Cruise Control Intermittently working-- stumped

viper69 · Post by **viper69** » 29 Sep 2018, 15:25

Here's what I know...

Previous to this week, I haven't used the Cruise Control in maybe 2 years.

A few weeks ago I had the battery changed to a brand new one, no issues. I've driven the car over a 100 miles since then, 30 in town, 70 highway. Oil changed recently too.

A week ago I had spark plugs changed- engine is no longer coughing.

Drove at 65 mph l last week for an hour- used cruise control (CC) no issues.
Driving back on a second day, I engaged the CC at the same speed, no issues, then went over a small bump, the car began to decelerate due to slope of the road, the CC had disengaged.

During this drive it would go on/off at the same rate of speed, not always due to a bump on the road.

Also, following day, the CC wouldn't engage at all. I gave up.

I don't have any engine/dashboard lights on or blinking. Nothing blinked when I went over the few small bumps.

What should I be looking for, suspects causing this error? I'm bringing it to my Volvo mechanic, he's from Sweden this is all he works on, never been wrong before.

Can a computer diagnostic reveal the issues?

It's a 1995 so I venture to guess at some point some things may just break. I'm the only owner too.

WhatAmIDoing · Post by **WhatAmIDoing** » 29 Sep 2018, 15:47

Cruise control is vacuum controlled. Would have to test pump (under the battery). Probably need to replace the vacuum lines. There's also a diaphragm that can break. Plus there are switches under the dash near the brake and clutch (tell CC whether you engage brake or clutch) that may need to be adjusted. That's about the extent of my knowledge. This is something I've been meaning to fix for 2 years now.

viper69 · Post by **viper69** » 29 Sep 2018, 17:03

I wonder how much this is going to cost..jeez

Post by **abscate** » 29 Sep 2018, 18:58

Very little for old vacuum hose replacement. 20 year old rubber hoses all pretty much need replacing.

Post by **JimBee** » 29 Sep 2018, 20:13

I looked this up recently, also, due to same symptoms in my 96 base 850. I haven't solved mine but I will explore one suggestion: the control module in the fuse box. Somebody confirm this, I think it's the one next to the headlight module. In that post I think someone suggested cleaning the contacts first. If that doesn't work I don't know if there's a way to test it off the pins.

I just found the link you need to troubleshoot all the parts of the cruise system. I'll be using it too. Read down the thread, erik posted the link:
viewtopic.php?f=1&t=65680

Post by **precopster** » 29 Sep 2018, 22:45

I had a CC module lying around so I opened it up. Inside there is a relay with contacts. Clean these with sandpaper preferably 400 grit.

WhatAmIDoing · Post by **WhatAmIDoing** » 30 Sep 2018, 00:29

The vacuum lines are a pretty good bet and cheap to replace. The switches for the clutch and brake are also an easy place to start. I don't know if there is any record of the CC pump or relay ever going bad, but those should be easy to test. Also should check the fuse for CC. Some users have reported the wiring for CC failing in the turn signal lever.

Edit:

JimBee wrote: ↑29 Sep 2018, 20:13 I just found the link you need to troubleshoot all the parts of the cruise system. I'll be using it too. Read down the thread, erik posted the link:
viewtopic.php?f=1&t=65680

This is a good guide. Does anyone know if there are difference for CC between a 95-96 and a 98?

Post by **abscate** » 30 Sep 2018, 02:16

I think the P80s are all the same, smirks from the 1999- crowd with fully electronic cruise control with no parts to fail notwithstanding...(basking in the warm glow of the ETS light)

Post by **precopster** » 30 Sep 2018, 02:26

Those Cruise modules date back to the early 80s. I had the same one on my 1989 turbo 740; not exactly miniaturization on the board.

The cut-out over a bump suggests an electrical contact problem.

markone · Post by **markone** » 30 Sep 2018, 04:42

I had a similar issue with my ‘97 850 CC. Turned out it was the battery sitting atop, and pinching, the vacuum line.