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1989 Pontiac Grand AM Intermittent Lurching Under Load

Sent to Car Experts May 20, 2006 8:34 a.m.

1989 Pontiac Grand AM with a 4 cylinder TBI engine runs well most of the time; however, under heavy load like hill climbing or passing, the car will lurch. Also, the check engine light flashes and the speedometer bounces from speed to zero. these happen with each lurch. Generrally, this stops within 30 to 45 seconds and the vehicle returns to normal operation. I have gone through the trouble section of the Haynes Manual and find nothing wrong. Does anyone have an idea as to what causes this to happen?

Optional Information:
1989 Pontiac Grand AM

Already Tried:
Changed fuel filter Checked, gapped and replaced spark plugs Checked plug wires Repaired several probable vacuum leaks thorough visual inspection of plug wires and vacuum lines

Customer (name blocked for privacy)

Status: Closed Value: $15

Answer
May 20, 2006 8:45 a.m. (10 minutes and 40 seconds later)

REPLIED

Check the speed sensor(in trans) if it is malfunctioning the trans will operate erradically. If you axtract the code it will most likley confirm. Some parts stores do this at no cost Autozone etc Does the speedo become erratic first then lurch, this would also be good indicator. If this has been helpful an accept would be appreciated if you need more info let me know Thanks Bill

Bill -- mechanic -- 100% Positive Feedback on 98 Car Accepts
mechanic since 1976

Reply to Bill
Sent May 20, 2006 9:36 a.m. (51 minutes and 49 seconds later)

Dear Bill,

Hello, thanks for responding so quickly.

The check engine only flashes, once operation returns to normal the check engine is not lit, so I don't think it is having a code set. Please correct me if I am wrong.

Also, it appears that the lurching precedes the speedometer bouncing and the check engine flashes on when the speed indication falls toward zero and extinguishes when the speed indication returns.

Another person, a GM technician, mentioned checking a type of speed switch that is located along the steering column. Does this sound feasible?

Thank you for you help.

Sincerely,

Joe M.

Customer (name blocked for privacy)

Answer
May 20, 2006 10:12 a.m. (35 minutes and 3 seconds later)

ACCEPTED

The code should still be in memory if it can be extracted would save alot of guess work, also check the modulator valve does it hold vacumm?

Bill -- mechanic -- 100% Positive Feedback on 98 Car Accepts
mechanic since 1976

Reply to Bill
Sent May 20, 2006 8:35 p.m. (10 hours and 23 minutes later)

Dear Bill

I have more questions.

Where is the speed sensor located, in or on the transmission?

Where is the modulator valve located, is it the electrical unit at the bottom right side of the transmission that is held in place with a wire spring clip?

Thanks again for your assistance.

Respectfully,

Joe M.

Customer (name blocked for privacy)

Answer
May 20, 2006 9:24 p.m. (49 minutes and 9 seconds later)

REPLIED

The modulator should be vacumm operated fromt side of transaxle a dashpot set up (small canister with vacumm line attached) the spped sensor, I should have asked before, what is your engine size 2.0,2.3, or 2.5?

Bill -- mechanic -- 100% Positive Feedback on 98 Car Accepts
mechanic since 1976

Reply to Bill
Sent May 20, 2006 9:48 p.m. (23 minutes and 44 seconds later)

Dear Bill,
The engine is a 2.5 liter, engine code U.
Joe M.

Customer (name blocked for privacy)

Answer
May 20, 2006 10:14 p.m. (26 minutes and 11 seconds later)

REPLIED

The vehichle speed sensor should be located on transaxle with a purple and a yellow wire going to it If you can,t find it let me know.

Bill -- mechanic -- 100% Positive Feedback on 98 Car Accepts
mechanic since 1976

Reply to Bill
Sent May 21, 2006 9:26 a.m. (11 hours and 11 minutes later)

Dear Bill,

Good morning, I found the VSS, is there any way to test it?

Still can't locate vacuum modulator valve?

Thanks,

Joe M.

Customer (name blocked for privacy)

Answer
May 21, 2006 9:55 a.m. (28 minutes and 35 seconds later)

REPLIED

The only test I can find right now is to jack drive wheels off the ground and back probe the vss with a volt meter on the ac scale put the car in gear vss voltage should increase smoothly as speed increases if no voltage or erradic change. You can try but in your case it might be tough where the problem is intermittent.I would also just give the wires a good look and jiggle while up in air make sure its not a bad wire

Bill -- mechanic -- 100% Positive Feedback on 98 Car Accepts
mechanic since 1976

Reply to Bill
Sent May 21, 2006 7:27 p.m. (9 hours and 31 minutes later)

Dear Bill,

I was able to test the speed sensor with a Wells unit at Auto Zone and it tested out as working.

Still went to the junkyard and tried two additional speed sensors. Both checked out good at Auto Zone, neither fixed the problem.

Will attempt to locate vacuum modulator vale, still unable to find it, also, I am looking into the transaxel clutch control.

Thanks for all your help.

Regards,

Joe M.

Customer (name blocked for privacy)

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