Intermittent Speedo and fan speed

[Intrepidation]

So the other day I noticed that the Prizm's blower motor kept changing it's speed despite being always set tof ull. It would slowly build up to normal speed, then go back down to a lower speed, then go back up. This would happen over a couple minutes, not like every few seconds. Because it works I'm not that worried...just curious if anyone might know why.

Of greater concern is the speedometer, which stopped working a few days ago as well. Then it started working again....then didn't. Tonight on the highway It would work, then drop down to zero, then a few minutes later jump back up to the correct speed. If the unit itself going or could it be something else?

[Blake Noble]

Our Impala had the same speedometer problems before we traded it off for a G6. First thing I would do is check the speedometer cable.

[Oracle of Delphi]

Sounds like a voltage issue to me ...

[fightingbee]

Is the blower speed somewhat related to engine speed?

If so, you may have an alt. that isn't totally fried but starts to get weak at idle. I had to replace the one in my Achieva for that reason this past fall. Volts were OK >1500 RPM but slowly fell while idling. One or a couple of the diodes had probably burnt up in it or started to "leak" AC current past them.

Also, if it does end up needing an alt. and has the Delco-Remy unit now, i'd swap in the Denso version instead. IIRC they're interchangable and the Denso tends to last longer.

Edited July 22, 2009 by fightingbee

[SAmadei]

Our Impala had the same speedometer problems before we traded it off for a G6. First thing I would do is check the speedometer cable.

Speedo? Does GM even use Speedo cables on anything anymore? I thought everything since the mid-90s would be VSS by now. Of course, the Prizm is from a different manufacturer and era, so its tough to say. If it does have a traditional cable, a fraying or semi-detacted cable can cause interesting results... but in my experience, its a noisy process. Turn off your blower fan and see if the changes in pitch still occur... perhaps they are the cable, not the blower. Checking this is pretty easy... disconnect the cable housing and watch the little nub on the cable inside as you drive... if you can stop it with your fingers, its broken somewhere. Assuming the jacket isn't damaged or kinked, this can be a easy fix... just leave the cable housing in place and get a junkyard cable... take the cable out from one end and thread the junkyard one in... the inner cable will have a little shaped piece crimped on it... in GM's it was a square... and you can get that to line up by just twisting the other end while gently pushing it in. Then drop some oil in and button it up.

If you have an electronic speedo, it could be a connector... most likely on the unshiny side of the car, but also possible on the back of the speedo gauge itself. If you have ABS, some electronic speedos are hooked into the ABS and/or tranny shifting, so a broken speedo can cause alot of trouble and even set a CEL (causing one with tough emission inspections big trouble). I like PCS's idea of low voltage, but I think you would see this with your headlights more than the speedo and/or blower.

As for the blower, see if your car has a replacable blower resistor pack. They actually list them at a lot of auto parts stores. A dying resistor pack could cause some speeds to not work, so I suppose they could also cause undesireable speed changes, as well. Finally, a dying motor is a possibility.

While it seems like a minor thing, I've found that those frigging blower motors are REAL important... like in humid weather or the winter when you need to

get moving and the damn windshield won't clear. My Caprice had a motor die... and I was stuck using a dirty towel on the windshield a few times, so I fixed it... then my sister, who, in her defense, has long legs, accidentally yanked one of the vacuum lines out... rendering the entire HVAC system useless until I got the chance, months later, to completely tear the dashboard apart to figure out where the line went. Ugh.

[fightingbee]

The Prizm used a VSS IIRC. The Suzuki-built Geos were prolly the last GMs to still have speedo cables.

[Intrepidation]

It's not a cable, the gauge cluster is electronic and plugs into a couple of connectors. It doesn't have ABS.

In other news, the Check Engine light came on today on the highway, but went off about 10 minutes after I got off of the highway and didn't come back on the rest of the day.

I can't get a break.

[fightingbee]

I'm still thinking alternator, mostly becuase of the blower speed varation, especally if it speeds up when the engine speed picks up (?)

My thought is that an alt. with a diode problem can cause odd electrical behavior if it is allowing some AC current leakage. The result is that the alt. then produces wavering, noisy DC power that can upset the signal from any sensor whose normal output signal is in the form of an AC sine or square wave. The and input/output speed sensors on electronically controlled auto trannies in particular, and possibly the VSS in your case.

The reason the blower speed would vary also is becuase the alt. also has less output if one of the diodes is not doing its job anymore.

Besides monitoring the voltage output across the battery at various engine speeds, If the Prizm has the Delco alt., it may be producing a faint but discernable whining noise that may also bleed into the AM radio reception if it has such a problem. Some places that load test batteries/alternators have a machine with an oscilloscope-like function to check such a condition as well.

Edited July 23, 2009 by fightingbee

[Intrepidation]

The fan increase/decrease doesn't really match the rpms of the car if I recall. I will have the alternator check out though.

[Intrepidation]

One the way down to Boston last week the Check Engine light came on, and the speedo would not always work. Oddly, since then everything has been working properly.