Auto Service World
Feature   February 1, 2000   by Craig Cook

Taking a Diagnostics Bath or an exercise infrustration can be a learning experience

After many hours of frustrating diagnosis and second guessing, this technician finally solved the problem. It would have taken much less time had the right part been delivered in the first place. Imagine if he didn't have access to car manufacturer TSBs and a Ford factory engineer?


It’s one thing to work on high tech vehicles with all the access to information that working at a new car dealership service department affords.

Life in the independent aftermarket is a lot tougher than that, as the following story about diagnosing a driveability problem with a 1992 Ford F 150 pick up truck illustrates.

Have you ever done everything possible and still come up short? That’s how I felt after spending hours trying to trace the source a surging problem at highway speed and an erratic speedometer.

A quick road test revealed that the speedo would bounce very erratically as soon as the vehicle got above 80 Km/h, and a noticeable surge would be present at the same time.

After a visual check under the hood, and associated wiring, I checked for trouble codes. None were present.

Then I performed a Key On Engine Running test, achieving a pass result. I then grabbed the wiring diagram and began my diagnosis.

According to the diagram the speedometer head gets its reading from the speed sensor located in the rear axle.

A brief description of Rear Speed Sensor

The speed sensor is of the permanent magnet design, utilizing a single wire wrapped around a soft bar magnet.

A reluctor wheel or exciter ring, as some people call it, attached to the ring gear in the rear axle spins as the vehicle moves. The resulting movement causes the speed sensor to produce an AC signal which the speedometer head, (called the Programmable Speedometer Odometer Module) uses to calculate road speed.

I got an acceptable reading of 1500 ohms when I checked the resistance of the sensor with my ohmmeter.

Afterwards, I hooked up my lab scope and drove the vehicle at 90 Km/h on the hoist. See figure 1 for test results.

Seeing as the results for the speed sensor were acceptable, I decided to check for technical service bulletins.

After careful checking for TSBs I found one that matched the problem. It was Ford TSB No. 96-21-11.

It stated the speedometer needle may waver ‘and /or’ a light surge may occur in some vehicles when speed control is used at highway speeds between 80 – 113 Km/h. This may be due to slight dents or chips in either the exciter ring or excessive air gaps between vehicle speed sensor and the exciter ring.

It said to replace the PSOM – if required, because the new PSOM had increased immunity to system variability.

Technicians were urged to refer to the following diagnostic procedure for details.

Diagnostic procedure

This bulletin refers to diagnostic procedures in manufacturer service manuals. Refer to the appropriate cruise control system articles in accessories/safety equipment section.

Perform normal PSOM diagnostics per Pinpoint Tests “H ” and “J” of the 1996 F-Series / Bronco Body / Chassis Service Manual.

As it turned out, the ultimate success or failure in finding the cause for this problem turned on an innocuous statement at the bottom of the TSB.

However, the absolute importance of that statement in the TSB only became evident much later. It said the dealership must tell the odometer exchange centre of the need for a PSOM 3 level cluster for a speedometer needle waver concern.

After reading the TSB, I was convinced that this was the fix for the problem I had in the shop.

I called Ford and priced a new PSOM Cluster.

After getting the green light from the customer, I ordered the PSOM from Ford.

When ordering the PSOM, the Ford parts person will fax over a form for you to fill in. It is very important that you fill this form in correctly, detailing specific tire size, axle ratio, kilometres and so on.

So I filled in the form and faxed it back.

Two days later the part arrived, I installed it and drove the vehicle onto the hoist again, where (I could hardly believe my eyes) the problem was still there!

Back I went with the lab scope for further diagnosis.

I double checked the speed sensor again. Everything looked fine. I even put my lab scope into record mode just in case the signal was dropping out and my eye wasn’t fast enough to catch it.

I grabbed the wiring diagram again and proceeded to do voltage drops and continuity checks on all the circuits leading to and from the PSOM cluster.

Everything checked out satisfactory. The customer complaint was that the vehicle would also surge on the highway.

Seeing as this vehicle didn’t have cruise contol and the TSB made note of a problem more noticeable with cruise control engaged, there had to be some input that the EEC 4 module was processing in order for the vehicle to have the surge condition.

See figure 2 for test results.

Figure 2 clearly shows the signal breaking down.

Seeing as computers work with digital signals, the PSOM cluster digitizes the AC Signal that comes from the speed sensor.

This signal is then sent to the EEC 4 module and cruise control module (if so equipped).

This signal is very important. The EEC 4 module uses it to compute Idle-air bypass, torque-converter lock up and, on some cars, the electric cooling fan is disabled above 72 Kph.

So what was the problem?

I spent another three hours going through the system, double checking everything and still no luck.

I even brought in another similar vehicle, put it up on the hoist opposite mine, and wired the speed sensor of that truck to drive the PSOM on the problem truck, and still no fix.

I finally made a phone call to the Ford service manager, in a last ditch effort for sanity.

This guy was more than helpful. He asked me to bring the truck over to the dealership.

With the vehicle I brought all my saved lab scope waveforms, wiring diagrams and details of the preceding horror story.

After talking for several minutes going through different scenarios, he called the Ford technical help line, a direct line to Detroit to speak directly with a Ford engineer. Nice set up.

After a few minutes, he asked if I had installed a Level 3 PSOM? I shrugged my shoulders, explaining that I installed the part that was sent to me.

The Ford engineer then explained how to differentiate between a Level 1 PSOM and a Level 3 PSOM.

To determine the level of PSOM that you have, remove the gear selector indicator from the bottom of the cluster. The speedometer needle mounts to a printed circuit board on the speedometer head.

If the printed circuit is brown then you have a Level 1 PSOM. If the printed circuit is green, then you have a Level 3 PSOM. I checked mine and sure enough (Murphy’s Law) it was Level 1.

It turns out I should have specified the Level 3 PSOM.

Like I would know the difference in the aftermarket.

Imagine if you had no access to TSBs?

Thinking back to the start of this job, I neglected to tell the parts guy that the speedo needle was erratic. I should have told him that I wanted the level 3 PSOM for the speedometer waver concern based on the TSB information.

A day later the Level 3 PSOM arrived. I plugged it in, drove the truck at 90 kph, with no speedo bounce!

It would have been nice to know that when you order a PSOM from Ford you have to tell them that you want a Level 3 PSOM for a speedometer waver concern.

Who would have thought that PSOM I was originally sent had been updated?

There I am in the aftermarket, having pinpointed the problem, installed the new part and it doesn’t fix the problem.

I start second guessing myself while suffering through a lot of frustration.

My diagnosis was right in the first place. Who said that being a mechanic was rewarding?

Note: All testing can be done under the hood. There is a test plug with two wires. See figure 4 to see how all speed sensor testing can be performed there. If you want to test the digital signal to the EEC 4 module, you can disconnect the cruise control module (if so equipped) and perform your testing. If the vehicle doesn’t have cruise control, there will be a harness located next to the speed sensor test plug. Do your testing there. SSGM


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