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Matt, Engineer
Category: Honda
Satisfied Customers: 21373
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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I have a problem regarding my transmission. My car is a 90

Customer Question

I have a problem regarding my transmission. My car is a 90 SM4 2.2 auto. The car revs too high about 4000-5000 before it shifts gear. From 1st gear till 4th. I tried change the TCU but it still happens. And I found out that the check engine light didnt turn on before I start the car. I'm afraid that the TCU didn't sync with the gearbox. Or do I have wiring problem?
Submitted: 1 year ago.
Category: Honda
Expert:  Matt replied 1 year ago.


I'd take a look at the colour of the transmission oil it should be a dark red colour if its black then its needs changing together with its oil filter. Also check its level with the gearbox warm and just after you've moved the selector through all the positions leaving it for a few seconds in each one and then back to park.

You can top up the box through the dipstick tube if required and make sure you use the correct oil for your car as the fluids are not all the same. Some manufacturers recommended a change of the type of fluid - Speak to the local dealer and parts dept - they will advise you on the updated fluid to use..

If that’s all OK then check all the electrical connections of the gearbox, remove each connector in turn and spray with contact cleaner. Check that the wiring to the gearbox is in good condition with no chafing or frayed wire

If this is OK then check the selector lever linkage / cable under the car for any broken parts and or missing bolts as if this is coming adrift then it can rub on the car body or just bind up

Then - you should top up if low and re try it - or, if its level is ok - and condition is ok - then the next step is to drain it off - open it up and clean the filters inside -- refill and retry it..

Often - just doing the above cures many troubles with these - and you certainly can't condemn the unit without doing the above...

t in insufficient fuel being delivered to the engine

Customer: replied 1 year ago.
Hi Matt,I had my ATF changed last December and it's in good condition with no leakage.Where is the selector lever linkage located?And I can't understand your last sentence "t in insufficient fuel being delivered to the engine"I will try to check the gearbox wiring first.
Customer: replied 1 year ago.
I have checked my gearbox wiring and it seems ok without any connectors loose out
Expert:  Matt replied 1 year ago.


the selector lever will run down from the gear selector in the car to a linkage on the transmission

you'll have to get under the car to check it

and the original post was foreshortened for some reason

as I also said that it would be worth checking for air / vacuum leaks after the MAF as the MAF signal is used to provide the main torque sensing into the ECU and so has a strong effect on the quality and control of the gearshift

it can also result in insufficient fuel being delivered to the engine

Customer: replied 1 year ago.
For the vacuum leaks do you mean to check the throttle body black hoses? If yes, I had check it and all the hoses are connected end to end. But for the MAF I haven't check yet.For the CEL, what is the cause that it didn't turn on before I start the engine?
Expert:  Matt replied 1 year ago.


any vacuum leak on the entire intake pipework and inlet manifold will produce this issue

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements

Customer: replied 1 year ago.
Hi MattCan I use WD-40 in exchange of propane?
Expert:  Matt replied 1 year ago.


not really , you need a gas to be drawn into the gaps

wd40 also isn't as combustible as propane or butane

Customer: replied 1 year ago.
Hello Matt,I've checked all the vacuum hoses and the leaking is nowhere to be found. No problem with the vacuum.
Expert:  Matt replied 1 year ago.


in that case I'd check that the throttle is closing off correctly

as it should idle at about 600rpm when warm, if its higher than this I'd clean out the idle speed control valve and also check that the throttle is cleaned out and snaps closed onto its stop

if you still have a high idle then adjust the throttle stop to close the throttle a little

Customer: replied 1 year ago.
IACV checked and cleaned. My rpm idle is perfect. Any possibility on the VSS?
Expert:  Matt replied 1 year ago.


OK if you have a fluctuating speedometer then yes a faulty VSS is a possible cause

so if this is true check the wiring and connections to the sensor on the transmisson

Customer: replied 1 year ago.
My speedo didn't fluctuate. Does my car have a throttle body map sensor?
Expert:  Matt replied 1 year ago.


I'd say its pretty likely that your car has the Map sensor on the throttle as its shown on this later car, might be worth disconnecting this and see how it effects your issue

Customer: replied 1 year ago.
Nope, still nothing changes.
Expert:  Matt replied 1 year ago.


in that case I'd say its time to get the car plugged into a dealer level diagnostic device to see if they can monitor the gearbox functionality and check the sensor inputs

Customer: replied 1 year ago.
How much would it cost roughly?
Expert:  Matt replied 1 year ago.

Usually their minimum labor charge as its only 10 minutes work

if you can find a independent specialist then they are usually half the labour charge of the dealer