rpms in first and it should come on. If not then your car don't have one or your bulb burned out and needs replacing.

 

WOT = 40 degrees of rotation??? More like 85-90. I'll look into the FSM and see what you're talking about later. Unfortunately I don't have time right now.

 

Found only on US Spec series 4 FCs