My 1990 Volvo 740 has a problem that I & the local dealer have not been able to solve. The problem is it misfires and the tachometer jumps. The tachometer jumps more often than it misfires. This happens most often when speeding-up. It seem to be worst the hotter the day and the engine is. Tried all the normal tune-up stuff: spark plugs, wires, cap etc. Also there is not any ignition trouble codes. Help! has one else had this problem before? What's the fix?

 

Has the speed sensor and the power stage been checked yet?

 

Thanks for the reply. About 6 months ago my daughter had the car in San Antonio and it would not start – I went over there one weekend and tried to trouble-shoot the problem. At that time I thought the problem might be with the power stage – I order one from the local NAPA store on Saturday and returned home to Houston. My daughter’s boy friend replaced the power stage when it arrived – but the car still did not start so I bit the bullet and had it towed to the Volvo dealer in San Antonio. The dealer replaced the crankshaft position sensor or RPM sensor. After that it started up fine but my daughter told me it didn’t drive as good as it use to. After she returned the car to me I found it had the problem I described. I’m looking at a manual that describes a test procedure for the RPM sensor that looks simple. It says I should check for resistance between 200-500 ohm at the plug end. Does that sound right to you? If the RPM sensor is good, do you know any way to test the power stage? The cheap manual I have does not have any test procedure for this.

 

Usually the best test for either unit ,is a substitution with a known good one. One other you can try is to re-do all of the grounds in the engine compartment. Remove, clean, replace and seal with di-electric grease. Even go so far as to replace the end connectors if they look suspect, just make sure that they are soldered and sealed if you do. Tho other to check would be what the ECU is 'seeing' by testing the connection at the ecu from the rpm sensor.

 

Ignaus, thanks for the reply! -- I'll definitely take your advice and check & clean the grounds. I don't how to check would be what the ECU is 'seeing' by testing the connection at the ecu from the rpm sensor.

 

You don't have a check engine light on do you?

You still have the new power stage on there don't you?

I also agree with cleaning all the ground wires.

Also look at the wiring harness at the big connector and see if any of the wires are bare.

 

Tech. Thank you for your reply. Yes -- I still have the new power stage on there and I don't have a check engine light on. I do have an update: I checked the grounds and found one loose. I remove the ground strap – clean and re-installed with the dielectric grease. I also check the RPM sensor and found that it had three post not the two my book showed. I have 195 ohms between post #1 and # 2 and just low between any other combination. I test drove the car after working on the grounds and checking the RPM sensor and while the performance of the engine while cold seemed better the problem was the same once it warmed up. What do think I should try next? Also where is the big wiring harness connector?

 

You don't happen to thave a Rex-Regina equipped car, by any chance? What doe the coil look like? Does it have an AMM, or a small two wire connector at the sensor in the air box snorkel?

 

The big plug for the engine harness is at the back of the intake manifold.It is a grey plug with 8 wires in it.

 

I think it's an EZK system the coil looks old fashion two small wires and the center distributor wire.

 

Yes it is the EZK The regina system uses a square coil.

 

Thanks to all,

Update: I've ordered a new power stage and rpm sensor. Please standby for more! I really appreciate and need all your help!

 

Thanks fro the update keep us posted.

 

Update: It's fixed! -- The new power stage did the trick -- The new rpm sensor gave me the same ohms reading right out of the box as the one I had so I didn't change it. Thanks to everyone for your help!

 

Glad it is fixed.